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+ <title>Data-Structure Genericity</title>
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+ <div id="page">
+ <h1>Data-Structure Genericity</h1>
+
+ <h2><a name="problem" id="problem">The Basic Problem</a></h2>
+
+ <p>The design attempts to address the following problem. When
+ writing a function manipulating a generic container object,
+ what is the behavior of the object? <i>E.g.</i>, suppose one
+ writes</p>
+ <pre>
+<b>template</b><<b>typename</b> Cntnr>
+<b>void</b>
+some_op_sequence(Cntnr &r_container)
+{
+ ...
+}
+</pre>then one needs to address the following questions in the body
+of <tt>some_op_sequence</tt>:
+
+ <ol>
+ <li>Which types and methods does <tt>Cntnr</tt> support?
+ Containers based on hash tables can be queries for the
+ hash-functor type and object; this is meaningless for
+ tree-based containers. Containers based on trees can be
+ split, joined, or can erase iterators and return the
+ following iterator; this cannot be done by hash-based
+ containers.</li>
+
+ <li>What are the guarantees of <tt>Cntnr</tt>? A container
+ based on a probing hash-table invalidates all iterators when
+ it is modified; this is not the case for containers based on
+ node-based trees. Containers based on a node-based tree can
+ be split or joined without exceptions; this is not the case
+ for containers based on vector-based trees.</li>
+
+ <li>How does the container maintain its elements? Tree-based
+ and Trie-based containers store elements by key order;
+ others, typically, do not. A container based on a splay trees
+ or lists with update policies "cache" "frequently accessed"
+ elements; containers based on most other underlying
+ data structures do not.</li>
+ </ol>
+
+ <p>The remainder of this section deals with these issues.</p>
+
+ <h2><a name="ds_hierarchy" id="ds_hierarchy">Container
+ Hierarchy</a></h2>
+
+ <p>Figure <a href="#cd">Container class hierarchy</a> shows the
+ container hierarchy.</p>
+
+ <h6 class="c1"><a name="cd" id="cd"><img src="container_cd.png" alt=
+ "no image" /></a></h6>
+
+ <h6 class="c1">Container class hierarchy.</h6>
+
+ <ol>
+ <li><a href=
+ "container_base.html"><tt>container_base</tt></a> is an
+ abstract base class for associative containers.</li>
+
+ <li>Tree-Like-Based Associative-Containers:
+
+ <ol>
+ <li><a href=
+ "basic_tree.html"><tt>basic_tree</tt></a>
+ is an abstract base class for tree-like-based
+ associative-containers</li>
+
+ <li><a href=
+ "tree.html"><tt>tree</tt></a>
+ is a concrete base class for tree-based
+ associative-containers</li>
+
+ <li><a href=
+ "trie.html"><tt>trie</tt></a>
+ is a concrete base class trie-based
+ associative-containers</li>
+ </ol>
+ </li>
+
+ <li>Hash-Based Associative-Containers:
+
+ <ol>
+ <li><a href=
+ "basic_hash_table.html"><tt>basic_hash_table</tt></a>
+ is an abstract base class for hash-based
+ associative-containers</li>
+
+ <li><a href=
+ "cc_hash_table.html"><tt>cc_hash_table</tt></a>
+ is a concrete collision-chaining hash-based
+ associative-containers</li>
+
+ <li><a href=
+ "gp_hash_table.html"><tt>gp_hash_table</tt></a>
+ is a concrete (general) probing hash-based
+ associative-containers</li>
+ </ol>
+ </li>
+
+ <li>List-Based Associative-Containers:
+
+ <ol>
+ <li><a href=
+ "list_update.html"><tt>list_update</tt></a> -
+ list-based update-policy associative container</li>
+ </ol>
+ </li>
+ </ol>
+
+ <p>The hierarchy is composed naturally so that commonality is
+ captured by base classes. Thus <tt><b>operator[]</b></tt> is
+ defined <a href=
+ "container_base.html"><tt>container_base</tt></a>, since
+ all containers support it. Conversely <tt>split</tt> is defined
+ in <a href=
+ "basic_tree.html"><tt>basic_tree</tt></a>,
+ since only tree-like containers support it. <a href=
+ "#container_traits">Data-Structure Tags and Traits</a> discusses how
+ to query which types and methods each container supports.</p>
+
+ <h2><a name="container_traits" id="container_traits">Data-Structure Tags and
+ Traits</a></h2>
+
+ <p>Tags and traits are very useful for manipulating generic
+ types. For example, if <tt>It</tt> is an iterator class, then
+ <tt><b>typename</b> It::iterator_category</tt> or
+ <tt><b>typename</b>
+ std::iterator_traits<It>::iterator_category</tt> will
+ yield its category, and <tt><b>typename</b>
+ std::iterator_traits<It>::value_type</tt> will yield its
+ value type.</p>
+
+ <p><tt>pb_ds</tt> contains a tag hierarchy corresponding to the
+ hierarchy in Figure <a href="#cd">Class hierarchy</a>. The tag
+ hierarchy is shown in Figure <a href=
+ "#tag_cd">Data-structure tag class hierarchy</a>.</p>
+
+ <h6 class="c1"><a name="tag_cd" id="tag_cd"><img src=
+ "assoc_container_tag_cd.png" alt="no image" /></a></h6>
+
+ <h6 class="c1">Data-structure tag class hierarchy.</h6>
+
+ <p><a href=
+ "container_base.html"><tt>container_base</tt></a>
+ publicly defines <tt>container_category</tt> as one of the classes in
+ Figure <a href="#tag_cd">Data-structure tag class
+ hierarchy</a>. Given any container <tt>Cntnr</tt>, the tag of
+ the underlying data structure can be found via
+ <tt><b>typename</b> Cntnr::container_category</tt>.</p>
+
+ <p>Additionally, a traits mechanism can be used to query a
+ container type for its attributes. Given any container
+ <tt>Cntnr</tt>, then <tt><a href=
+ "assoc_container_traits.html">__gnu_pbds::container_traits</a><Cntnr></tt>
+ is a traits class identifying the properties of the
+ container.</p>
+
+ <p>To find if a container can throw when a key is erased (which
+ is true for vector-based trees, for example), one can
+ use</p><a href=
+ "assoc_container_traits.html"><tt>container_traits</tt></a><tt><Cntnr>::erase_can_throw</tt>,
+ for example.
+
+ <p>Some of the definitions in <a href=
+ "assoc_container_traits.html"><tt>container_traits</tt></a> are
+ dependent on other definitions. <i>E.g.</i>, if <a href=
+ "assoc_container_traits.html"><tt>container_traits</tt></a><tt><Cntnr>::order_preserving</tt>
+ is <tt><b>true</b></tt> (which is the case for containers based
+ on trees and tries), then the container can be split or joined;
+ in this case, <a href=
+ "assoc_container_traits.html"><tt>container_traits</tt></a><tt><Cntnr>::split_join_can_throw</tt>
+ indicates whether splits or joins can throw exceptions (which
+ is true for vector-based trees); otherwise <a href=
+ "assoc_container_traits.html"><tt>container_traits</tt></a><tt><Cntnr>::split_join_can_throw</tt>
+ will yield a compilation error. (This is somewhat similar to a
+ compile-time version of the COM model [<a href=
+ "references.html#mscom">mscom</a>]).</p>
+
+ <h2><a name="find_range" id="find_range">Point-Type and
+ Range-Type Methods and Iterators</a></h2>
+
+ <h3><a name="it_unordered" id="it_unordered">Iterators in
+ Unordered Container Types</a></h3>
+
+ <p><tt>pb_ds</tt> differentiates between two types of methods
+ and iterators: point-type methods and iterators, and range-type
+ methods and iterators (see <a href=
+ "motivation.html#assoc_diff_it">Motivation::Associative
+ Containers::Differentiating between Iterator Types</a> and
+ <a href="tutorial.html#assoc_find_range">Tutorial::Associative
+ Containers::Point-Type and Range-Type Methods and
+ Iterators</a>). Each associative container's interface includes
+ the methods:</p>
+ <pre>
+const_point_iterator
+find(const_key_reference r_key) const;
+
+point_iterator
+find(const_key_reference r_key);
+
+std::pair<point_iterator,<b>bool</b>>
+insert(const_reference r_val);
+</pre>
+
+ <p>The relationship between these iterator types varies between
+ container types. Figure <a href=
+ "#point_iterators_cd">Point-type and range-type iterators</a>-A
+ shows the most general invariant between point-type and
+ range-type iterators: <tt>iterator</tt>, <i>e.g.</i>, can
+ always be converted to <tt>point_iterator</tt>. Figure <a href=
+ "#point_iterators_cd">Point-type and range-type iterators</a>-B
+ shows invariants for order-preserving containers: point-type
+ iterators are synonymous with range-type iterators.
+ Orthogonally, Figure <a href="#point_iterators_cd">Point-type
+ and range-type iterators</a>-C shows invariants for "set"
+ containers: iterators are synonymous with const iterators.</p>
+
+ <h6 class="c1"><a name="point_iterators_cd" id=
+ "point_iterators_cd"><img src="point_iterators_cd.png" alt=
+ "no image" /></a></h6>
+
+ <h6 class="c1">Point-type and range-type iterators.</h6>
+
+ <p>Note that point-type iterators in self-organizing containers
+ (<i>e.g.</i>, hash-based associative containers) lack movement
+ operators, such as <tt><b>operator++</b></tt> - in fact, this
+ is the reason why <tt>pb_ds</tt> differentiates from the STL's
+ design on this point.</p>
+
+ <p>Typically, one can determine an iterator's movement
+ capabilities in the STL using
+ <tt>std::iterator_traits<It>iterator_category</tt>, which
+ is a <tt><b>struct</b></tt> indicating the iterator's movement
+ capabilities. Unfortunately, none of the STL's predefined
+ categories reflect a pointer's <u>not</u> having any movement
+ capabilities whatsoever. Consequently, <tt>pb_ds</tt> adds a
+ type <a href=
+ "trivial_iterator_tag.html"><tt>trivial_iterator_tag</tt></a>
+ (whose name is taken from a concept in [<a href=
+ "references.html#sgi_stl">sgi_stl</a>]), which is the category
+ of iterators with no movement capabilities. All other STL tags,
+ such as <tt>forward_iterator_tag</tt> retain their common
+ use.</p>
+
+ <h3><a name="inv_guar" id="inv_guar">Invalidation
+ Guarantees</a></h3>
+
+ <p><a href=
+ "motivation.html#assoc_inv_guar">Motivation::Associative
+ Containers::Differentiating between Iterator
+ Types::Invalidation Guarantees</a> posed a problem. Given three
+ different types of associative containers, a modifying
+ operation (in that example, <tt>erase</tt>) invalidated
+ iterators in three different ways: the iterator of one
+ container remained completely valid - it could be de-referenced
+ and incremented; the iterator of a different container could
+ not even be de-referenced; the iterator of the third container
+ could be de-referenced, but its "next" iterator changed
+ unpredictably.</p>
+
+ <p>Distinguishing between find and range types allows
+ fine-grained invalidation guarantees, because these questions
+ correspond exactly to the question of whether point-type
+ iterators and range-type iterators are valid. <a href=
+ "#invalidation_guarantee_cd">Invalidation guarantees class
+ hierarchy</a> shows tags corresponding to different types of
+ invalidation guarantees.</p>
+
+ <h6 class="c1"><a name="invalidation_guarantee_cd" id=
+ "invalidation_guarantee_cd"><img src=
+ "invalidation_guarantee_cd.png" alt="no image" /></a></h6>
+
+ <h6 class="c1">Invalidation guarantees class hierarchy.</h6>
+
+ <ol>
+ <li><a href=
+ "basic_invalidation_guarantee.html"><tt>basic_invalidation_guarantee</tt></a>
+ corresponds to a basic guarantee that a point-type iterator,
+ a found pointer, or a found reference, remains valid as long
+ as the container object is not modified.</li>
+
+ <li><a href=
+ "point_invalidation_guarantee.html"><tt>point_invalidation_guarantee</tt></a>
+ corresponds to a guarantee that a point-type iterator, a
+ found pointer, or a found reference, remains valid even if
+ the container object is modified.</li>
+
+ <li><a href=
+ "range_invalidation_guarantee.html"><tt>range_invalidation_guarantee</tt></a>
+ corresponds to a guarantee that a range-type iterator remains
+ valid even if the container object is modified.</li>
+ </ol>
+
+ <p>As shown in <a href=
+ "tutorial.html#assoc_find_range">Tutorial::Associative
+ Containers::Point-Type and Range-Type Methods and
+ Iterators</a>, to find the invalidation guarantee of a
+ container, one can use</p>
+ <pre>
+<b>typename</b> <a href=
+"assoc_container_traits.html">container_traits</a><Cntnr>::invalidation_guarantee
+</pre>
+
+ <p>which is one of the classes in Figure <a href=
+ "#invalidation_guarantee_cd">Invalidation guarantees class
+ hierarchy</a>.</p>
+
+ <p>Note that this hierarchy corresponds to the logic it
+ represents: if a container has range-invalidation guarantees,
+ then it must also have find invalidation guarantees;
+ correspondingly, its invalidation guarantee (in this case
+ <a href=
+ "range_invalidation_guarantee.html"><tt>range_invalidation_guarantee</tt></a>)
+ can be cast to its base class (in this case <a href=
+ "point_invalidation_guarantee.html"><tt>point_invalidation_guarantee</tt></a>).
+ This means that this this hierarchy can be used easily using
+ standard metaprogramming techniques, by specializing on the
+ type of <tt>invalidation_guarantee</tt>.</p>
+
+ <p>(These types of problems were addressed, in a more general
+ setting, in [<a href=
+ "references.html#meyers96more">meyers96more</a>] - Item 2. In
+ our opinion, an invalidation-guarantee hierarchy would solve
+ these problems in all container types - not just associative
+ containers.)</p>
+ </div>
+</body>
+</html>
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