<!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" xml:lang="en" lang="en"> <head> <meta name="generator" content= "HTML Tidy for Linux/x86 (vers 12 April 2005), see www.w3.org" /> <title>Data-Structure Genericity</title> <meta http-equiv="Content-Type" content= "text/html; charset=us-ascii" /> </head> <body> <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>