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+ <title>Concepts</title>
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+ <div id="page">
+ <h1>Concepts</h1>
+
+ <h2><a name="concepts_find_and_range_iterators" id=
+ "concepts_find_and_range_iterators">Point and Range Methods and
+ Iterators</a></h2>
+
+ <p>A point-type iterator is an iterator that refers to a
+ specific element, <i>e.g.</i> as returned through an
+ associative-container's <tt>find</tt> method; a range-type
+ iterator is an iterator that is used to go over a sequence of
+ elements, <i>e.g.</i>, as returned by a container's
+ <tt>find</tt> method. A point-type method is a method that
+ returns a point-type iterator; a range-type method is a method
+ that returns a range-type iterator.</p>
+
+ <p>For most containers, these types are synonymous; for
+ self-organizing containers, such as hash-based containers or
+ priority queues, these are inherently different (in any
+ implementation, including that of the STL), but in
+ <tt>pb_ds</tt> this is made explicit - they are distinct
+ types.</p>
+
+
+ <h2><a name="invalidation_guarantees" id=
+ "invalidation_guarantees">Invalidation Guarantees</a></h2>
+
+ <p>If one manipulates a container object, then iterators
+ previously obtained from it can be invalidated. In some cases a
+ previously-obtained iterator cannot be de-referenced; in other
+ cases, the iterator's next or previous element might have
+ changed unpredictably. This corresponds exactly to the question
+ whether a point-type or range-type iterator (see previous
+ concept) is valid or not. In <tt>pb_ds</tt> one can query a
+ container (in compile time) what are its invalidation
+ guarantees.</p>
+
+ <h2><a name="prm_sec" id="prm_sec">Primary and Secondary Keys
+ and Associative Containers</a></h2>
+
+ <p>In <tt>pb_ds</tt> there are no associative containers which
+ allow multiple values with equivalent keys (such as the STL's
+ <tt>std::multimap</tt>, for example). Instead, one maps the
+ unique part of a key - the primary key, into an
+ associative-container of the (originally) non-unique parts of
+ the key - the secondary key. A primary associative-container is
+ an associative container of primary keys; a secondary
+ associative-container is an associative container of secondary
+ keys.</p>
+
+
+ <h2><a name="concepts_null_policies" id=
+ "concepts_null_policies">Null Policy Classes</a></h2>
+
+ <p>Associative containers are typically parametrized by
+ various policies. For example, a hash-based associative
+ container is parametrized by a hash-functor, transforming each
+ key into an non-negative numerical type. Each such value is
+ then further mapped into a position within the table. The
+ mapping of a key into a position within the table is therefore
+ a two-step process.</p>
+
+ <p>In some cases, instantiations are <i>redundant</i>. For
+ example, when the keys are integers, it is possible to use a
+ <i>redundant</i> hash policy, which transforms each key into
+ its value.</p>
+
+ <p>In some other cases, these policies are <i>irrelevant</i>.
+ For example, a hash-based associative container might transform
+ keys into positions within a table by a different method than
+ the two-step method described above. In such a case, the hash
+ functor is simply irrelevant.</p>
+
+ <p><tt>pb_ds</tt> uses special pre-defined "null policies"
+ classes for these cases. Some null policies in <tt>pb_ds</tt>
+ are:</p>
+
+ <ol>
+ <li><a href=
+ "null_mapped_type.html"><tt>null_mapped_type</tt></a></li>
+
+ <li><a href=
+ "null_tree_node_update.html"><tt>null_tree_node_update</tt></a></li>
+
+ <li><a href=
+ "null_trie_node_update.html"><tt>null_trie_node_update</tt></a></li>
+
+ <li><a href=
+ "null_hash_fn.html"><tt>null_hash_fn</tt></a></li>
+
+ <li><a href=
+ "null_probe_fn.html"><tt>null_probe_fn</tt></a></li>
+ </ol>
+
+ <p>A "set" in <tt>pb_ds</tt>, for example, is an associative
+ container with its <tt>Data_Parameter</tt> instantiated by
+ <a href="null_mapped_type.html"><tt>null_mapped_type</tt></a>.
+ <a href=
+ "tree_based_containers.html#invariants">Design::Tree-Based
+ Containers::Node Invariants</a> explains another case where a
+ null policy is needed.</p>
+ </div>
+</body>
+</html>
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