<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.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>Concepts</title> <meta http-equiv="Content-Type" content= "text/html; charset=us-ascii" /> </head> <body> <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>