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<!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>Using</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, policy, container, data, structure, associated, tree, trie, hash, metaprogramming" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="policy_data_structures.html" title="Chapter 22. Policy-Based Data Structures" /><link rel="prev" href="policy_data_structures.html" title="Chapter 22. Policy-Based Data Structures" /><link rel="next" href="policy_data_structures_design.html" title="Design" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Using</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="policy_data_structures.html">Prev</a> </td><th width="60%" align="center">Chapter 22. Policy-Based Data Structures</th><td width="20%" align="right"> <a accesskey="n" href="policy_data_structures_design.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="containers.pbds.using"></a>Using</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="pbds.using.prereq"></a>Prerequisites</h3></div></div></div><p>The library contains only header files, and does not require any
other libraries except the standard C++ library . All classes are
defined in namespace <code class="code">__gnu_pbds</code>. The library internally
uses macros beginning with <code class="code">PB_DS</code>, but
<code class="code">#undef</code>s anything it <code class="code">#define</code>s (except for
header guards). Compiling the library in an environment where macros
beginning in <code class="code">PB_DS</code> are defined, may yield unpredictable
results in compilation, execution, or both.</p><p>
Further dependencies are necessary to create the visual output
for the performance tests. To create these graphs, an
additional package is needed: <span class="command"><strong>pychart</strong></span>.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="pbds.using.organization"></a>Organization</h3></div></div></div><p>
The various data structures are organized as follows.
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Branch-Based
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p>
<code class="classname">basic_branch</code>
is an abstract base class for branched-based
associative-containers
</p></li><li class="listitem"><p>
<code class="classname">tree</code>
is a concrete base class for tree-based
associative-containers
</p></li><li class="listitem"><p>
<code class="classname">trie</code>
is a concrete base class trie-based
associative-containers
</p></li></ul></div></li><li class="listitem"><p>
Hash-Based
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p>
<code class="classname">basic_hash_table</code>
is an abstract base class for hash-based
associative-containers
</p></li><li class="listitem"><p>
<code class="classname">cc_hash_table</code>
is a concrete collision-chaining hash-based
associative-containers
</p></li><li class="listitem"><p>
<code class="classname">gp_hash_table</code>
is a concrete (general) probing hash-based
associative-containers
</p></li></ul></div></li><li class="listitem"><p>
List-Based
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p>
<code class="classname">list_update</code>
list-based update-policy associative container
</p></li></ul></div></li><li class="listitem"><p>
Heap-Based
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p>
<code class="classname">priority_queue</code>
A priority queue.
</p></li></ul></div></li></ul></div><p>
The hierarchy is composed naturally so that commonality is
captured by base classes. Thus <code class="function">operator[]</code>
is defined at the base of any hierarchy, since all derived
containers support it. Conversely <code class="function">split</code> is
defined in <code class="classname">basic_branch</code>, since only
tree-like containers support it.
</p><p>
In addition, there are the following diagnostics classes,
used to report errors specific to this library's data
structures.
</p><div class="figure"><a id="id-1.3.5.9.3.3.6"></a><p class="title"><strong>Figure 22.7. Exception Hierarchy</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="../images/pbds_exception_hierarchy.png" align="middle" alt="Exception Hierarchy" /></div></div></div><br class="figure-break" /></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="pbds.using.tutorial"></a>Tutorial</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.tutorial.basic"></a>Basic Use</h4></div></div></div><p>
For the most part, the policy-based containers containers in
namespace <code class="literal">__gnu_pbds</code> have the same interface as
the equivalent containers in the standard C++ library, except for
the names used for the container classes themselves. For example,
this shows basic operations on a collision-chaining hash-based
container:
</p><pre class="programlisting">
#include <ext/pb_ds/assoc_container.h>
int main()
{
__gnu_pbds::cc_hash_table<int, char> c;
c[2] = 'b';
assert(c.find(1) == c.end());
};
</pre><p>
The container is called
<code class="classname">__gnu_pbds::cc_hash_table</code> instead of
<code class="classname">std::unordered_map</code>, since <span class="quote">“<span class="quote">unordered
map</span>”</span> does not necessarily mean a hash-based map as implied by
the C++ library (C++11 or TR1). For example, list-based associative
containers, which are very useful for the construction of
"multimaps," are also unordered.
</p><p>This snippet shows a red-black tree based container:</p><pre class="programlisting">
#include <ext/pb_ds/assoc_container.h>
int main()
{
__gnu_pbds::tree<int, char> c;
c[2] = 'b';
assert(c.find(2) != c.end());
};
</pre><p>The container is called <code class="classname">tree</code> instead of
<code class="classname">map</code> since the underlying data structures are
being named with specificity.
</p><p>
The member function naming convention is to strive to be the same as
the equivalent member functions in other C++ standard library
containers. The familiar methods are unchanged:
<code class="function">begin</code>, <code class="function">end</code>,
<code class="function">size</code>, <code class="function">empty</code>, and
<code class="function">clear</code>.
</p><p>
This isn't to say that things are exactly as one would expect, given
the container requirments and interfaces in the C++ standard.
</p><p>
The names of containers' policies and policy accessors are
different then the usual. For example, if <span class="type">hash_type</span> is
some type of hash-based container, then</p><pre class="programlisting">
hash_type::hash_fn
</pre><p>
gives the type of its hash functor, and if <code class="varname">obj</code> is
some hash-based container object, then
</p><pre class="programlisting">
obj.get_hash_fn()
</pre><p>will return a reference to its hash-functor object.</p><p>
Similarly, if <span class="type">tree_type</span> is some type of tree-based
container, then
</p><pre class="programlisting">
tree_type::cmp_fn
</pre><p>
gives the type of its comparison functor, and if
<code class="varname">obj</code> is some tree-based container object,
then
</p><pre class="programlisting">
obj.get_cmp_fn()
</pre><p>will return a reference to its comparison-functor object.</p><p>
It would be nice to give names consistent with those in the existing
C++ standard (inclusive of TR1). Unfortunately, these standard
containers don't consistently name types and methods. For example,
<code class="classname">std::tr1::unordered_map</code> uses
<span class="type">hasher</span> for the hash functor, but
<code class="classname">std::map</code> uses <span class="type">key_compare</span> for
the comparison functor. Also, we could not find an accessor for
<code class="classname">std::tr1::unordered_map</code>'s hash functor, but
<code class="classname">std::map</code> uses <code class="classname">compare</code>
for accessing the comparison functor.
</p><p>
Instead, <code class="literal">__gnu_pbds</code> attempts to be internally
consistent, and uses standard-derived terminology if possible.
</p><p>
Another source of difference is in scope:
<code class="literal">__gnu_pbds</code> contains more types of associative
containers than the standard C++ library, and more opportunities
to configure these new containers, since different types of
associative containers are useful in different settings.
</p><p>
Namespace <code class="literal">__gnu_pbds</code> contains different classes for
hash-based containers, tree-based containers, trie-based containers,
and list-based containers.
</p><p>
Since associative containers share parts of their interface, they
are organized as a class hierarchy.
</p><p>Each type or method is defined in the most-common ancestor
in which it makes sense.
</p><p>For example, all associative containers support iteration
expressed in the following form:
</p><pre class="programlisting">
const_iterator
begin() const;
iterator
begin();
const_iterator
end() const;
iterator
end();
</pre><p>
But not all containers contain or use hash functors. Yet, both
collision-chaining and (general) probing hash-based associative
containers have a hash functor, so
<code class="classname">basic_hash_table</code> contains the interface:
</p><pre class="programlisting">
const hash_fn&
get_hash_fn() const;
hash_fn&
get_hash_fn();
</pre><p>
so all hash-based associative containers inherit the same
hash-functor accessor methods.
</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.tutorial.configuring"></a>
Configuring via Template Parameters
</h4></div></div></div><p>
In general, each of this library's containers is
parametrized by more policies than those of the standard library. For
example, the standard hash-based container is parametrized as
follows:
</p><pre class="programlisting">
template<typename Key, typename Mapped, typename Hash,
typename Pred, typename Allocator, bool Cache_Hashe_Code>
class unordered_map;
</pre><p>
and so can be configured by key type, mapped type, a functor
that translates keys to unsigned integral types, an equivalence
predicate, an allocator, and an indicator whether to store hash
values with each entry. this library's collision-chaining
hash-based container is parametrized as
</p><pre class="programlisting">
template<typename Key, typename Mapped, typename Hash_Fn,
typename Eq_Fn, typename Comb_Hash_Fn,
typename Resize_Policy, bool Store_Hash
typename Allocator>
class cc_hash_table;
</pre><p>
and so can be configured by the first four types of
<code class="classname">std::tr1::unordered_map</code>, then a
policy for translating the key-hash result into a position
within the table, then a policy by which the table resizes,
an indicator whether to store hash values with each entry,
and an allocator (which is typically the last template
parameter in standard containers).
</p><p>
Nearly all policy parameters have default values, so this
need not be considered for casual use. It is important to
note, however, that hash-based containers' policies can
dramatically alter their performance in different settings,
and that tree-based containers' policies can make them
useful for other purposes than just look-up.
</p><p>As opposed to associative containers, priority queues have
relatively few configuration options. The priority queue is
parametrized as follows:</p><pre class="programlisting">
template<typename Value_Type, typename Cmp_Fn,typename Tag,
typename Allocator>
class priority_queue;
</pre><p>The <code class="classname">Value_Type</code>, <code class="classname">Cmp_Fn</code>, and
<code class="classname">Allocator</code> parameters are the container's value type,
comparison-functor type, and allocator type, respectively;
these are very similar to the standard's priority queue. The
<code class="classname">Tag</code> parameter is different: there are a number of
pre-defined tag types corresponding to binary heaps, binomial
heaps, etc., and <code class="classname">Tag</code> should be instantiated
by one of them.</p><p>Note that as opposed to the
<code class="classname">std::priority_queue</code>,
<code class="classname">__gnu_pbds::priority_queue</code> is not a
sequence-adapter; it is a regular container.</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.tutorial.traits"></a>
Querying Container Attributes
</h4></div></div></div><p></p><p>A containers underlying data structure
affect their performance; Unfortunately, they can also affect
their interface. When manipulating generically associative
containers, it is often useful to be able to statically
determine what they can support and what the cannot.
</p><p>Happily, the standard provides a good solution to a similar
problem - that of the different behavior of iterators. If
<code class="classname">It</code> is an iterator, then
</p><pre class="programlisting">
typename std::iterator_traits<It>::iterator_category
</pre><p>is one of a small number of pre-defined tag classes, and
</p><pre class="programlisting">
typename std::iterator_traits<It>::value_type
</pre><p>is the value type to which the iterator "points".</p><p>
Similarly, in this library, if <span class="type">C</span> is a
container, then <code class="classname">container_traits</code> is a
trait class that stores information about the kind of
container that is implemented.
</p><pre class="programlisting">
typename container_traits<C>::container_category
</pre><p>
is one of a small number of predefined tag structures that
uniquely identifies the type of underlying data structure.
</p><p>In most cases, however, the exact underlying data
structure is not really important, but what is important is
one of its other attributes: whether it guarantees storing
elements by key order, for example. For this one can
use</p><pre class="programlisting">
typename container_traits<C>::order_preserving
</pre><p>
Also,
</p><pre class="programlisting">
typename container_traits<C>::invalidation_guarantee
</pre><p>is the container's invalidation guarantee. Invalidation
guarantees are especially important regarding priority queues,
since in this library's design, iterators are practically the
only way to manipulate them.</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.tutorial.point_range_iteration"></a>
Point and Range Iteration
</h4></div></div></div><p></p><p>This library differentiates between two types of methods
and iterators: point-type, and range-type. For example,
<code class="function">find</code> and <code class="function">insert</code> are point-type methods, since
they each deal with a specific element; their returned
iterators are point-type iterators. <code class="function">begin</code> and
<code class="function">end</code> are range-type methods, since they are not used to
find a specific element, but rather to go over all elements in
a container object; their returned iterators are range-type
iterators.
</p><p>Most containers store elements in an order that is
determined by their interface. Correspondingly, it is fine that
their point-type iterators are synonymous with their range-type
iterators. For example, in the following snippet
</p><pre class="programlisting">
std::for_each(c.find(1), c.find(5), foo);
</pre><p>
two point-type iterators (returned by <code class="function">find</code>) are used
for a range-type purpose - going over all elements whose key is
between 1 and 5.
</p><p>
Conversely, the above snippet makes no sense for
self-organizing containers - ones that order (and reorder)
their elements by implementation. It would be nice to have a
uniform iterator system that would allow the above snippet to
compile only if it made sense.
</p><p>
This could trivially be done by specializing
<code class="function">std::for_each</code> for the case of iterators returned by
<code class="classname">std::tr1::unordered_map</code>, but this would only solve the
problem for one algorithm and one container. Fundamentally, the
problem is that one can loop using a self-organizing
container's point-type iterators.
</p><p>
This library's containers define two families of
iterators: <span class="type">point_const_iterator</span> and
<span class="type">point_iterator</span> are the iterator types returned by
point-type methods; <span class="type">const_iterator</span> and
<span class="type">iterator</span> are the iterator types returned by range-type
methods.
</p><pre class="programlisting">
class <- some container ->
{
public:
...
typedef <- something -> const_iterator;
typedef <- something -> iterator;
typedef <- something -> point_const_iterator;
typedef <- something -> point_iterator;
...
public:
...
const_iterator begin () const;
iterator begin();
point_const_iterator find(...) const;
point_iterator find(...);
};
</pre><p>For
containers whose interface defines sequence order , it
is very simple: point-type and range-type iterators are exactly
the same, which means that the above snippet will compile if it
is used for an order-preserving associative container.
</p><p>
For self-organizing containers, however, (hash-based
containers as a special example), the preceding snippet will
not compile, because their point-type iterators do not support
<code class="function">operator++</code>.
</p><p>In any case, both for order-preserving and self-organizing
containers, the following snippet will compile:
</p><pre class="programlisting">
typename Cntnr::point_iterator it = c.find(2);
</pre><p>
because a range-type iterator can always be converted to a
point-type iterator.
</p><p>Distingushing between iterator types also
raises the point that a container's iterators might have
different invalidation rules concerning their de-referencing
abilities and movement abilities. This now corresponds exactly
to the question of whether point-type and range-type iterators
are valid. As explained above, <code class="classname">container_traits</code> allows
querying a container for its data structure attributes. The
iterator-invalidation guarantees are certainly a property of
the underlying data structure, and so
</p><pre class="programlisting">
container_traits<C>::invalidation_guarantee
</pre><p>
gives one of three pre-determined types that answer this
query.
</p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="pbds.using.examples"></a>Examples</h3></div></div></div><p>
Additional code examples are provided in the source
distribution, as part of the regression and performance
testsuite.
</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.examples.basic"></a>Intermediate Use</h4></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Basic use of maps:
<code class="filename">basic_map.cc</code>
</p></li><li class="listitem"><p>
Basic use of sets:
<code class="filename">basic_set.cc</code>
</p></li><li class="listitem"><p>
Conditionally erasing values from an associative container object:
<code class="filename">erase_if.cc</code>
</p></li><li class="listitem"><p>
Basic use of multimaps:
<code class="filename">basic_multimap.cc</code>
</p></li><li class="listitem"><p>
Basic use of multisets:
<code class="filename">basic_multiset.cc</code>
</p></li><li class="listitem"><p>
Basic use of priority queues:
<code class="filename">basic_priority_queue.cc</code>
</p></li><li class="listitem"><p>
Splitting and joining priority queues:
<code class="filename">priority_queue_split_join.cc</code>
</p></li><li class="listitem"><p>
Conditionally erasing values from a priority queue:
<code class="filename">priority_queue_erase_if.cc</code>
</p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.examples.query"></a>Querying with <code class="classname">container_traits</code> </h4></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Using <code class="classname">container_traits</code> to query
about underlying data structure behavior:
<code class="filename">assoc_container_traits.cc</code>
</p></li><li class="listitem"><p>
A non-compiling example showing wrong use of finding keys in
hash-based containers: <code class="filename">hash_find_neg.cc</code>
</p></li><li class="listitem"><p>
Using <code class="classname">container_traits</code>
to query about underlying data structure behavior:
<code class="filename">priority_queue_container_traits.cc</code>
</p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="pbds.using.examples.container"></a>By Container Method</h4></div></div></div><p></p><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="pbds.using.examples.container.hash"></a>Hash-Based</h5></div></div></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="pbds.using.examples.container.hash.resize"></a>size Related</h6></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Setting the initial size of a hash-based container
object:
<code class="filename">hash_initial_size.cc</code>
</p></li><li class="listitem"><p>
A non-compiling example showing how not to resize a
hash-based container object:
<code class="filename">hash_resize_neg.cc</code>
</p></li><li class="listitem"><p>
Resizing the size of a hash-based container object:
<code class="filename">hash_resize.cc</code>
</p></li><li class="listitem"><p>
Showing an illegal resize of a hash-based container
object:
<code class="filename">hash_illegal_resize.cc</code>
</p></li><li class="listitem"><p>
Changing the load factors of a hash-based container
object: <code class="filename">hash_load_set_change.cc</code>
</p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="pbds.using.examples.container.hash.hashor"></a>Hashing Function Related</h6></div></div></div><p></p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Using a modulo range-hashing function for the case of an
unknown skewed key distribution:
<code class="filename">hash_mod.cc</code>
</p></li><li class="listitem"><p>
Writing a range-hashing functor for the case of a known
skewed key distribution:
<code class="filename">shift_mask.cc</code>
</p></li><li class="listitem"><p>
Storing the hash value along with each key:
<code class="filename">store_hash.cc</code>
</p></li><li class="listitem"><p>
Writing a ranged-hash functor:
<code class="filename">ranged_hash.cc</code>
</p></li></ul></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="pbds.using.examples.container.branch"></a>Branch-Based</h5></div></div></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="pbds.using.examples.container.branch.split"></a>split or join Related</h6></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Joining two tree-based container objects:
<code class="filename">tree_join.cc</code>
</p></li><li class="listitem"><p>
Splitting a PATRICIA trie container object:
<code class="filename">trie_split.cc</code>
</p></li><li class="listitem"><p>
Order statistics while joining two tree-based container
objects:
<code class="filename">tree_order_statistics_join.cc</code>
</p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="pbds.using.examples.container.branch.invariants"></a>Node Invariants</h6></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Using trees for order statistics:
<code class="filename">tree_order_statistics.cc</code>
</p></li><li class="listitem"><p>
Augmenting trees to support operations on line
intervals:
<code class="filename">tree_intervals.cc</code>
</p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h6 class="title"><a id="pbds.using.examples.container.branch.trie"></a>trie</h6></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Using a PATRICIA trie for DNA strings:
<code class="filename">trie_dna.cc</code>
</p></li><li class="listitem"><p>
Using a PATRICIA
trie for finding all entries whose key matches a given prefix:
<code class="filename">trie_prefix_search.cc</code>
</p></li></ul></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="pbds.using.examples.container.priority_queue"></a>Priority Queues</h5></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Cross referencing an associative container and a priority
queue: <code class="filename">priority_queue_xref.cc</code>
</p></li><li class="listitem"><p>
Cross referencing a vector and a priority queue using a
very simple version of Dijkstra's shortest path
algorithm:
<code class="filename">priority_queue_dijkstra.cc</code>
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