119 lines
4.6 KiB
HTML
119 lines
4.6 KiB
HTML
<!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>
|