<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Working with primitive types</title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> <link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" /> <link rel="up" href="stl.html" title="Chapter 7. Standard Template Library API" /> <link rel="prev" href="stl_mt_usage.html" title="Using dbstl in multithreaded applications" /> <link rel="next" href="stl_complex_rw.html" title="Store and Retrieve data or objects of complex types" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Working with primitive types </th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="stl_mt_usage.html">Prev</a> </td> <th width="60%" align="center">Chapter 7. Standard Template Library API</th> <td width="20%" align="right"> <a accesskey="n" href="stl_complex_rw.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="stl_primitive_rw"></a>Working with primitive types </h2> </div> </div> </div> <div class="toc"> <dl> <dt> <span class="sect2"> <a href="stl_primitive_rw.html#id1600210">Storing strings</a> </span> </dt> </dl> </div> <p> To store simple primitive types such as <code class="literal">int</code>, <code class="literal">long</code>, <code class="literal">double</code>, and so forth, an additional type parameter for the container class templates is needed. For example, to store an <code class="literal">int</code> in a <code class="classname">db_vector</code>, use this container class: </p> <pre class="programlisting">db_vector<int, ElementHolder<int> >;</pre> <p> To map integers to doubles, use this: </p> <pre class="programlisting">db_map<int, double, ElementHolder<double> >;</pre> <p> To store a <code class="literal">char*</code> string with <code class="literal">long</code> keys, use this: </p> <pre class="programlisting">db_map<long, char*, ElementHolder<char*> >;</pre> <p> Use this for <code class="literal">const char*</code> strings: </p> <pre class="programlisting">db_map<long, const char*, ElementHolder<const char*> >;</pre> <p> To map one const string to another, use this type: </p> <pre class="programlisting">db_map<const char*, const char*, ElementHolder<const char*> >;</pre> <p> The <code class="methodname">TestVector::test_primitive()</code> method demonstrates more of these examples. You can find this method implemented in the dbstl test suite. </p> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h3 class="title"><a id="id1600210"></a>Storing strings</h3> </div> </div> </div> <p> For <code class="literal">char*</code> and <code class="literal">wchar_t*</code> strings, <code class="methodname">_DB_STL_StoreElement()</code> must be called following partial or total modifications before iterator movement, <code class="literal">container::operator[]</code> or <code class="literal">iterator::operator*/-></code> calls. Without the <code class="methodname">_DB_STL_StoreElement()</code> call, the modified change will be lost. If storing an new value like this: </p> <pre class="programlisting">*iterator = new_char_star_string;</pre> <p> the call to <code class="methodname">_DB_STL_StoreElement()</code> is not needed. </p> <p> Note that passing a NULL pointer to a container of <code class="literal">char*</code> type or passing a <code class="classname">std::string</code> with no contents at all will insert an empty string of zero length into the database. </p> <p> The string returned from a container will not live beyond the next iterator movement call, <code class="literal">container::operator[]</code> or <code class="literal">iterator::operator*/-></code> call. </p> <p> A <span class="bold"><strong>db_map::value_type::second_type</strong></span> or <span class="bold"><strong>db_map::datatype_wrap</strong></span> should be used to hold a reference to a <code class="literal">container::operator[]</code> return value. Then the reference should be used for repeated references to that value. The *iterator is of type <code class="literal">ElementHolder<char *></code>, which can be automatically converted to a <code class="literal">char *</code> pointer using its type conversion operator. Wherever an auto conversion is done by the compiler, the conversion operator of <code class="literal">ElementHolder<T></code> is called. This avoids almost all explicit conversions, except for two use cases: </p> <div class="orderedlist"> <ol type="1"> <li> <p> The *iterator is used as a "..." parameter like this: </p> <pre class="programlisting">printf("this is the special case %s", *iterator);</pre> <p> This compiles but causes errors. Instead, an explicit cast should be used: </p> <pre class="programlisting">printf("this is the special case %s", (char *)*iterator);</pre> </li> <li> <p> For some old compilers, such as gcc3.4.6, the *iterator cannot be used with the ternary <code class="literal">?</code> operator, like this: </p> <pre class="programlisting">expr ? *iterator : var</pre> <p> Even when <span class="bold"><strong>var</strong></span> is the same type as the iterator's <code class="literal">value_type</code>, the compiler fails to perform an auto conversion. </p> </li> </ol> </div> <p> When using <code class="classname">std::string</code> or <code class="classname">std::wstring</code> as the data type for dbstl containers — that is, <code class="classname">db_vector<string></code>, and <code class="classname">db_map<string, wstring></code> — the string's content rather than the string object itself is stored in order to maintain persistence. </p> <p> You can find example code demonstrating string storage in the <code class="methodname">TestAssoc::test_char_star_string_storage()</code> and <code class="methodname">TestAssoc::test_storing_std_strings()</code> methods. These are available in the dbstl test suite. </p> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="stl_mt_usage.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="stl.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="stl_complex_rw.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Using dbstl in multithreaded applications </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Store and Retrieve data or objects of complex types </td> </tr> </table> </div> </body> </html>