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 <span id="projectnumber">3.9.0</span>
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<div class="title">Crash Course: service locator </div> </div>
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<div class="textblock"><h1><a class="anchor" id="autotoc_md114"></a>
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Introduction</h1>
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<p>Usually service locators are tightly bound to the services they expose and it's hard to define a general purpose solution. This template based implementation tries to fill the gap and to get rid of the burden of defining a different specific locator for each application.<br />
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This class is tiny, partially unsafe and thus risky to use. Moreover it doesn't fit probably most of the scenarios in which a service locator is required. Look at it as a small tool that can sometimes be useful if users know how to handle it.</p>
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<h1><a class="anchor" id="autotoc_md115"></a>
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Service locator</h1>
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<p>The API is straightforward. The basic idea is that services are implemented by means of interfaces and rely on polymorphism.<br />
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The locator is instantiated with the base type of the service if any and a concrete implementation is provided along with all the parameters required to initialize it. As an example:</p>
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<div class="fragment"><div class="line"><span class="comment">// the service has no base type, a locator is used to treat it as a kind of singleton</span></div>
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<div class="line"><a class="code" href="structentt_1_1service__locator.html#a1cab7cf8cff562d6ec95b7aaeaf12366">entt::service_locator<my_service>::set</a>(params...);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// sets up an opaque service</span></div>
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<div class="line"><a class="code" href="structentt_1_1service__locator.html">entt::service_locator<audio_interface>::set<audio_implementation></a>(params...);</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// resets (destroys) the service</span></div>
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<div class="line"><a class="code" href="structentt_1_1service__locator.html#a4d655acab54d87058ad41a266830d0e8">entt::service_locator<audio_interface>::reset</a>();</div>
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<div class="ttc" id="astructentt_1_1service__locator_html"><div class="ttname"><a href="structentt_1_1service__locator.html">entt::service_locator</a></div><div class="ttdoc">Service locator, nothing more.</div><div class="ttdef"><b>Definition:</b> <a href="locator_8hpp_source.html#l00022">locator.hpp:22</a></div></div>
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<div class="ttc" id="astructentt_1_1service__locator_html_a1cab7cf8cff562d6ec95b7aaeaf12366"><div class="ttname"><a href="structentt_1_1service__locator.html#a1cab7cf8cff562d6ec95b7aaeaf12366">entt::service_locator::set</a></div><div class="ttdeci">static void set(Args &&...args)</div><div class="ttdoc">Sets or replaces a service.</div><div class="ttdef"><b>Definition:</b> <a href="locator_8hpp_source.html#l00078">locator.hpp:78</a></div></div>
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<div class="ttc" id="astructentt_1_1service__locator_html_a4d655acab54d87058ad41a266830d0e8"><div class="ttname"><a href="structentt_1_1service__locator.html#a4d655acab54d87058ad41a266830d0e8">entt::service_locator::reset</a></div><div class="ttdeci">static void reset()</div><div class="ttdoc">Resets a service.</div><div class="ttdef"><b>Definition:</b> <a href="locator_8hpp_source.html#l00096">locator.hpp:96</a></div></div>
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</div><!-- fragment --><p>The locator can also be queried to know if an active service is currently set and to retrieve it if necessary (either as a pointer or as a reference):</p>
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<div class="fragment"><div class="line"><span class="comment">// no service currently set</span></div>
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<div class="line"><span class="keyword">auto</span> empty = <a class="code" href="structentt_1_1service__locator.html#ad96474e6acfb0edd0e123c584f79296b">entt::service_locator<audio_interface>::empty</a>();</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// gets a (possibly empty) shared pointer to the service ...</span></div>
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<div class="line">std::shared_ptr<audio_interface> ptr = <a class="code" href="structentt_1_1service__locator.html#a88134390e70265359dda08f5aba2877f">entt::service_locator<audio_interface>::get</a>();</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// ... or a reference, but it's undefined behaviour if the service isn't set yet</span></div>
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<div class="line">audio_interface &ref = <a class="code" href="structentt_1_1service__locator.html#a5379f8d798c2de5b1c41756ba3959e06">entt::service_locator<audio_interface>::ref</a>();</div>
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<div class="ttc" id="astructentt_1_1service__locator_html_a5379f8d798c2de5b1c41756ba3959e06"><div class="ttname"><a href="structentt_1_1service__locator.html#a5379f8d798c2de5b1c41756ba3959e06">entt::service_locator::ref</a></div><div class="ttdeci">static Service & ref()</div><div class="ttdoc">Returns a weak reference to a service implementation, if any.</div><div class="ttdef"><b>Definition:</b> <a href="locator_8hpp_source.html#l00067">locator.hpp:67</a></div></div>
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<div class="ttc" id="astructentt_1_1service__locator_html_a88134390e70265359dda08f5aba2877f"><div class="ttname"><a href="structentt_1_1service__locator.html#a88134390e70265359dda08f5aba2877f">entt::service_locator::get</a></div><div class="ttdeci">static std::weak_ptr< Service > get()</div><div class="ttdoc">Returns a weak pointer to a service implementation, if any.</div><div class="ttdef"><b>Definition:</b> <a href="locator_8hpp_source.html#l00049">locator.hpp:49</a></div></div>
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<div class="ttc" id="astructentt_1_1service__locator_html_ad96474e6acfb0edd0e123c584f79296b"><div class="ttname"><a href="structentt_1_1service__locator.html#ad96474e6acfb0edd0e123c584f79296b">entt::service_locator::empty</a></div><div class="ttdeci">static bool empty()</div><div class="ttdoc">Tests if a valid service implementation is set.</div><div class="ttdef"><b>Definition:</b> <a href="locator_8hpp_source.html#l00035">locator.hpp:35</a></div></div>
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</div><!-- fragment --><p>A common use is to wrap the different locators in a container class, creating aliases for the various services:</p>
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<div class="fragment"><div class="line"><span class="keyword">struct </span>locator {</div>
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<div class="line"> <span class="keyword">using</span> camera = <a class="code" href="structentt_1_1service__locator.html">entt::service_locator<camera_interface></a>;</div>
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<div class="line"> <span class="keyword">using</span> audio = <a class="code" href="structentt_1_1service__locator.html">entt::service_locator<audio_interface></a>;</div>
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<div class="line"> <span class="comment">// ...</span></div>
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<div class="line">};</div>
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<div class="line"> </div>
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<div class="line"><span class="comment">// ...</span></div>
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<div class="line"> </div>
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<div class="line"><span class="keywordtype">void</span> init() {</div>
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<div class="line"> locator::camera::set<camera_null>();</div>
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<div class="line"> locator::audio::set<audio_implementation>(params...);</div>
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<div class="line"> <span class="comment">// ...</span></div>
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<div class="line">}</div>
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