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<h1>Overview</h1>
<p>
The <a href = "introduction.html">Introduction</a> Section described some challenges
in designing associative containers. This section describes the <tt>pb_assoc</tt>'s solution.
</p>
<p>
Figure
<a href = "#cd">Class hierarchy</a>
shows a class diagram of <tt>pb_assoc's associative containers.</tt>
Associative container classes subclass other associative container classes such that
base classes capture common types and methods
[<a href = "references.html#stroustrup97cpp">stroustrup97cpp</a>]. The type <tt>hash_fn</tt> is defined in <a href = "basic_hash_assoc_cntnr.html"><tt>basic_hash_assoc_cntnr</tt></a>, for example, since all hash-based containers employ a hash function;
<a href = "cc_hash_assoc_cntnr.html"><tt>cc_hash_assoc_cntnr</tt></a>
and
<a href = "gp_hash_assoc_cntnr.html"><tt>gp_hash_assoc_cntnr</tt></a>,
subclasses encapsulating a collision-chaining and (general) probing hash table, respectively, each define other types specific for their underlying data-structure.
This is described further in
<a href = "ds_gen.html">Data-Structure Genericity</a>.
</p>
<h6 align = "center">
<a name = "cd">
<img src = "cd.jpg" width = "70%" alt = "no image">
</h6>
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<h6 align = "center">
Class hierarchy.
</h6>
<p>
It is sometimes useful to know the underlying data-structure.
Associative containers internally define <tt>ds_category</tt> as a class describing this. Two classes might be different instantiations
of
<a href = "tree_assoc_cntnr.html"><tt>tree_assoc_cntnr</tt></a>, but one might be based on a red-black tree while another might be based on a splay tree. (This might affect the way tree objects should be manipulated.) <tt><b>typename</b> Cntnr::ds_category</tt>
yields a "tag" class for the underlying data-structure of some type
<tt>Cntnr</tt>.
This is described further in
<a href = "ds_gen.html">Data-Structure Genericity</a>.
</p>
<p>
When manipulating generic containers, it is useful to know which types, methods, and guarantees they support. For example, tree-based containers can support split and join operations, while containers based on most other underlying data-structures cannot.
These questions can be answered in compile time through a traits mechanism.
<a href = "ds_traits.html"><tt>ds_traits</tt><Cntnr>::split_join</a>, for example, answers the above question.
This is described further in
<a href = "ds_gen.html">Data-Structure Genericity</a>;
<a href = "../example/ds_traits_example.cpp"><tt>ds_traits_example.cpp</tt></a>-
shows an example.
</p>
<p>
<tt>pb_assoc</tt> does not contain separate containers for different mapping semantics,
as the STL does (<i>e.g.</i>, <tt>std::map</tt> and <tt>std::multimap</tt>). Rather, containers are parameterized by a <tt>Data</tt> parameter, and this parameter is a policy for the mapping semantics.
</p>
<ol>
<li>
Instantiating a container's <tt>Data</tt> parameter by all but two distingished types, will make a "map". Thus
<pre>
<a href = "cc_hash_assoc_cntnr.html">cc_hash_assoc_cntnr</a><
<b>int</b>,
<b>char</b>>
</pre> is a type mapping each <tt><b>int</b></tt> value to a <tt><b>char</b></tt>
value.
<a href = "../example/basic_map_example.cpp"><tt>basic_map_example.cpp</tt></a>
shows an example.
</li>
<li>
Instantiating a container's <tt>Data</tt> parameter by <a href = "null_data_type.html"><tt>null_data_type</tt></a> will make a "set". Thus
<pre>
<a href = "cc_hash_assoc_cntnr.html">cc_hash_assoc_cntnr</a><
<b>int</b>,
<a href = "null_data_type.html">null_data_type</a>>
</pre>
is a type storing unique <tt><b>int</b></tt> values.
<a href = "../example/basic_set_example.cpp"><tt>basic_set_example.cpp</tt></a> shows an example.
</li>
<li>
Instantiating a container's <tt>Data</tt> parameter by <a href = "compound_data_type.html"><tt>compound_data_type</tt></a><tt><Cntnr></tt>, where <tt>Cntnr</tt> is a different associative container, will make a "(multi)+map". Thus
<pre>
<a href = "cc_hash_assoc_cntnr.html">cc_hash_assoc_cntnr</a><
<b>int</b>,
<a href = "compound_data_type.html">compound_data_type</a><
<a href = "cc_hash_assoc_cntnr.html">cc_hash_assoc_cntnr</a><
<b>char</b>,
<a href = "null_data_type.html">null_data_type</a>> > >
</pre>
is a type
mapping each <tt><b>int</b></tt> value to a "set" of <tt><b>char</b></tt>
values.
<a href = "../example/basic_multimap_example.cpp"><tt>basic_multimap_example.cpp</tt></a> shows an example.
This composition is recursive, however, and more complex relationships can be built.
<a href = "../example/mapping_level_example.cpp"><tt>mapping_level_example.cpp</tt></a> shows an example.
</li>
</ol>
<p>
The associative-container classes derive each from one of the three
<a href = "basic_assoc_cntnr.html"><tt>basic_assoc_cntnr</tt></a> classes, depending
on the data policy. These three base classes define different types and methods. For example, the "map" specialization of
<a href = "basic_assoc_cntnr.html"><tt>basic_assoc_cntnr</tt></a>
defines <tt><b>operator</b>[]</tt>, wherase the "set" specialization does not.
This is described further in
<a href = "ms_gen.html">Mapping-Semantic Genericity</a>.
</p>
<p>
<tt>pb_assoc</tt>'s design contains the concept of a <i>mapping level</i>. "Map" and "set" types have a single mapping level; A container
mapping integers to "maps" mapping characters to floats has two mapping levels, since it can be viewed as a type mapping each integer to a "map", or as a type mapping each pair of integer and character to a float. <tt>pb_assoc</tt> contains traits and rebind mechanisms for querying and altering the mapping levels.
This is described further in
<a href = "ms_gen.html">Mapping-Semantic Genericity</a>.
</p>
<p>
The leaf classes in Figure
<a href = "#cd">Class hierarchy</a>
are each parameterized by policies, easing configuring containers for different settings.
<a href = "hash_based_containers.html">Hash-Based Containers</a> describes the design and policies of hash-based containers,
<a href = "tree_based_containers.html">Tree-Based Containers</a> describes the design and policies of tree-based containers, and
<a href = "lu_based_containers.html">List-Based Containers</a> describes the design and policies of list-based containers with update policies.
</p>
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