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<div id="projectname">EnTT<span id="projectnumber"> 3.15.0</span>
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<div class="headertitle"><div class="title">Push EnTT across boundaries</div></div>
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<div class="textblock"><p><a class="anchor" id="push-entt-across-boundaries"></a></p>
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<h1><a class="anchor" id="table-of-contents-6"></a>
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Table of Contents</h1>
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<ul>
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<li><a class="el" href="#working-across-boundaries">Working across boundaries</a><ul>
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<li><a class="el" href="#smooth-until-proven-otherwise">Smooth until proven otherwise</a></li>
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<li><a class="el" href="#meta-context">Meta context</a></li>
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<li><a class="el" href="#memory-management">Memory management</a></li>
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<h1><a class="anchor" id="working-across-boundaries"></a>
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Working across boundaries</h1>
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<p><code>EnTT</code> has historically had a limit when used across boundaries on Windows in general and on GNU/Linux when default visibility was set to hidden. The limitation was mainly due to a custom utility used to assign unique, sequential identifiers with different types.<br />
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Fortunately, nowadays <code>EnTT</code> works smoothly across boundaries.</p>
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<h2><a class="anchor" id="smooth-until-proven-otherwise"></a>
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Smooth until proven otherwise</h2>
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<p>Many classes in <code>EnTT</code> make extensive use of type erasure for their purposes. This raises the need to identify objects whose type has been erased.<br />
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The <code>type_hash</code> class template is how identifiers are generated and thus made available to the rest of the library. In general, this class arouses little interest. The only exception is when a conflict between identifiers occurs (definitely uncommon though) or when the default solution proposed by <code>EnTT</code> is not suitable for the user's purposes.<br />
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The section dedicated to <code>type_info</code> contains all the details to get around the issue in a concise and elegant way. Please refer to the specific documentation.</p>
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<p>When working with linked libraries, compile definitions <code>ENTT_API_EXPORT</code> and <code>ENTT_API_IMPORT</code> are to import or export symbols, so as to make everything work nicely across boundaries.<br />
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On the other hand, everything should run smoothly when working with plugins or shared libraries that do not export any symbols.</p>
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<p>For those who need more details, the test suite contains many examples covering the most common cases (see the <code>lib</code> directory for all details).<br />
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It goes without saying that it is impossible to cover <b>all</b> possible cases. However, what is offered should hopefully serve as a basis for all of them.</p>
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<h2><a class="anchor" id="meta-context"></a>
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Meta context</h2>
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<p>The runtime reflection system deserves a special mention when it comes to using it across boundaries.<br />
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Since it is linked already to a static context to which the elements are attached and different contexts do not relate to each other, they must be <em>shared</em> to allow the use of meta types across boundaries.</p>
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<p>Fortunately, sharing a context is also trivial to do. First of all, the local one is acquired in the main space:</p>
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<div class="fragment"><div class="line"><span class="keyword">auto</span> handle = <a class="code hl_function" href="classentt_1_1locator.html#a09b8c4659e08c600c113ce9e5b8fa7d3">entt::locator<entt::meta_ctx>::handle</a>();</div>
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<div class="ttc" id="aclassentt_1_1locator_html_a09b8c4659e08c600c113ce9e5b8fa7d3"><div class="ttname"><a href="classentt_1_1locator.html#a09b8c4659e08c600c113ce9e5b8fa7d3">entt::locator::handle</a></div><div class="ttdeci">static node_type handle() noexcept</div><div class="ttdoc">Returns a handle to the underlying service.</div><div class="ttdef"><b>Definition</b> <a href="locator_8hpp_source.html#l00124">locator.hpp:124</a></div></div>
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</div><!-- fragment --><p>Then, it is passed to the receiving space that sets it as its default context, thus discarding or storing aside the local one:</p>
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<div class="fragment"><div class="line"><a class="code hl_function" href="classentt_1_1locator.html#a51c0f72339094183a1779da8bf01ee22">entt::locator<entt::meta_ctx>::reset</a>(handle);</div>
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<div class="ttc" id="aclassentt_1_1locator_html_a51c0f72339094183a1779da8bf01ee22"><div class="ttname"><a href="classentt_1_1locator.html#a51c0f72339094183a1779da8bf01ee22">entt::locator::reset</a></div><div class="ttdeci">static void reset(const node_type &other={}) noexcept</div><div class="ttdoc">Resets or replaces a service.</div><div class="ttdef"><b>Definition</b> <a href="locator_8hpp_source.html#l00134">locator.hpp:134</a></div></div>
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</div><!-- fragment --><p>From now on, both spaces refer to the same context and to it are all new meta types attached, no matter where they are created.<br />
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Note that <em>replacing</em> the main context does not also propagate changes across boundaries. In other words, replacing a context results in the decoupling of the two sides and therefore a divergence in the contents.</p>
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<h2><a class="anchor" id="memory-management"></a>
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Memory Management</h2>
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<p>There is another subtle problem due to memory management that can lead to headaches.<br />
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It can occur where there are pools of objects (such as components or events) dynamically created on demand. This is usually not a problem when working with linked libraries that rely on the same dynamic runtime. However, it can occur in the case of plugins or statically linked runtimes.</p>
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<p>As an example, imagine creating an instance of <code>registry</code> in the main executable and sharing it with a plugin. If the latter starts working with a component that is unknown to the former, a dedicated pool is created within the registry on first use.<br />
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As one can guess, this pool is instantiated on a different side of the boundary from the <code>registry</code>. Therefore, the instance is now managing memory from different spaces, and this can quickly lead to crashes if not properly addressed.</p>
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<p>To overcome the risk, it is recommended to use well-defined interfaces that make fundamental types pass through the boundaries, isolating the instances of the <code>EnTT</code> classes from time to time and as appropriate.<br />
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Refer to the test suite for some examples, read the documentation available online about this type of issues or consult someone who has already had such experiences to avoid problems. </p>
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