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140 lines
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<title>Additional Features</title>
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<link rel="home" href="../../index.html" title="Chapter 1. Boost.Bimap">
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<link rel="up" href="../rationale.html" title="Rationale">
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<div class="section boost_bimap_rationale_additional_features">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_bimap.rationale.additional_features"></a><a class="link" href="additional_features.html" title="Additional Features">Additional
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Features</a>
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</h3></div></div></div>
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<h5>
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<a name="boost_bimap.rationale.additional_features.h0"></a>
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<span class="phrase"><a name="boost_bimap.rationale.additional_features.n_1__n_n__hashed_maps"></a></span><a class="link" href="additional_features.html#boost_bimap.rationale.additional_features.n_1__n_n__hashed_maps">N-1,
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N-N, hashed maps</a>
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</h5>
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<p>
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This is a very interesting point of the design. The framework introduced
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in <span class="emphasis"><em>std::set theory</em></span> permits the management of the different
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constraints with a very simple and conceptual approach. It is easy both to
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remember and to learn. The idea here is to allow the user to specify the
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collection type of each key directly. In order to implement this feature,
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we have to solve two problems:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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The index types of the <code class="computeroutput"><span class="identifier">multi_index_container</span></code>
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core now depends on the collection type used for each key.
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</li>
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<li class="listitem">
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The map views now change their semantics according to the collection
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type chosen.
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</li>
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</ul></div>
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<p>
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Boost.Bimap relies heavily on Boost.MPL to implement all of the metaprogramming
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necessary to make this framework work. By default, if the user does not specify
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the kind of the set, a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span></code> type
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is used.
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../images/bimap/bimap.structures.png" alt="bimap.structures"></span>
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</p>
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<h5>
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<a name="boost_bimap.rationale.additional_features.h1"></a>
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<span class="phrase"><a name="boost_bimap.rationale.additional_features.collection_type_of_relation_constraints"></a></span><a class="link" href="additional_features.html#boost_bimap.rationale.additional_features.collection_type_of_relation_constraints">Collection
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type of relation constraints</a>
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</h5>
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<p>
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The constraints of the bimap set view are another very important feature.
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In general, Boost.Bimap users will base the set view type on one of the two
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collection types of their keys. It may be useful however to give this set
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other constraints or simply to order it differently. By default, Boost.Bimap
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bases the collection type of relations on the left collection type, but the
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user may choose between:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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left_based
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</li>
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<li class="listitem">
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right_based
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</li>
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<li class="listitem">
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set_of_relation<>
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</li>
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<li class="listitem">
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multiset_of_relation<>
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</li>
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<li class="listitem">
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unordered_set_of_relation<>
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</li>
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<li class="listitem">
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unordered_multiset_of_relation<>
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</li>
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<li class="listitem">
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list_of
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</li>
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<li class="listitem">
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vector_of
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</li>
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</ul></div>
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<p>
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In the first two cases, there are only two indices in the <code class="computeroutput"><span class="identifier">multi_index_core</span></code>,
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and for this reason, these are the preferred options. The implementation
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uses further metaprogramming to define a new index if necessary.
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</p>
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<h5>
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<a name="boost_bimap.rationale.additional_features.h2"></a>
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<span class="phrase"><a name="boost_bimap.rationale.additional_features.tagged"></a></span><a class="link" href="additional_features.html#boost_bimap.rationale.additional_features.tagged">Tagged</a>
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</h5>
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<p>
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The idea of using tags instead of the <code class="literal">member_at::side</code>
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idiom is very appealing since code that uses it is more readable. The only
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cost is compile time. <span class="emphasis"><em>boost/bimap/tagged</em></span> is the implementation
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of a non-invasive tagged idiom. The <code class="literal">relation</code> class is
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built in such a way that even when the user uses tags, the <code class="literal">member_at::side</code>
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idiom continues to work. This is good since an user can start tagging even
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before completing the coding of the algorithm, and the untagged code continues
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to work. The development becomes a little more complicated when user-defined
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tags are included, but there are many handy metafunctions defined in the
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<code class="literal">tagged</code> idiom that help to keep things simple enough.
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../images/bimap/tagged.png" alt="tagged"></span>
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2006-2012 Matias Capeletto<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></td>
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</tr></table>
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