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<div id="projectname">EnTT<span id="projectnumber"> 3.12.0</span>
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<div class="headertitle"><div class="title">Crash Course: containers </div></div>
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<div class="textblock"><h1><a class="anchor" id="autotoc_md14"></a>
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Introduction</h1>
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<p>The standard C++ library offers a wide range of containers and it's really difficult to do better (although it's very easy to do worse, as many examples available online demonstrate).<br />
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<code>EnTT</code> doesn't try in any way to replace what is offered by the standard. Quite the opposite, given the widespread use that is made of standard containers.<br />
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However, the library also tries to fill a gap in features and functionalities by making available some containers initially developed for internal use.</p>
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<p>This section of the library is likely to grow larger over time. However, for the moment it's quite small and mainly aimed at satisfying some internal needs.<br />
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For all containers made available, full test coverage and stability over time is guaranteed as usual.</p>
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<h1><a class="anchor" id="autotoc_md15"></a>
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Containers</h1>
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<h2><a class="anchor" id="autotoc_md16"></a>
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Dense map</h2>
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<p>The dense map made available in <code>EnTT</code> is a hash map that aims to return a packed array of elements, so as to reduce the number of jumps in memory during iterations.<br />
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The implementation is based on <em>sparse sets</em> and each bucket is identified by an implicit list within the packed array itself.</p>
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<p>The interface is very close to its counterpart in the standard library, that is, the <code>std::unordered_map</code> class.<br />
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However, both local and non-local iterators returned by a dense map belong to the input iterator category although they respectively model the concepts of a <em>forward iterator</em> type and a <em>random access iterator</em> type.<br />
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This is because they return a pair of references rather than a reference to a pair. In other words, dense maps return a so called <em>proxy iterator</em> the value type of which is:</p>
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<ul>
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<li><code>std::pair<const Key &, Type &></code> for non-const iterator types.</li>
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<li><code>std::pair<const Key &, const Type &></code> for const iterator types.</li>
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</ul>
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<p>This is quite different from what any standard library map returns and should be taken into account when looking for a drop-in replacement.</p>
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<h2><a class="anchor" id="autotoc_md17"></a>
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Dense set</h2>
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<p>The dense set made available in <code>EnTT</code> is a hash set that aims to return a packed array of elements, so as to reduce the number of jumps in memory during iterations.<br />
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The implementation is based on <em>sparse sets</em> and each bucket is identified by an implicit list within the packed array itself.</p>
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<p>The interface is in all respects similar to its counterpart in the standard library, that is, the <code>std::unordered_set</code> class.<br />
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Therefore, there is no need to go into the API description. </p>
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