<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Boost.MultiIndex Documentation - Tutorial - Basics</title> <link rel="stylesheet" href="../style.css" type="text/css"> <link rel="start" href="../index.html"> <link rel="prev" href="index.html"> <link rel="up" href="index.html"> <link rel="next" href="indices.html"> </head> <body> <h1><img src="../../../../boost.png" alt="boost.png (6897 bytes)" align= "middle" width="277" height="86">Boost.MultiIndex Tutorial: Basics</h1> <div class="prev_link"><a href="index.html"><img src="../prev.gif" alt="Boost.MultiIndex tutorial" border="0"><br> Boost.MultiIndex tutorial </a></div> <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.MultiIndex tutorial" border="0"><br> Boost.MultiIndex tutorial </a></div> <div class="next_link"><a href="indices.html"><img src="../next.gif" alt="index types" border="0"><br> Index types </a></div><br clear="all" style="clear: all;"> <hr> <h2>Contents</h2> <ul> <li><a href="#intro">Introduction</a> <ul> <li><a href="#multiple_sort">Multiple sorts on a single set</a></li> <li><a href="#list_fast_lookup">A bidirectional list with fast lookup</a></li> </ul> </li> <li><a href="#index_spec">Index specification</a></li> <li><a href="#tagging">Tagging</a></li> <li><a href="#iterator_access">Iterator access</a></li> <li><a href="#index_types">Index types</a> <ul> <li><a href="#ord_indices">Ordered indices</a> <ul> <li><a href="#unique_non_unique">Unique and non-unique variants</a></li> <li><a href="#ord_spec">Specification</a></li> <li><a href="#key_extraction">Key extraction</a></li> <li><a href="#comparison_predicates">Comparison predicates</a></li> <li><a href="#special_lookup">Special lookup operations</a></li> <li><a href="#range">Retrieval of ranges</a></li> <li><a href="#ord_updating">Updating</a></li> </ul> </li> <li><a href="#seq_indices">Sequenced indices</a> <ul> <li><a href="#seq_spec">Specification</a></li> <li><a href="#list_ops">List operations</a></li> <li><a href="#seq_updating">Updating</a></li> </ul> </li> </ul> </li> <li><a href="#projection">Projection of iterators</a></li> <li><a href="#complexity">Complexity and exception safety</a></li> </ul> <h2><a name="intro">Introduction</a></h2> <p> We introduce the main concepts of Boost.MultiIndex through the study of two typical use cases. </p> <h3><a name="multiple_sort">Multiple sorts on a single set</a></h3> <p> STL sets and multisets are varying-length containers where elements are efficiently sorted according to a given comparison predicate. These container classes fall short of functionality when the programmer wishes to efficiently sort and look up the elements following a different sorting criterion. Consider for instance: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>employee</span> <span class=special>{</span> <span class=keyword>int</span> <span class=identifier>id</span><span class=special>;</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>name</span><span class=special>;</span> <span class=identifier>employee</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>id</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>name</span><span class=special>):</span><span class=identifier>id</span><span class=special>(</span><span class=identifier>id</span><span class=special>),</span><span class=identifier>name</span><span class=special>(</span><span class=identifier>name</span><span class=special>){}</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special><(</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&</span> <span class=identifier>e</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>id</span><span class=special><</span><span class=identifier>e</span><span class=special>.</span><span class=identifier>id</span><span class=special>;}</span> <span class=special>};</span> </pre></blockquote> <p>The fact that IDs are unique to each employee is reflected by the way <code>operator<</code> is defined, so a natural data structure for storing of <code>employee</code>s is just a <code>std::set<employee></code>. Now, if one wishes to print out a listing of all employees in alphabetical order, available solutions may have disadvantages either in terms of storage space, complexity or ease of maintenance: <ul> <li>Copy the employee set into a vector or similar and sort this by a comparison functor dependent upon <code>employee::name</code>. <li>Keep a secondary data structure of pointers to the elements of the set, appropriately sorted by name. </ul> Of these, probably the second solution will be the one adopted by most programmers concerned about efficiency, but it imposes the annoying burden of keeping the two structures in sync. If the code is additionally required to be exception-safe, this construct easily becomes unmaintainable. </p> <p> Boost.MultiIndex features <a href="#ord_indices"><i>ordered indices</i></a>, which sort the elements according to a particular key, and are designed to help programmers in need of sequences of elements for which <i>more than one</i> sorting criteria are relevant. We do so by defining a <code>multi_index_container</code> instantiation composed of several ordered indices: each index, viewed in isolation, behaves much as an ordered <code>std::set</code> (or <code>std::multiset</code>), whilst the overall integrity of the entire data structure is preserved. Our example problem thus can be solved with Boost.MultiIndex as follows: </p> <blockquote><pre> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>identity</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>member</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=comment>// define a multiply indexed set with indices by id and name</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>employee</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=comment>// sort by employee::operator<</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>>,</span> <span class=comment>// sort by less<string> on name</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> <span class=keyword>void</span> <span class=identifier>print_out_by_name</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>employee_set</span><span class=special>&</span> <span class=identifier>es</span><span class=special>)</span> <span class=special>{</span> <span class=comment>// get a view to index #1 (name)</span> <span class=keyword>const</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>();</span> <span class=comment>// use name_index as a regular std::set</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>copy</span><span class=special>(</span> <span class=identifier>name_index</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(),</span><span class=identifier>name_index</span><span class=special>.</span><span class=identifier>end</span><span class=special>(),</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>ostream_iterator</span><span class=special><</span><span class=identifier>employee</span><span class=special>>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>cout</span><span class=special>));</span> <span class=special>}</span> </pre></blockquote> <p> Instead of a single comparison predicate type, as it happens for STL associative containers, <code>multi_index_container</code> is passed a <a href="../reference/multi_index_container.html#multi_index_container">list</a> of index specifications (<code>indexed_by</code>), each one inducing the corresponding index. Indices are accessed via <a href="../reference/multi_index_container.html#index_retrieval"><code>get</code></a><code><N>()</code> where <i>N</i> ranges between 0 and the number of comparison predicates minus one. The functionality of index #0 can be accessed directly from a <code>multi_index_container</code> object without using <code>get<0>()</code>: for instance, <code>es.begin()</code> is equivalent to <code>es.get<0>().begin()</code>. </p> <p> Note that <code>get</code> returns a <i>reference</i> to the index, and not an index object. Indices cannot be constructed as separate objects from the container they belong to, so the following </p> <blockquote><pre> <span class=comment>// Wrong: we forgot the & after employee_set::nth_index<1>::type</span> <span class=keyword>const</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>();</span> </pre></blockquote> <p> does not compile, since it is trying to construct the index object <code>name_index</code>. This is a common source of errors in user code. </p> <h3><a name="list_fast_lookup">A bidirectional list with fast lookup</a></h3> <p> This study case allows us to introduce so-called <a href="#seq_indices"><i>sequenced indices</i></a>, and how they interact with ordered indices to construct powerful containers. Suppose we have a text parsed into words and stored in a list like this: </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>list</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>></span> <span class=identifier>text_container</span><span class=special>;</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>text</span><span class=special>=</span> <span class=string>"Alice was beginning to get very tired of sitting by her sister on the "</span> <span class=string>"bank, and of having nothing to do: once or twice she had peeped into the "</span> <span class=string>"book her sister was reading, but it had no pictures or conversations in "</span> <span class=string>"it, 'and what is the use of a book,' thought Alice 'without pictures or "</span> <span class=string>"conversation?'"</span><span class=special>;</span> <span class=comment>// feed the text into the list</span> <span class=identifier>text_container</span> <span class=identifier>tc</span><span class=special>;</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>tokenizer</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>char_separator</span><span class=special><</span><span class=keyword>char</span><span class=special>></span> <span class=special>></span> <span class=identifier>tok</span> <span class=special>(</span><span class=identifier>text</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>char_separator</span><span class=special><</span><span class=keyword>char</span><span class=special>>(</span><span class=string>" \t\n.,;:!?'\"-"</span><span class=special>));</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>copy</span><span class=special>(</span><span class=identifier>tok</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(),</span><span class=identifier>tok</span><span class=special>.</span><span class=identifier>end</span><span class=special>(),</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>back_inserter</span><span class=special>(</span><span class=identifier>tc</span><span class=special>));</span> </pre></blockquote> <p> If we want to count the occurrences of a given word into the text we will resort to <code>std::count</code>: </p> <blockquote><pre> <span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span> <span class=identifier>occurrences</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>word</span><span class=special>)</span> <span class=special>{</span> <span class=keyword>return</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>count</span><span class=special>(</span><span class=identifier>tc</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(),</span><span class=identifier>tc</span><span class=special>.</span><span class=identifier>end</span><span class=special>(),</span><span class=identifier>word</span><span class=special>);</span> <span class=special>}</span> </pre></blockquote> <p> But this implementation of <code>occurrences</code> performs in linear time, which could be unacceptable for large quantities of text. Similarly, other operations like deletion of selected words are just too costly to carry out on a <code>std::list</code>: </p> <blockquote><pre> <span class=keyword>void</span> <span class=identifier>delete_word</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>word</span><span class=special>)</span> <span class=special>{</span> <span class=identifier>tc</span><span class=special>.</span><span class=identifier>remove</span><span class=special>(</span><span class=identifier>word</span><span class=special>);</span> <span class=comment>// scans the entire list looking for word</span> <span class=special>}</span> </pre></blockquote> <p> When performance is a concern, we will need an additional data structure that indexes the elements in <code>tc</code>, presumably in alphabetical order. Boost.MultiIndex does precisely this through the combination of sequenced and ordered indices: </p> <blockquote><pre> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index_container</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>sequenced_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>ordered_index</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=preprocessor>#include</span> <span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>multi_index</span><span class=special>/</span><span class=identifier>identity</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>></span> <span class=comment>// define a multi_index_container with a list-like index and an ordered index</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>sequenced</span><span class=special><>,</span> <span class=comment>// list-like index</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>></span> <span class=special>></span> <span class=comment>// words by alphabetical order</span> <span class=special>></span> <span class=special>></span> <span class=identifier>text_container</span><span class=special>;</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>text</span><span class=special>=...</span> <span class=comment>// feed the text into the list</span> <span class=identifier>text_container</span> <span class=identifier>tc</span><span class=special>;</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>tokenizer</span><span class=special><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>char_separator</span><span class=special><</span><span class=keyword>char</span><span class=special>></span> <span class=special>></span> <span class=identifier>tok</span> <span class=special>(</span><span class=identifier>text</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>char_separator</span><span class=special><</span><span class=keyword>char</span><span class=special>>(</span><span class=string>" \t\n.,;:!?'\"-"</span><span class=special>));</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>copy</span><span class=special>(</span><span class=identifier>tok</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(),</span><span class=identifier>tok</span><span class=special>.</span><span class=identifier>end</span><span class=special>(),</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>back_inserter</span><span class=special>(</span><span class=identifier>tc</span><span class=special>));</span> </pre></blockquote> <p> So far, the substitution of <code>multi_index_container</code> for <code>std::list</code> does not show any advantage. The code for inserting the text into the container does not change as sequenced indices provide an interface similar to that of <code>std::list</code> (no explicit access to this index through <code>get<0>()</code> is needed as <code>multi_index_container</code> inherits the functionality of index #0.) But the specification of an additional ordered index allows us to implement <code>occurrences</code> and <code>delete_word</code> in a much more efficient manner: </p> <blockquote><pre> <span class=identifier>std</span><span class=special>::</span><span class=identifier>size_t</span> <span class=identifier>occurrences</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>word</span><span class=special>)</span> <span class=special>{</span> <span class=comment>// get a view to index #1</span> <span class=identifier>text_container</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>&</span> <span class=identifier>sorted_index</span><span class=special>=</span><span class=identifier>tc</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>();</span> <span class=comment>// use sorted_index as a regular std::set</span> <span class=keyword>return</span> <span class=identifier>sorted_index</span><span class=special>.</span><span class=identifier>count</span><span class=special>(</span><span class=identifier>word</span><span class=special>);</span> <span class=special>}</span> <span class=keyword>void</span> <span class=identifier>delete_word</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>word</span><span class=special>)</span> <span class=special>{</span> <span class=comment>// get a view to index #1</span> <span class=identifier>text_container</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>&</span> <span class=identifier>sorted_index</span><span class=special>=</span><span class=identifier>tc</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>();</span> <span class=comment>// use sorted_index as a regular std::set</span> <span class=identifier>sorted_index</span><span class=special>.</span><span class=identifier>erase</span><span class=special>(</span><span class=identifier>word</span><span class=special>);</span> <span class=special>}</span> </pre></blockquote> <p> Now, <code>occurrences</code> and <code>delete_word</code> have logarithmic complexity. The programmer can use index #0 for accessing the text as with <code>std::list</code>, and use index #1 when logarithmic lookup is needed. </p> <h2> <a name="index_spec">Index specification</a> </h2> <p> The indices of a <code>multi_index_container</code> instantiation are specified by means of the <a href="../reference/indices.html#indexed_by"> <code>indexed_by</code></a> construct. For instance, the instantiation </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>employee</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>>,</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> </pre></blockquote> <p> is comprised of a <a href="#unique_non_unique">unique ordered index</a> and a <a href="#unique_non_unique">non-unique ordered index</a>, while in </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>sequenced</span><span class=special><>,</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>text_container</span><span class=special>;</span> </pre></blockquote> <p> we specifiy two indices, the first of <a href="#seq_indices">sequenced type</a>, the second a non-unique <a href="#ord_indices">ordered index</a>. In general, we can specify an arbitrary number of indices: each of the arguments of <code>indexed_by</code> is called an <a href="../reference/indices.html#index_specification"><i>index specifier</i></a>. Depending on the type of index being specified, the corresponding specifier will need additional information: for instance, the specifiers <code>ordered_unique</code> and <code>ordered_non_unique</code> are provided with a <a href="#key_extraction">key extractor</a> and an optional <a href="#comparison_predicates">comparison predicate</a> which jointly indicate how the sorting of elements will be performed. </p> <p> A <code>multi_index_container</code> instantiation can be declared without supplying the <code>indexed_by</code> part: in this case, default index values are taken so that the resulting type is equivalent to a regular <code>std::set</code>. Concretely, the instantiation </p> <blockquote><pre> <span class=identifier>multi_index_container</span><span class=special><</span><i>(element)</i><span class=special>></span> </pre></blockquote> <p> is equivalent to </p> <blockquote><pre> <span class=identifier>multi_index_container</span><span class=special><</span> <i>(element)</i><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><(</span><span class=identifier>element</span><span class=special>)></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> </pre></blockquote> <h2><a name="tagging">Tagging</a></h2> <p> In order to retrieve (a reference to) an index of a given <code>multi_index_container</code>, the programmer must provide its order number, which is cumbersome and not very self-descriptive. Optionally, indices can be assigned <i>tags</i> (C++ types) that act as more convenient mnemonics. If provided, tags must be passed as the first parameter of the corresponding index specifier. The following is a revised version of <code>employee_set</code> with inclusion of tags: </p> <blockquote><pre> <span class=comment>// tags</span> <span class=keyword>struct</span> <span class=identifier>name</span><span class=special>{};</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>employee</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>>,</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>tag</span><span class=special><</span><span class=identifier>name</span><span class=special>>,</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> </pre></blockquote> <p> Tags have to be passed inside the <a href="../reference/indices.html#tag"><code>tag</code></a> construct. Any type can be used as a tag for an index, although in general one will choose names that are descriptive of the index they are associated with. The tagging mechanism allows us to write expressions like</p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>().</span><span class=identifier>begin</span><span class=special>();</span> </pre></blockquote> <p> If no tag is provided for an index (as is the case for index #0 of the previous example), access to that index can only be performed by number. Note the existence of two different <code>typedef</code>s <code>nth_index</code> and <code>index</code> for referring to an index by number and by tag, respectively; for instance, <ul> <li><code>employee_set::nth_index<1>::type</code> is the type of index #1,</li> <li><code>employee_set::index<name>::type</code> is the type of the index tagged with <code>name</code> (the same index #1 in this case.)</li> </ul> <code>get()</code>, on the other hand, is overloaded to serve both styles of access: </p> <blockquote><pre> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>&</span> <span class=identifier>name_index2</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>();</span> <span class=comment>// same index</span> </pre></blockquote> <p> Additionally, the <code>tag</code> class template accepts several tags for one index, that we can use interchangeably: for instance, the specification of index #1 in the previous example can be rewritten to hold two different tags <code>name</code> and <code>by_name</code>: </p> <blockquote><pre> <span class=comment>// tags</span> <span class=keyword>struct</span> <span class=identifier>name</span><span class=special>{};</span> <span class=keyword>struct</span> <span class=identifier>by_name</span><span class=special>{};</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=special>...</span> <span class=identifier>ordered_non_unique</span><span class=special><</span> <span class=identifier>tag</span><span class=special><</span><span class=identifier>name</span><span class=special>,</span><span class=identifier>by_name</span><span class=special>>,</span> <span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>></span> <span class=special>...</span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> </pre></blockquote> <h2><a name="iterator_access">Iterator access</a></h2> <p> Each index of a <code>multi_index_container</code> uses its own iterator types, which are different from those of another indices. As is the rule with STL containers, these iterators are defined as nested types of the index: </p> <blockquote><pre> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span> <span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>().</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Judy Smith"</span><span class=special>);</span> </pre></blockquote> <p> This kind of expressions can be rendered more readable by means of user-defined <code>typedef</code>s: </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span> <span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>().</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Judy Smith"</span><span class=special>);</span> </pre></blockquote> <h2> <a name="index_types">Index types</a> </h2> <p> Currently, Boost.MultiIndex provides the following index types: <ul> <li>Ordered indices sort the elements like <code>std::set</code>s do and provide a similar interface. There are <i>unique</i> and <i>non-unique</i> variants: the former do not allow for duplicates, while the latter permit them (like <code>std::multiset</code>.)</li> <li>Sequenced indices are modeled after the semantics and interface of <code>std::list</code>: they arrange the elements as if in a bidirectional list.</li> <li>Hashed indices provide fast access to the elements through hashing tecnhiques, in a similar way as non-standard <code>hash_set</code>s provided by some vendors. Recently, <i>unordered associative containers</i> have been proposed as part of an extension of the C++ standard library known in the standardization commitee as TR1. Hashed indices closely model this proposal.</li> <li>Random access indices provide an interface similar to that of sequenced indices, and additionally feature random access iterators and positional access to the elements.</li> </ul> The examples in the <a href="#intro">introduction</a> exercise ordered and sequenced indices, which are the most commonly used; the other kinds of indices are presented in the <a href="indices.html">index types</a> section of the tutorial. </p> <h3> <a name="ord_indices">Ordered indices</a> </h3> <p> Ordered indices sort the elements in a <code>multi_index_container</code> according to a specified key and an associated comparison predicate. These indices can be viewed as analogues of the standard container <code>std::set</code>, and in fact they do replicate its interface, albeit with some minor differences dictated by the general constraints of Boost.MultiIndex. </p> <h4> <a name="unique_non_unique">Unique and non-unique variants</a> </h4> <p> Ordered indices are classified into <i>unique</i>, which prohibit two elements to have the same key value, and <i>non-unique</i> indices, which allow for duplicates. Consider again the definition </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>employee</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>>,</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> </pre></blockquote> <p> In this instantiation of <code>multi_index_container</code>, the first index is to be treated as unique (since IDs are exclusive to each employee) and thus is declared using <code>ordered_unique</code>, whereas the second index is non-unique (as the possibility exists that say two John Smiths are hired in the same company), which is specified by the use of <code>ordered_non_unique</code>. </p> <p> The classification of ordered indices in unique and non-unique has an impact on which elements are allowed to be inserted into a given <code>multi_index_container</code>; briefly put, unique ordered indices mimic the behavior of <code>std::set</code>s while non-unique ordered indices are similar to <code>std::multiset</code>s. For instance, an <code>employee_set</code> can hold the objects <code>employee(0,"George Brown")</code> and <code>employee(1,"George Brown")</code>, but will not accept the insertion of an <code>employee</code> object whose ID coincides with that of some previously inserted employee. </p> <p> More than one unique index can be specified. For instance, if we augment <code>employee</code> to include an additional member for the Social Security number, which is reasonably treated as unique, the following captures this design: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>employee</span> <span class=special>{</span> <span class=keyword>int</span> <span class=identifier>id</span><span class=special>;</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>name</span><span class=special>;</span> <span class=keyword>int</span> <span class=identifier>ssnumber</span><span class=special>;</span> <span class=identifier>employee</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>id</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>name</span><span class=special>,</span><span class=keyword>int</span> <span class=identifier>ssnumber</span><span class=special>):</span> <span class=identifier>id</span><span class=special>(</span><span class=identifier>id</span><span class=special>),</span><span class=identifier>name</span><span class=special>(</span><span class=identifier>name</span><span class=special>),</span><span class=identifier>ssnumber</span><span class=special>(</span><span class=identifier>ssnumber</span><span class=special>){}</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special><(</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&</span> <span class=identifier>e</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>id</span><span class=special><</span><span class=identifier>e</span><span class=special>.</span><span class=identifier>id</span><span class=special>;}</span> <span class=special>};</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>employee</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=comment>// sort by employee::operator<</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>>,</span> <span class=comment>// sort by less<string> on name</span> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>>,</span> <span class=comment>// sort by less<int> on ssnumber</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=keyword>int</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>ssnumber</span><span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> </pre></blockquote> <h4> <a name="ord_spec">Specification</a> </h4> <p> Ordered index specifiers in <code>indexed_by</code> must conform to one of the following syntaxes: </p> <blockquote><pre> <span class=special>(</span><span class=identifier>ordered_unique</span> <span class=special>|</span> <span class=identifier>ordered_non_unique</span><span class=special>) </span><span class=special><[</span><i>(tag)</i><span class=special>[,</span><i>(key extractor)</i><span class=special>[,</span><i>(comparison predicate)</i><span class=special>]]]></span> <span class=special>(</span><span class=identifier>ordered_unique</span> <span class=special>|</span> <span class=identifier>ordered_non_unique</span><span class=special>)</span> <span class=special><[</span><i>(key extractor)</i><span class=special>[,</span><i>(comparison predicate)</i><span class=special>]]></span> </pre></blockquote> <p> The first optional argument is used if <a href="#tagging">tags</a> are associated with the index. We now proceed to briefly discuss the remaining arguments of an ordered index specifier. </p> <h4> <a name="key_extraction">Key extraction</a> </h4> <p> The first template parameter (or the second, if tags are supplied) in the specification of an ordered index provides a <i>key extraction</i> predicate. This predicate takes a whole element (in our example, a reference to an <code>employee</code> object) and returns the piece of information by which the sorting is performed. In most cases, one of the following two situations arises: <ul> <li>The whole element serves as the key, as is the case of the first index in <code>employee_set</code>. The predefined <a href="key_extraction.html#identity"><code>identity</code></a> predicate can be used here as a key extractor; <code>identity</code> returns as the key the same object passed as argument.</li> <li>The comparison is performed on a particular data member of the element; this closely follows the specification of indices on a column of a table in relational databases. Boost.MultiIndex provides <a href="key_extraction.html#member"><code>member</code></a>, which returns as the key a member of the element specified by a given pointer.</li> </ul> As an example, consider again the definition of <code>employee_set</code>. The definition of the first index: </p> <blockquote><pre> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>></span> </pre></blockquote> <p> specifies by means of <code>identity</code> that <code>element</code> objects themselves serve as key for this index. On the other hand, in the second index: </p> <blockquote><pre> <span class=identifier>ordered_non_unique</span><span class=special><</span><span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>></span> <span class=special>></span> </pre></blockquote> <p> we use <code>member</code> to extract the <code>name</code> part of the <code>employee</code> object. The key type of this index is then <code>std::string</code>. </p> <p> Apart from <code>identity</code> and <code>member</code>, Boost.MultiIndex provides several other predefined key extractors and powerful ways to combine them. Key extractors can also be defined by the user. Consult the <a href="key_extraction.html">key extraction section</a> of the tutorial for a more detailed exposition of this topic. </p> <h4><a name="comparison_predicates">Comparison predicates</a></h4> <p> The last part of the specification of an ordered index is the associated <i>comparison predicate</i>, which must order the keys in a less-than fashion. These comparison predicates are not different from those used by STL containers like <code>std::set</code>. By default (i.e. if no comparison predicate is provided), an index with keys of type <code>key_type</code> sorts the elements by <code>std::less<key_type></code>. Should other comparison criteria be needed, they can be specified as an additional parameter in the index declaration: </p> <blockquote><pre> <span class=comment>// define a multiply indexed set with indices by id and by name // in reverse alphabetical order</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=identifier>employee</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=identifier>employee</span><span class=special>></span> <span class=special>>,</span> <span class=comment>// as usual</span> <span class=identifier>ordered_non_unique</span><span class=special><</span> <span class=identifier>member</span><span class=special><</span><span class=identifier>employee</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>,&</span><span class=identifier>employee</span><span class=special>::</span><span class=identifier>name</span><span class=special>>,</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>greater</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>></span> <span class=comment>// default would be std::less<std::string></span> <span class=special>></span> <span class=special>></span> <span class=special>></span> <span class=identifier>employee_set</span><span class=special>;</span> </pre></blockquote> <h4><a name="special_lookup">Special lookup operations</a></h4> <p> A given ordered index allows for lookup based on its key type, rather than the whole element. For instance, to find Veronica Cruz in an <code>employee_set</code> one would write: </p> <blockquote><pre> <span class=identifier>employee_set</span> <span class=identifier>es</span><span class=special>;</span> <span class=special>...</span> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>().</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Veronica Cruz"</span><span class=special>);</span> </pre></blockquote> <p>As a plus, Boost.MultiIndex provides lookup operations accepting search keys different from the <code>key_type</code> of the index, which is a specially useful facility when <code>key_type</code> objects are expensive to create. Ordered STL containers fail to provide this functionality, which often leads to inelegant workarounds: consider for instance the problem of determining the employees whose IDs fall in the range [0,100]. Given that the key of <code>employee_set</code> index #0 is <code>employee</code> itself, on a first approach one would write the following: </p> <blockquote><pre> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>p0</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>lower_bound</span><span class=special>(</span><span class=identifier>employee</span><span class=special>(</span><span class=number>0</span><span class=special>,</span><span class=string>""</span><span class=special>));</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>p1</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>upper_bound</span><span class=special>(</span><span class=identifier>employee</span><span class=special>(</span><span class=number>100</span><span class=special>,</span><span class=string>""</span><span class=special>));</span> </pre></blockquote> <p> Note however that <code>std::less<employee></code> actually only depends on the IDs of the employees, so it would be more convenient to avoid the creation of entire <code>employee</code> objects just for the sake of their IDs. Boost.MultiIndex allows for this: define an appropriate comparison predicate </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>comp_id</span> <span class=special>{</span> <span class=comment>// compare an ID and an employee</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&</span> <span class=identifier>e2</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>x</span><span class=special><</span><span class=identifier>e2</span><span class=special>.</span><span class=identifier>id</span><span class=special>;}</span> <span class=comment>// compare an employee and an ID</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>const</span> <span class=identifier>employee</span><span class=special>&</span> <span class=identifier>e1</span><span class=special>,</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>e1</span><span class=special>.</span><span class=identifier>id</span><span class=special><</span><span class=identifier>x</span><span class=special>;}</span> <span class=special>};</span> </pre></blockquote> <p>and now write the search as</p> <blockquote><pre> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>p0</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>lower_bound</span><span class=special>(</span><span class=number>0</span><span class=special>,</span><span class=identifier>comp_id</span><span class=special>());</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>p1</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>upper_bound</span><span class=special>(</span><span class=number>100</span><span class=special>,</span><span class=identifier>comp_id</span><span class=special>());</span> </pre></blockquote> <p> Here we are not only passing IDs instead of <code>employee</code> objects: an alternative comparison predicate is passed as well. In general, lookup operations of ordered indices are overloaded to accept <a href="../reference/ord_indices.html#set_operations"><i>compatible sorting criteria</i></a>. The somewhat cumbersone definition of compatibility in this context is given in the reference, but roughly speaking we say that a comparison predicate <code>C1</code> is compatible with <code>C2</code> if any sequence sorted by <code>C2</code> is also sorted with respect to <code>C1</code>. The following shows a more interesting use of compatible predicates: </p> <blockquote><pre> <span class=comment>// sorting by name's initial</span> <span class=keyword>struct</span> <span class=identifier>comp_initial</span> <span class=special>{</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>char</span> <span class=identifier>ch</span><span class=special>,</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>s</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span> <span class=keyword>if</span><span class=special>(</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>empty</span><span class=special>())</span><span class=keyword>return</span> <span class=keyword>false</span><span class=special>;</span> <span class=keyword>return</span> <span class=identifier>ch</span><span class=special><</span><span class=identifier>s</span><span class=special>[</span><span class=number>0</span><span class=special>];</span> <span class=special>}</span> <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>s</span><span class=special>,</span><span class=keyword>char</span> <span class=identifier>ch</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span> <span class=keyword>if</span><span class=special>(</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>empty</span><span class=special>())</span><span class=keyword>return</span> <span class=keyword>true</span><span class=special>;</span> <span class=keyword>return</span> <span class=identifier>s</span><span class=special>[</span><span class=number>0</span><span class=special>]<</span><span class=identifier>ch</span><span class=special>;</span> <span class=special>}</span> <span class=special>};</span> <span class=comment>// obtain first employee whose name begins with 'J' (ordered by name)</span> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>const_iterator</span> <span class=identifier>it</span><span class=special>=</span> <span class=identifier>name_index</span><span class=special>.</span><span class=identifier>lower_bound</span><span class=special>(</span><span class=literal>'J'</span><span class=special>,</span><span class=identifier>comp_initial</span><span class=special>());</span> </pre></blockquote> <h4><a name="range">Retrieval of ranges</a></h4> <p> Range searching, i.e. the lookup of all elements in a given interval, is a very frequent operation for which standard <code>lower_bound</code> and <code>upper_bound</code> can be resorted to, though in a cumbersome manner. For instance, the following code retrieves the elements of an <code>multi_index_container<double></code> in the interval [100,200]: </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span><span class=keyword>double</span><span class=special>></span> <span class=identifier>double_set</span><span class=special>;</span> <span class=comment>// note: default template parameters resolve to // multi_index_container<double,indexed_by<unique<identity<double> > > >.</span> <span class=identifier>double_set</span> <span class=identifier>s</span><span class=special>;</span> <span class=special>...</span> <span class=identifier>double_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it0</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>lower_bound</span><span class=special>(</span><span class=number>100.0</span><span class=special>);</span> <span class=identifier>double_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it1</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>upper_bound</span><span class=special>(</span><span class=number>200.0</span><span class=special>);</span> <span class=comment>// range [it0,it1) contains the elements in [100,200]</span> </pre></blockquote> <p> Subtle changes to the code are required when strict inequalities are considered. To retrieve the elements <i>greater</i> than 100 and <i>less</i> than 200, the code has to be rewritten as </p> <blockquote><pre> <span class=identifier>double_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it0</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>upper_bound</span><span class=special>(</span><span class=number>100.0</span><span class=special>);</span> <span class=identifier>double_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it1</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>lower_bound</span><span class=special>(</span><span class=number>200.0</span><span class=special>);</span> <span class=comment>// range [it0,it1) contains the elements in (100,200)</span> </pre></blockquote> <p> To add to this complexity, the careful programmer has to take into account that the lower and upper bounds of the interval searched be compatible: for instance, if the lower bound is 200 and the upper bound is 100, the iterators <code>it0</code> and <code>it1</code> produced by the code above will be in reverse order, with possibly catastrophic results if a traversal from <code>it0</code> to <code>it1</code> is tried. All these details make range searching a tedious and error prone task. </p> <p> The <a href="../reference/ord_indices.html#range_operations"><code>range</code></a> member function, often in combination with <a href="../../../../libs/lambda/index.html">Boost.Lambda</a> expressions, can greatly help alleviate this situation: </p> <blockquote><pre> <span class=keyword>using</span> <span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>lambda</span><span class=special>;</span> <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special><</span><span class=keyword>double</span><span class=special>></span> <span class=identifier>double_set</span><span class=special>;</span> <span class=identifier>double_set</span> <span class=identifier>s</span><span class=special>;</span> <span class=special>...</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special><</span><span class=identifier>double_set</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>,</span><span class=identifier>double_set</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>></span> <span class=identifier>p</span><span class=special>=</span> <span class=identifier>s</span><span class=special>.</span><span class=identifier>range</span><span class=special>(</span><span class=number>100.0</span><span class=special><=</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>_1</span><span class=special><=</span><span class=number>200</span><span class=special>);</span> <span class=comment>// 100<= x <=200</span> <span class=special>...</span> <span class=identifier>p</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>range</span><span class=special>(</span><span class=number>100.0</span><span class=special><</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>_1</span><span class=special><</span><span class=number>200</span><span class=special>);</span> <span class=comment>// 100< x < 200</span> <span class=special>...</span> <span class=identifier>p</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>range</span><span class=special>(</span><span class=number>100.0</span><span class=special><=</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>_1</span><span class=special><</span><span class=number>200</span><span class=special>);</span> <span class=comment>// 100<= x < 200</span> </pre></blockquote> <p> <code>range</code> simply accepts predicates specifying the lower and upper bounds of the interval searched. Please consult the reference for a detailed explanation of the permissible predicates passed to <code>range</code>.</p> <p> One or both bounds can be omitted with the special <code>unbounded</code> marker: </p> <blockquote><pre> <span class=identifier>p</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>range</span><span class=special>(</span><span class=number>100.0</span><span class=special><=</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>unbounded</span><span class=special>);</span> <span class=comment>// 100 <= x</span> <span class=identifier>p</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>range</span><span class=special>(</span><span class=identifier>unbounded</span><span class=special>,</span><span class=identifier>_1</span><span class=special><</span><span class=number>200.0</span><span class=special>);</span> <span class=comment>// x < 200</span> <span class=identifier>p</span><span class=special>=</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>range</span><span class=special>(</span><span class=identifier>unbounded</span><span class=special>,</span><span class=identifier>unbounded</span><span class=special>);</span> <span class=comment>// equiv. to std::make_pair(s.begin(),s.end())</span> </pre></blockquote> <h4><a name="ord_updating">Updating</a></h4> <p> The <a href="../reference/ord_indices.html#replace"><code>replace</code></a> member function performs in-place replacement of a given element as the following example shows: </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>index</span><span class=special><</span><span class=identifier>employee_set</span><span class=special>,</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>name_index</span><span class=special>.</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Anna Jones"</span><span class=special>);</span> <span class=identifier>employee</span> <span class=identifier>anna</span><span class=special>=*</span><span class=identifier>it</span><span class=special>;</span> <span class=identifier>anna</span><span class=special>.</span><span class=identifier>name</span><span class=special>=</span><span class=string>"Anna Smith"</span><span class=special>;</span> <span class=comment>// she just got married to Calvin Smith</span> <span class=identifier>name_index</span><span class=special>.</span><span class=identifier>replace</span><span class=special>(</span><span class=identifier>it</span><span class=special>,</span><span class=identifier>anna</span><span class=special>);</span> <span class=comment>// update her record</span> </pre></blockquote> <p> <code>replace</code> performs this substitution in such a manner that: <ul> <li>The complexity is constant time if the changed element retains its original order with respect to all indices; it is logarithmic otherwise. <li>Iterator and reference validity are preserved. <li>The operation is strongly exception-safe, i.e. the <code>multi_index_container</code> remains unchanged if some exception (originated by the system or the user's data types) is thrown. </ul> <code>replace</code> is a powerful operation not provided by standard STL containers, and one that is specially handy when strong exception-safety is required. </p> <p> The observant reader might have noticed that the convenience of <code>replace</code> comes at a cost: namely the whole element has to be copied <i>twice</i> to do the updating (when retrieving it and inside <code>replace</code>). If elements are expensive to copy, this may be quite a computational cost for the modification of just a tiny part of the object. To cope with this situation, Boost.MultiIndex provides an alternative updating mechanism called <a href="../reference/ord_indices.html#modify"><code>modify</code></a>: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>change_name</span> <span class=special>{</span> <span class=identifier>change_name</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>new_name</span><span class=special>):</span><span class=identifier>new_name</span><span class=special>(</span><span class=identifier>new_name</span><span class=special>){}</span> <span class=keyword>void</span> <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>employee</span><span class=special>&</span> <span class=identifier>e</span><span class=special>)</span> <span class=special>{</span> <span class=identifier>e</span><span class=special>.</span><span class=identifier>name</span><span class=special>=</span><span class=identifier>new_name</span><span class=special>;</span> <span class=special>}</span> <span class=keyword>private</span><span class=special>:</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>new_name</span><span class=special>;</span> <span class=special>};</span> <span class=special>...</span> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>name_index</span><span class=special>.</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Anna Jones"</span><span class=special>);</span> <span class=identifier>name_index</span><span class=special>.</span><span class=identifier>modify</span><span class=special>(</span><span class=identifier>it</span><span class=special>,</span><span class=identifier>change_name</span><span class=special>(</span><span class=string>"Anna Smith"</span><span class=special>));</span> </pre></blockquote> <p><code>modify</code> accepts a functor (or pointer to function) that is passed a reference to the element to be changed, thus eliminating the need for spurious copies. Like <code>replace</code>, <code>modify</code> does preserve the internal orderings of all the indices of the <code>multi_index_container</code>. However, the semantics of <code>modify</code> is not entirely equivalent to <code>replace</code>. Consider what happens if a collision occurs as a result of modifying the element, i.e. the modified element clashes with another with respect to some unique ordered index. In the case of <code>replace</code>, the original value is kept and the method returns without altering the container, but <code>modify</code> cannot afford such an approach, since the modifying functor leaves no trace of the previous value of the element. Integrity constraints thus lead to the following policy: when a collision happens in the process of calling <code>modify</code>, the element is erased and the method returns <code>false</code>. There is a further version of <code>modify</code> which accepts a <i>rollback</i> functor to undo the changes in case of collision: </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>change_id</span> <span class=special>{</span> <span class=identifier>change_id</span><span class=special>(</span><span class=keyword>int</span> <span class=identifier>new_id</span><span class=special>):</span><span class=identifier>new_id</span><span class=special>(</span><span class=identifier>new_id</span><span class=special>){}</span> <span class=keyword>void</span> <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>employee</span><span class=special>&</span> <span class=identifier>e</span><span class=special>)</span> <span class=special>{</span> <span class=identifier>e</span><span class=special>.</span><span class=identifier>id</span><span class=special>=</span><span class=identifier>new_id</span><span class=special>;</span> <span class=special>}</span> <span class=keyword>private</span><span class=special>:</span> <span class=keyword>int</span> <span class=identifier>new_id</span><span class=special>;</span> <span class=special>};</span> <span class=special>...</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=...</span> <span class=keyword>int</span> <span class=identifier>old_id</span><span class=special>=</span><span class=identifier>it</span><span class=special>-></span><span class=identifier>id</span><span class=special>;</span> <span class=comment>// keep the original id // try to modify the id, restore it in case of collisions</span> <span class=identifier>es</span><span class=special>.</span><span class=identifier>modify</span><span class=special>(</span><span class=identifier>it</span><span class=special>,</span><span class=identifier>change_id</span><span class=special>(</span><span class=number>321</span><span class=special>),</span><span class=identifier>change_id</span><span class=special>(</span><span class=identifier>old_id</span><span class=special>));</span> </pre></blockquote> <p>In the example, <code>change_id(old_id)</code> is invoked to restore the original conditions when the modification results in collisions with some other element. The differences in behavior between <code>replace</code>, <code>modify</code> and <code>modify</code> with rollback have to be considered by the programmer on a case-by-case basis to determine the best updating mechanism. </p> <p align="center"> <table cellspacing="0"> <caption><b>Behavior of the different updating mechanisms.</b></caption> <tr> <th align="center">updating function</th> <th>If there is a collision...</th> </tr> <tr> <td align="center"><code>replace(it,x)</code></td> <td>replacement does not take place.</td> </tr> <tr class="odd_tr"> <td align="center"><code>modify(it,mod)</code></td> <td>the element is erased.</td> </tr> <tr> <td align="center"><code>modify(it,mod,back)</code></td> <td><code>back</code> is used to restore the original conditions. (If <code>back</code> throws, the element is erased.) </td> </tr> </table> </p> <p> Key-based versions of <code>modify</code>, named <a href="../reference/ord_indices.html#modify_key"><code>modify_key</code></a>, are provided as well. In this case, the modifying functors are passed a reference to the <code>key_type</code> part of the element instead of the whole object. </p> <blockquote><pre> <span class=keyword>struct</span> <span class=identifier>change_str</span> <span class=special>{</span> <span class=identifier>change_str</span><span class=special>(</span><span class=keyword>const</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>new_str</span><span class=special>):</span><span class=identifier>new_str</span><span class=special>(</span><span class=identifier>new_str</span><span class=special>){}</span> <span class=comment>// note this is passed a string, not an employee</span> <span class=keyword>void</span> <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&</span> <span class=identifier>str</span><span class=special>)</span> <span class=special>{</span> <span class=identifier>str</span><span class=special>=</span><span class=identifier>new_str</span><span class=special>;</span> <span class=special>}</span> <span class=keyword>private</span><span class=special>:</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span> <span class=identifier>new_str</span><span class=special>;</span> <span class=special>};</span> <span class=special>...</span> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>name_index</span><span class=special>.</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Anna Jones"</span><span class=special>);</span> <span class=identifier>name_index</span><span class=special>.</span><span class=identifier>modify_key</span><span class=special>(</span><span class=identifier>it</span><span class=special>,</span><span class=identifier>change_str</span><span class=special>(</span><span class=string>"Anna Smith"</span><span class=special>));</span> </pre></blockquote> <p> Like <code>modify</code>, there are versions of <code>modify_key</code> with and without rollback. <code>modify</code> and <code>modify_key</code> are particularly well suited to use in conjunction to <a href="../../../../libs/lambda/index.html">Boost.Lambda</a> for defining the modifying functors: </p> <blockquote><pre> <span class=keyword>using</span> <span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>::</span><span class=identifier>lambda</span><span class=special>;</span> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it</span><span class=special>=</span><span class=identifier>name_index</span><span class=special>.</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Anna Jones"</span><span class=special>);</span> <span class=identifier>name_index</span><span class=special>.</span><span class=identifier>modify_key</span><span class=special>(</span><span class=identifier>it</span><span class=special>,</span><span class=identifier>_1</span><span class=special>=</span><span class=string>"Anna Smith"</span><span class=special>);</span> </pre></blockquote> <p> <code>modify_key</code> requires that the key extractor be of a special type called <a href="key_extraction.html#read_write_key_extractors">read/write</a>: this is usually, but not always, the case. </p> <h3> <a name="seq_indices">Sequenced indices</a> </h3> <p> Unlike ordered indices, sequenced indices do not impose a fixed order on the elements: instead, these can be arranged in any position on the sequence, in the same way as <code>std::list</code> permits. The interface of sequenced indices is thus designed upon that of <code>std::list</code>; nearly every operation provided in the standard container is replicated here, occasionally with changes in the syntax and/or semantics to cope with the constraints imposed by Boost.MultiIndex. In particular, there is an important limitation of sequenced indices with respect to <code>std::list</code>s, namely that elements of an <code>multi_index_container</code> are not mutable through an iterator: </p> <blockquote><pre> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=keyword>int</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span><span class=identifier>sequenced</span><span class=special><></span> <span class=special>></span> <span class=special>></span> <span class=identifier>s</span><span class=special>;</span> <span class=comment>// list-like container</span> <span class=identifier>s</span><span class=special>.</span><span class=identifier>push_front</span><span class=special>(</span><span class=number>0</span><span class=special>);</span> <span class=special>*(</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>begin</span><span class=special>())=</span><span class=number>1</span><span class=special>;</span> <span class=comment>// ERROR: the element cannot be changed</span> </pre></blockquote> <p> That is, iterators of a sequenced index (of all types of indices, actually) point to constant elements. This limitation might come as a surprise, but it is imposed by the way <code>multi_index_container</code>s work; if elements were allowed to be changed in this manner, we could introduce inconsistencies in other ordered indices of the <code>multi_index_container</code>. Element modification can nevertheless be done by means of <a href="#seq_updating">update operations</a>. </p> <p> Consider a <code>multi_index_container</code> with two or more indices, one of them of sequenced type. If an element is inserted through another index, then it will be automatically appended to the end of the sequenced index. An example will help to clarify this: </p> <blockquote><pre> <span class=identifier>multi_index_container</span><span class=special><</span> <span class=keyword>int</span><span class=special>,</span> <span class=identifier>indexed_by</span><span class=special><</span> <span class=identifier>sequenced</span><span class=special><>,</span> <span class=comment>// sequenced type</span> <span class=identifier>ordered_unique</span><span class=special><</span><span class=identifier>identity</span><span class=special><</span><span class=keyword>int</span><span class=special>></span> <span class=special>></span> <span class=comment>// another index</span> <span class=special>></span> <span class=special>></span> <span class=identifier>s</span><span class=special>;</span> <span class=identifier>s</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>().</span><span class=identifier>insert</span><span class=special>(</span><span class=number>1</span><span class=special>);</span> <span class=comment>// insert 1 through index #1</span> <span class=identifier>s</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>().</span><span class=identifier>insert</span><span class=special>(</span><span class=number>0</span><span class=special>);</span> <span class=comment>// insert 0 through index #1 // list elements through sequenced index #0</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>copy</span><span class=special>(</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(),</span><span class=identifier>s</span><span class=special>.</span><span class=identifier>end</span><span class=special>(),</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>ostream_iterator</span><span class=special><</span><span class=keyword>int</span><span class=special>>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>cout</span><span class=special>));</span> <span class=comment>// result: 1 0</span> </pre></blockquote> <p> Thus the behavior of sequenced indices when insertions are not made through them is to preserve insertion order. </p> <h4><a name="seq_spec">Specification</a></h4> <p> Sequenced indices are specified with the <code>sequenced</code> construct: </p> <blockquote><pre> <span class=identifier>sequenced</span><span class=special><[</span><i>(tag)</i><span class=special>]></span> </pre></blockquote> <p> The <a href="#tagging">tag</a> parameter is optional. </p> <h4><a name="list_ops">List operations</a></h4> <p> As mentioned before, sequenced indices mimic the interface of <code>std::list</code>, and most of the original operations therein are provided as well. The semantics and complexity of these operations, however, do not always coincide with those of the standard container. Differences result mainly from the fact that insertions into a sequenced index are not guaranteed to succeed, due to the possible banning by other indices of the <code>multi_index_container</code>. Consult the <a href="../reference/seq_indices.html">reference</a> for further details. </p> <h4><a name="seq_updating">Updating</a></h4> <p> Like ordered indices, sequenced indices provide <a href="../reference/seq_indices.html#replace"><code>replace</code></a> and <a href="../reference/seq_indices.html#modify"><code>modify</code></a> operations, with identical functionality. There is however no analogous <code>modify_key</code>, since sequenced indices are not key-based. </p> <h2><a name="projection">Projection of iterators</a></h2> <p> Given indices <code>i1</code> and <code>i2</code> on the same <code>multi_index_container</code>, <a href="../reference/multi_index_container.html#projection"><code>project</code></a> can be used to retrieve an <code>i2</code>-iterator from an <code>i1</code>-iterator, both of them pointing to the same element of the container. This functionality allows the programmer to move between different indices of the same <code>multi_index_container</code> when performing elaborate operations: </p> <blockquote><pre> <span class=keyword>typedef</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>index</span><span class=special><</span><span class=identifier>name</span><span class=special>>::</span><span class=identifier>type</span> <span class=identifier>employee_set_by_name</span><span class=special>;</span> <span class=identifier>employee_set_by_name</span><span class=special>&</span> <span class=identifier>name_index</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=identifier>name</span><span class=special>>();</span> <span class=comment>// list employees by ID starting from Robert Brown's ID</span> <span class=identifier>employee_set_by_name</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it1</span><span class=special>=</span><span class=identifier>name_index</span><span class=special>.</span><span class=identifier>find</span><span class=special>(</span><span class=string>"Robert Brown"</span><span class=special>);</span> <span class=comment>// obtain an iterator of index #0 from it1</span> <span class=identifier>employee_set</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it2</span><span class=special>=</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>project</span><span class=special><</span><span class=number>0</span><span class=special>>(</span><span class=identifier>it1</span><span class=special>);</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>copy</span><span class=special>(</span><span class=identifier>it2</span><span class=special>,</span><span class=identifier>es</span><span class=special>.</span><span class=identifier>end</span><span class=special>(),</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>ostream_iterator</span><span class=special><</span><span class=identifier>employee</span><span class=special>>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>cout</span><span class=special>));</span> </pre></blockquote> <p> A slightly more interesting example: </p> <blockquote><pre> <span class=identifier>text_container</span> <span class=identifier>tc</span><span class=special>;</span> <span class=comment>// get a view to index #1 (ordered index on the words)</span> <span class=identifier>text_container</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>&</span> <span class=identifier>sorted_index</span><span class=special>=</span><span class=identifier>tc</span><span class=special>.</span><span class=identifier>get</span><span class=special><</span><span class=number>1</span><span class=special>>();</span> <span class=comment>// prepend "older" to all occurrences of "sister"</span> <span class=identifier>text_container</span><span class=special>::</span><span class=identifier>nth_index</span><span class=special><</span><span class=number>1</span><span class=special>>::</span><span class=identifier>type</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it1</span><span class=special>=</span> <span class=identifier>sorted_index</span><span class=special>.</span><span class=identifier>lower_bound</span><span class=special>(</span><span class=string>"sister"</span><span class=special>);</span> <span class=keyword>while</span><span class=special>(</span><span class=identifier>it1</span><span class=special>!=</span><span class=identifier>sorted_index</span><span class=special>.</span><span class=identifier>end</span><span class=special>()&&*</span><span class=identifier>it1</span><span class=special>==</span><span class=string>"sister"</span><span class=special>){</span> <span class=comment>// convert to an iterator to the sequenced index</span> <span class=identifier>text_container</span><span class=special>::</span><span class=identifier>iterator</span> <span class=identifier>it2</span><span class=special>=</span><span class=identifier>tc</span><span class=special>.</span><span class=identifier>project</span><span class=special><</span><span class=number>0</span><span class=special>>(</span><span class=identifier>it1</span><span class=special>);</span> <span class=identifier>tc</span><span class=special>.</span><span class=identifier>insert</span><span class=special>(</span><span class=identifier>it2</span><span class=special>,</span><span class=string>"older"</span><span class=special>);</span> <span class=special>++</span><span class=identifier>it1</span><span class=special>;</span> <span class=special>}</span> </pre></blockquote> <p> When provided, <code>project</code> can also be used with <a href="#tagging">tags</a>. </p> <h2><a name="complexity">Complexity and exception safety</a></h2> <p> <code>multi_index_container</code> provides the same complexity and exception safety guarantees as the equivalent STL containers do. Iterator and reference validity is preserved in the face of insertions, even for replace and modify operations. </p> <p> Appropriate instantiations of <code>multi_index_container</code> can in fact simulate <code>std::set</code>, <code>std::multiset</code> and (with more limitations) <code>std::list</code>, as shown in the <a href="techniques.html#emulate_std_containers">techniques</a> section. These simulations are as nearly as efficient as the original STL containers; consult the <a href="../reference/index.html">reference</a> for further information on complexity guarantees and the <a href="../performance.html">performance section</a> for practical measurements of efficiency. </p> <hr> <div class="prev_link"><a href="index.html"><img src="../prev.gif" alt="tutorial" border="0"><br> Boost.MultiIndex Tutorial </a></div> <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.MultiIndex tutorial" border="0"><br> Boost.MultiIndex tutorial </a></div> <div class="next_link"><a href="indices.html"><img src="../next.gif" alt="index types" border="0"><br> Index types </a></div><br clear="all" style="clear: all;"> <br> <p>Revised November 7th 2008</p> <p>© Copyright 2003-2008 Joaquín M López Muñoz. Distributed under the Boost Software License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt"> LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> http://www.boost.org/LICENSE_1_0.txt</a>) </p> </body> </html>