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    <h1>Short Tutorial</h1>

    <p>Following is a short tutorial illustrating the main points
    of <tt>pb_ds</tt>. <a href="concepts.html">Concepts</a>
    describes and summarizes some concepts.</p>

    <h2><a name="assoc_main" id="assoc_main">Associative
    Containers</a></h2>

    <h3><a name="assoc_basic" id="assoc_basic">Basic Use</a></h3>

    <p>For the most part, <tt>pb_ds</tt>'s containers have the same
    interface as the STL's, 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>
<a href=
"cc_hash_table.html">cc_hash_table</a>&lt;<b>int</b>, <b>char</b>&gt; c;

c[2] = 'b';

assert(c.find(1) == c.end());
</pre>

    <p>The container is called <a href=
    "cc_hash_table.html"><tt>cc_hash_table</tt></a> as
    opposed to <tt>unordered_map</tt>, since "unordered map" does
    not necessarily mean a hash-based map (as the STL implicitly
    implies). For example, list-based associative containers, which
    are very useful for the construction of "multimaps" (see
    <a href=
    "assoc_performance_tests.html#msc">Associative-Container
    Performance Tests::Observations::Mapping-Semantics
    Considerations</a>), are also unordered. It is also not called
    <tt>hash_map</tt> since there are more ways than one to
    implement hash tables.</p>

    <p>This snippet shows a red-black tree based container:</p>
    <pre>
<a href=
"tree.html">tree</a>&lt;<b>int</b>, <b>char</b>&gt; c;

c[2] = 'b';

assert(c.find(2) != c.end());
</pre>

    <p>The container is called <a href=
    "tree.html"><tt>tree</tt></a>
    as opposed to <tt>map</tt>, since "map" doesn't say that
    much.</p>

    <p>Most of the STL's familiar methods are unchanged.
    <i>E.g.</i>, <tt>being</tt>, <tt>end</tt>, <tt>size</tt>,
    <tt>empty</tt>, and <tt>clear</tt>, do just the same as is
    customary. <a href=
    "assoc_examples.html#basic_usage">Associative-Container
    Examples::Basic use</a>, and especially <a href=
    "../../../../testsuite/ext/pb_ds/example/basic_map.cc"><tt>basic_map.cc</tt></a>,
    show examples of this.</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 than those of the STL. For example, if <tt>C</tt> is
    some type of hash-based container, then</p>
    <pre>
C::hash_fn
</pre>gives the type of its hash functor, and if <tt>c</tt> is some
hash-based container object, then
    <pre>
c.get_hash_fn()
</pre>

    <p>will return a reference to its hash-functor object.</p>

    <p>Similarly, if <tt>C</tt> is some type of tree-based
    container, then</p>
    <pre>
C::cmp_fn
</pre>gives the type of its comparison functor, and if <tt>c</tt>
is some tree-based container object, then
    <pre>
c.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, <tt>std::tr1::unordered_map</tt> uses
    <tt>hasher</tt> for the hash functor, but <tt>std::map</tt> uses
    <tt>key_compare</tt> for the comparison functor. Also, we could
    not find an accessor for <tt>std::tr1::unordered_map</tt>'s hash
    functor, but <tt>std::map</tt> uses <tt>compare</tt> for accessing
    the comparison functor.</p> 

<p>Instead, <tt>pb_ds</tt> attempts to be internally consistent, and
uses standard-derived terminology if possible.
</p>

    <p>Another source of difference is in scope: <tt>pb_ds</tt>
    contains more types of associative containers than the STL, and
    more opportunities to configure these new containers, since
    different types of associative containers are useful in different
    settings (see <a href=
    "assoc_performance_tests.html#dss_family_choice">Associative-Container
    Performance Tests::Observations::Underlying Data-Structure
    Families</a>).</p>

    <p><tt>pb_ds</tt> contains different classes for hash-based containers,
    tree-based containers, trie-based containers, and list-based
    containers. <a href=
    "interface.html#containers_assoc">Inteface::Containers::Associative
    Containers</a> lists the containers. <a href=
    "hash_based_containers.html">Design::Associative
    Containers::Hash-Based Containers</a>, <a href=
    "tree_based_containers.html">Design::Associative
    Containers::Tree-Based Containers</a>, <a href=
    "trie_based_containers.html">Design::Associative
    Containers::Trie-Based Containers</a>, and <a href=
    "lu_based_containers.html">Design::Associative
    Containers::List-Based Containers</a>, explain some more about
    these types of containers, respectively.</p>

    <p>Since associative containers share parts of their interface,
    they are organized as a class hierarchy; it is shown in Figure
    <a href="#cd">Class hierarchy</a>.</p>

    <h6 class="c1"><a name="cd" id="cd"><img src="container_cd.png" alt=
    "no image" /></a></h6>

    <h6 class="c1">Class hierarchy.</h6>

    <p>Each type or method is defined in the most-common ancestor
    in which it makes sense:
  <a href=
    "../../../../testsuite/ext/pb_ds/example/basic_map.cc"><tt>basic_map.cc</tt></a>
    shows an example of most of the associative-container
    types.</p>

 
    <p>For example, all associative containers support iteration.
    Consequently, <a href=
    "container_base.html"><tt>container_base</tt></a> has the
    interface:</p>
    <pre>
<b>template</b>&lt;...&gt;
<b>class</b> <a href="container_base.html">container_base</a>
{
    ...
    
<b>public</b>:
    ...
    
    const_iterator
    begin() <b>const</b>;
    
    iterator
    begin();

    const_iterator
    end() <b>const</b>;
    
    iterator
    end();
        
    ...
};
</pre>

    <p>and so all associative containers inherent this method.
    Conversely, both collision-chaining and (general) probing
    hash-based associative containers have a hash functor, so
    <a href=
    "basic_hash_table.html"><tt>basic_hash_table</tt></a>
    has the interface:</p>
    <pre>
<b>template</b>&lt;...&gt;
<b>class</b> <a href="basic_hash_table.html">basic_hash_table</a> : <b>public</b> <a href="container_base.html">container_base</a>
{
    ...
    
<b>public</b>:
    ...
    
    const hash_fn&amp;
    get_hash_fn() const;
        
    hash_fn&amp;
    get_hash_fn();
    ...
};
</pre>

    <p>and so all hash-based associative containers inherit the
    same hash-functor accessor methods.</p>

    <p>This is discussed further in <a href=
    "ds_gen.html">Design::Associative Containers::Data-Structure
    Genericity</a>.</p>

    <h3><a name="assoc_policies" id="assoc_policies">Configuring
    Associative Containers</a></h3>

    <p>In general, each of <tt>pb_ds</tt>'s containers is
    parametrized by more policies than those of the STL's. For
    example, the STL's hash-based container is parametrized as
    follows:</p>
    <pre>
<b>template</b>&lt;
    <b>typename</b> Key,
    <b>typename</b> Mapped,
    <b>typename</b> Hash,
    <b>typename</b> Pred,
    <b>typename</b> Allocator,
    <b>bool</b> Cache_Hashe_Code&gt;
<b>class</b> 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. <tt>pb_ds</tt>'s collision-chaining
    hash-based container is parametrized as</p>
    <pre>
<b>template</b>&lt;
    <b>typename</b> Key,
    <b>typename</b> Mapped,
    <b>typename</b> Hash_Fn,
    <b>typename</b> Eq_Fn,
    <b>typename</b> Comb_Hash_Fn,
    <b>typename</b> Resize_Policy
    <b>bool</b> Store_Hash
    <b>typename</b> Allocator&gt;
<b>class</b> <a href=
"cc_hash_table.html">cc_hash_table</a>;
</pre>

    <p>and so can be configured by the first four types of
    <tt>std::tr1::unordered_map</tt>, 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 STL 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><a href="hash_based_containers.html">Design::Associative
    Containers::Hash-Based Containers</a>, <a href=
    "tree_based_containers.html">Design::Associative
    Containers::Tree-Based Containers</a>, <a href=
    "trie_based_containers.html">Design::Associative
    Containers::Trie-Based Containers</a>, and <a href=
    "lu_based_containers.html">Design::Associative
    Containers::List-Based Containers</a>, explain some more about
    configuring hash based, tree based, trie based, and list base
    containers, respectively. <a href=
    "interface.html#ds_policy_classes">Interface::Container Policy
    Classes</a> shows the different policy classes for configuring
    associative containers. <a href=
    "assoc_examples.html#hash_based">Examples::Hash-Based
    Containers</a>, <a href=
    "assoc_examples.html#tree_like_based">Examples::Tree-Like-Based
    Containers</a>, and <a href=
    "assoc_examples.html#trie_based">Examples::Trie-Based
    Containers</a> show examples for this.</p>

    <h3><a name="assoc_ds_gen" id="assoc_ds_gen">Determining
    Containers' Attributes</a></h3>

    <p>Associative-containers' underlying data structures obviously
    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. (This was
    discussed in <a href=
    "motivation.html#assoc_ds_genericity">Motivation::Associative
    Containers::Data-Structure Genericity</a>.)</p>

    <p>Happily, the STL provides a good solution to a similar
    problem - that of the different behavior of iterators. If
    <tt>It</tt> is an iterator, then</p>
    <pre>
<b>typename</b> std::iterator_traits&lt;It&gt;::iterator_category
</pre>

    <p>is one of a small number of pre-defined
    <tt><b>struct</b></tt>s, and,</p>
    <pre>
<b>typename</b> std::iterator_traits&lt;It&gt;::value_type
</pre>

    <p>is the value type to which the iterator "points".</p>

    <p>Similarly, in <tt>pb_ds</tt>, if <tt>C</tt> is an
    associative container, then</p>
    <pre>
<b>typename</b> <a href=
"assoc_container_traits.html"><tt>container_traits</tt></a>&lt;C&gt;::container_category
</pre>is one of a small number of pre-defined
<tt><b>struct</b></tt>s, each one corresponding to a class in
Figure <a href="#cd">Class hierarchy</a>. These tags are listed in
<a href="interface.html#ds_ts_assoc">Interface::Associative
Containers::Data-Structure Tags and Traits::Data-Structure
Tags::Associative-Containers</a>; <a href="ds_gen.html#container_traits">
    Design::Associative Containers::Data-Structure Tags and
    Traits</a> explains this further; <a href=
    "ds_gen.html#tag_cd">Design::Associative
    Containers::Data-Structure Tags and Traits::Data-structure tag
    class hierarchy</a> shows a class diagram.

    <p>In most cases, however, the exact underlying data structure
    is not really important, but only one of its attributes:
    whether it guarantees storing elements by key order, for
    example. For this one can use</p>
    <pre>
<b>typename</b> <a href=
"assoc_container_traits.html"><tt>container_traits</tt></a>&lt;C&gt;::order_preserving
</pre>

    <p>This is described further in <a href=
    "ds_gen.html">Design::Data-Structure Genericity</a>; <a href=
    "../../../../testsuite/ext/pb_ds/example/assoc_container_traits.cc"><tt>assoc_container_traits.cc</tt></a>
    shows an example of querying containers' attributes.</p>

    <h3><a name="assoc_find_range" id="assoc_find_range">Point-Type
    and Range-Type Methods and Iterators</a></h3>(This subsection
    addresses points from <a href=
    "motivation.html#assoc_diff_it">Motivation::Associative
    Containers::Differentiating between Iterator Types</a>.)

    <p><tt>pb_ds</tt> differentiates between two types of methods
    and iterators: point-type, and range-type. For example,
    <tt>find</tt> and <tt>insert</tt> are point-type methods, since
    they each deal with a specific element; their returned
    iterators are point-type iterators. <tt>begin</tt> and
    <tt>end</tt> 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>
std::for_each(c.find(1), c.find(5), foo);
</pre>two point-type iterators (returned by <tt>find</tt>) are used
for a range-type purpose - going over all elements whose key is
between 1 and 5.

    <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
    <tt>std::for_each</tt> for the case of iterators returned by
    <tt>std::tr1::unordered_map</tt>, 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><tt>pb_ds</tt>'s containers define two families of
    iterators: <tt>const_point_iterator</tt> and
    <tt>point_iterator</tt> are the iterator types returned by
    point-type methods; <tt>const_iterator</tt> and
    <tt>iterator</tt> are the iterator types returned by range-type
    methods.</p>
    <pre>
<b>class</b> <i>&lt;- some container -&gt;</i>
{
<b>public</b>:
    ...

    <b>typedef</b> <i>&lt;- something -&gt;</i> const_iterator;

    <b>typedef</b> <i>&lt;- something -&gt;</i> iterator;

    <b>typedef</b> <i>&lt;- something -&gt;</i> const_point_iterator;

    <b>typedef</b> <i>&lt;- something -&gt;</i> point_iterator;
 
    ...

<b>public</b>:
    ...

    const_iterator begin () <b>const</b>;

    iterator begin();

    const_point_iterator find(...) <b>const</b>;

    point_iterator find(...);
};
</pre>

    <p><a href="ds_gen.html#find_range">Design::Associative
    Containers::Data-Structure Genericity::Point-Type and
    Range-Type Methods and Iterators</a> discusses the relationship
    between point-type and range-type iterators in general; for
    containers whose interface defines sequence order, however, 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
    <tt><b>operator</b>++</tt>.</p>

    <p>In any case, both for order-preserving and self-organizing
    containers, the following snippet will compile:</p>
    <pre>
<b>typename</b> 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><a href="ds_gen.html#find_range">Design::Associative
    Containers::Data-Structure Genericity::Point-Type and
    Range-Type Methods and Iterators</a> discusses this
    further.</p>

    <p><a href=
    "motivation.html#assoc_diff_it">Motivation::Associative
    Containers::Differentiating between Iterator Types</a> also
    raised 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 in <a href="#assoc_ds_gen">Determining
    Containers' Attributes</a>, <a href=
    "assoc_container_traits.html"><tt>container_traits</tt></a> 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>
<a href=
"assoc_container_traits.html">container_traits</a>&lt;C&gt;::invalidation_guarantee
</pre>

    <p>gives one of three pre-determined types that answer this
    query. This is explained further in <a href=
    "ds_gen.html#find_range">Design::Associative
    Containers::Data-Structure Genericity::Point-Type and
    Range-Type Methods and Iterators</a>.</p>

    <h3><a name="assoc_ms" id="assoc_ms">Distinguishing between Maps and Sets</a></h3>

    <p>Anyone familiar with the STL knows that there are four kinds
    of associative containers: maps, sets, multimaps, and
    multisets. <a href="#assoc_basic">Basic Use</a> discussed how
    to use maps, <i>i.e.</i> containers that associate each key to
    some data.</p>

    <p>Sets are associative containers that simply store keys -
    they do not map them to anything. In the STL, each map class
    has a corresponding set class. <i>E.g.</i>,
    <tt>std::map&lt;<b>int</b>, <b>char</b>&gt;</tt> maps each
    <tt><b>int</b></tt> to a <tt><b>char</b></tt>, but
    <tt>std::set&lt;<b>int</b>, <b>char</b>&gt;</tt> simply stores
    <tt><b>int</b></tt>s. In <tt>pb_ds</tt>, however, there are no
    distinct classes for maps and sets. Instead, an associative
    container's <tt>Mapped</tt> template parameter is a policy: if
    it is instantiated by <a href=
    "null_mapped_type.html"><tt>null_mapped_type</tt></a>, then it
    is a "set"; otherwise, it is a "map". <i>E.g.</i>,</p>
    <pre>
<a href="cc_hash_table.html">cc_hash_table</a>&lt;<b>int</b>, <b>char</b>&gt;
</pre>is a "map" mapping each <tt><b>int</b></tt> value to a <tt>
    <b>char</b></tt>, but
    <pre>
<a href="cc_hash_table.html">cc_hash_table</a>&lt;<b>int</b>, <a href="null_mapped_type.html">null_mapped_type</a>&gt;
</pre>is a type that uniquely stores <tt><b>int</b></tt> values.

    <p>Once the <tt>Mapped</tt> template parameter is instantiated
    by <a href="null_mapped_type.html">null_mapped_type</a>, then
    the "set" acts very similarly to the STL's sets - it does not
    map each key to a distinct <a href=
    "null_mapped_type.html">null_mapped_type</a> object. Also,
   , the container's <tt>value_type</tt> is essentially
    its <tt>key_type</tt> - just as with the STL's sets. For a simple example, see <a href=
    "../../../../testsuite/ext/pb_ds/example/basic_set.cc"><tt>basic_set.cc</tt></a>
   .</p>

    <p>The STL's multimaps and multisets allow, respectively,
    non-uniquely mapping keys and non-uniquely storing keys. As
    discussed in <a href=
    "motivation.html#assoc_mapping_semantics">Motivation::Associative
    Containers::Alternative to Multiple Equivalent Keys</a>, the
    reasons why this might be necessary are 1) that a key might be
    decomposed into a primary key and a secondary key, 2) that a
    key might appear more than once, or 3) any arbitrary
    combination of 1)s and 2)s. Correspondingly,
    one should use 1) "maps" mapping primary keys to secondary
    keys, 2) "maps" mapping keys to size types, or 3) any arbitrary
    combination of 1)s and 2)s. Thus, for example, an
    <tt>std::multiset&lt;<b>int</b>&gt;</tt> might be used to store
    multiple instances of integers, but using <tt>pb_ds</tt>'s
    containers, one might use</p>
    <pre>
<a href=
"tree.html">tree</a>&lt;<b>int</b>, size_t&gt;
</pre><i>i.e.</i>, a "map" of <tt><b>int</b></tt>s to
<tt>size_t</tt>s.

    <p><a href="assoc_examples.html#mmaps">Associative-Container
    Examples::"Multimaps" and "Multisets"</a> shows some simple
    examples.</p>

    <p>These "multimaps" and "multisets" might be confusing to
    anyone familiar with the STL's <tt>std::multimap</tt> and
    <tt>std::multiset</tt>, because there is no clear
    correspondence between the two. For example, in some cases
    where one uses <tt>std::multiset</tt> in the STL, one might use
    in <tt>pb_ds</tt> a "multimap" of "multisets" - <i>i.e.</i>, a
    container that maps primary keys each to an associative
    container that maps each secondary key to the number of times
    it occurs.</p>

    <p>When one uses a "multimap," one should choose with care the
    type of container used for secondary keys. This is further
    explained in <a href=
    "assoc_performance_tests.html#msc">Associative-Container
    Performance Tests::Observations::Mapping-Semantics
    Considerations</a>.</p>

<hr>
    <h2><a name="pq" id="pq">Priority Queues</a></h2>

    <h3><a name="pq_basic" id="pq_basic">Basic Use</a></h3>

    <p><tt>pb_ds</tt>'s priority_queue container is
    similar to the STL's in interface. For example:</p>
    <pre>
<a href=
"priority_queue.html">priority_queue</a>&lt;<b>int</b>&gt; p;

p.push(2);
p.push(4);
p.push(1);

assert(p.top() == 4);

p.pop();

assert(p.top() == 2);

assert(p.size() == 2);
assert(!p.empty());
</pre>

    <h3><a name="pq_policies" id="pq_policies">Configuring Priority
    Queues</a></h3>

    <p>As opposed to associative containers, priority queues have
    relatively few configuration options. The priority queue is
    parametrized as follows:</p>
    <pre>
<b>template</b>&lt;
    <b>typename</b> Value_Type,
    <b>typename</b> Cmp_Fn,
    <b>typename</b> Tag,
    <b>typename</b> Allocator&gt;
<b>class</b> <a href="priority_queue.html">priority_queue</a>;
</pre>

    <p>The <tt>Value_Type</tt>, <tt>Cmp_Fn</tt>, and
    <tt>Allocator</tt> parameters are the container's value type,
    comparison-functor type, and allocator type, respectively;
    these are very similar to the STL's priority queue. The
    <tt>Tag</tt> parameter is different: there are a number of
    pre-defined tag types corresponding to binary heaps, binomial
    heaps, <i>etc.</i>, and <tt>Tag</tt> should be instantiated
    by one of them. <a href=
    "interface.html#ds_ts_pq">Interface::Data-Structure Tags and
    Traits::Data Structure Tags::Priority-Queues</a> lists the
    possible types, <a href="pq_design.html">Priority-Queue
    Design</a> explains this further, and <a href=
    "../../../../testsuite/ext/pb_ds/example/basic_priority_queue.cc"><tt>basic_priority_queue.cc</tt></a>
    shows an example.</p>

    <p>Note that as opposed to the STL's priority queue, <a href=
    "priority_queue.html"><tt>priority_queue</tt></a> is not a
    sequence-adapter; it is a regular container.</p>

    <h3><a name="pq_ds_more_ops" id="pq_ds_more_ops">Supporting
    More Operations</a></h3>

    <p><a href="priority_queue.html"><tt>priority_queue</tt></a>'s
    <tt>push</tt> method returns a point-type iterator, which can
    be used for modifying or erasing arbitrary values. For
    example:</p>
    <pre>
<a href=
"priority_queue.html">priority_queue</a>&lt;<b>int</b>&gt; p;

<a href=
"priority_queue.html">priority_queue</a>&lt;<b>int</b>&gt;::point_iterator it = p.push(3);

p.modify(it, 4);
</pre>

    <p>These types of operations are necessary for making priority
    queues useful for different applications, especially graph
    applications. <a href="pq_examples.html#xref">Priority-Queue
    Examples::Cross-Referencing</a> gives some examples.</p>

    <h3><a name="pq_ds_gen" id="pq_ds_gen">Determining Container
    Attributes</a></h3>

    <p>Similarly to <a href=
    "assoc_container_traits.html"><tt>container_traits</tt></a> (described
    in <a href="#assoc_ds_gen">Associative Containers::Determining
    Containers' Attributes</a>), <a href=
    "pq_container_traits.html"><tt>container_traits</tt></a> can be used to
    statically determine priority-queues' attributes:</p>
    <pre>
<a href=
"pq_container_traits.html">container_traits</a>&lt;C&gt;::container_category
</pre>is one of a small number of predefined tag structures that
identifies the underlying data structure, and
    <pre>
<a href=
"pq_container_traits.html">container_traits</a>&lt;C&gt;::invalidation_guarantee
</pre>

    <p>is its invalidation guarantee. Invalidation guarantees are
    especially important regarding priority queues, since in
    <tt>pb_ds</tt>'s design, iterators are practically the only way
    to manipulate them.</p>

    <p><a href="pq_design.html#pq_traits">Design::Priority
    Queues::Traits</a> discusses this further. <a href=
    "pq_examples.html#generics">Priority-Queue
    Examples::Generics</a> shows an example.</p>
  </div>
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