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<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno"> 1</span> # The EnTT Framework</div><div class="line"><a name="l00002"></a><span class="lineno"> 2</span> </div><div class="line"><a name="l00003"></a><span class="lineno"> 3</span> [](https://travis-ci.org/skypjack/entt)</div><div class="line"><a name="l00004"></a><span class="lineno"> 4</span> [](https://ci.appveyor.com/project/skypjack/entt)</div><div class="line"><a name="l00005"></a><span class="lineno"> 5</span> [](https://coveralls.io/github/skypjack/entt?branch=master)</div><div class="line"><a name="l00006"></a><span class="lineno"> 6</span> [](https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=W2HF9FESD5LJY&lc=IT&item_name=Michele%20Caini&currency_code=EUR&bn=PP%2dDonationsBF%3abtn_donateCC_LG%2egif%3aNonHosted)</div><div class="line"><a name="l00007"></a><span class="lineno"> 7</span> </div><div class="line"><a name="l00008"></a><span class="lineno"> 8</span> # Introduction</div><div class="line"><a name="l00009"></a><span class="lineno"> 9</span> </div><div class="line"><a name="l00010"></a><span class="lineno"> 10</span> `EnTT` is a header-only, tiny and easy to use framework written in modern</div><div class="line"><a name="l00011"></a><span class="lineno"> 11</span> C++.<br/></div><div class="line"><a name="l00012"></a><span class="lineno"> 12</span> It's entirely designed around an architectural pattern pattern called _ECS_ that</div><div class="line"><a name="l00013"></a><span class="lineno"> 13</span> is used mostly in game development. For further details:</div><div class="line"><a name="l00014"></a><span class="lineno"> 14</span> </div><div class="line"><a name="l00015"></a><span class="lineno"> 15</span> * [Entity Systems Wiki](http://entity-systems.wikidot.com/)</div><div class="line"><a name="l00016"></a><span class="lineno"> 16</span> * [Evolve Your Hierarchy](http://cowboyprogramming.com/2007/01/05/evolve-your-heirachy/)</div><div class="line"><a name="l00017"></a><span class="lineno"> 17</span> * [ECS on Wikipedia](https://en.wikipedia.org/wiki/Entity%E2%80%93component%E2%80%93system)</div><div class="line"><a name="l00018"></a><span class="lineno"> 18</span> </div><div class="line"><a name="l00019"></a><span class="lineno"> 19</span> Originally, `EnTT` was written as a faster alternative to other well known and</div><div class="line"><a name="l00020"></a><span class="lineno"> 20</span> open source entity-component systems.<br/></div><div class="line"><a name="l00021"></a><span class="lineno"> 21</span> After a while the codebase has grown and more features have become part of the</div><div class="line"><a name="l00022"></a><span class="lineno"> 22</span> framework.</div><div class="line"><a name="l00023"></a><span class="lineno"> 23</span> </div><div class="line"><a name="l00024"></a><span class="lineno"> 24</span> ## Code Example</div><div class="line"><a name="l00025"></a><span class="lineno"> 25</span> </div><div class="line"><a name="l00026"></a><span class="lineno"> 26</span> ```cpp</div><div class="line"><a name="l00027"></a><span class="lineno"> 27</span> #include <registry.hpp></div><div class="line"><a name="l00028"></a><span class="lineno"> 28</span> </div><div class="line"><a name="l00029"></a><span class="lineno"> 29</span> struct Position {</div><div class="line"><a name="l00030"></a><span class="lineno"> 30</span>  float x;</div><div class="line"><a name="l00031"></a><span class="lineno"> 31</span>  float y;</div><div class="line"><a name="l00032"></a><span class="lineno"> 32</span> };</div><div class="line"><a name="l00033"></a><span class="lineno"> 33</span> </div><div class="line"><a name="l00034"></a><span class="lineno"> 34</span> struct Velocity {</div><div class="line"><a name="l00035"></a><span class="lineno"> 35</span>  float dx;</div><div class="line"><a name="l00036"></a><span class="lineno"> 36</span>  float dy;</div><div class="line"><a name="l00037"></a><span class="lineno"> 37</span> };</div><div class="line"><a name="l00038"></a><span class="lineno"> 38</span> </div><div class="line"><a name="l00039"></a><span class="lineno"> 39</span> void update(entt::DefaultRegistry &registry) {</div><div class="line"><a name="l00040"></a><span class="lineno"> 40</span>  auto view = registry.view<Position, Velocity>();</div><div class="line"><a name="l00041"></a><span class="lineno"> 41</span> </div><div class="line"><a name="l00042"></a><span class="lineno"> 42</span>  for(auto entity: view) {</div><div class="line"><a name="l00043"></a><span class="lineno"> 43</span>  auto &position = view.get<Position>(entity);</div><div class="line"><a name="l00044"></a><span class="lineno"> 44</span>  auto &velocity = view.get<Velocity>(entity);</div><div class="line"><a name="l00045"></a><span class="lineno"> 45</span>  // ...</div><div class="line"><a name="l00046"></a><span class="lineno"> 46</span>  }</div><div class="line"><a name="l00047"></a><span class="lineno"> 47</span> }</div><div class="line"><a name="l00048"></a><span class="lineno"> 48</span> </div><div class="line"><a name="l00049"></a><span class="lineno"> 49</span> int main() {</div><div class="line"><a name="l00050"></a><span class="lineno"> 50</span>  entt::DefaultRegistry registry;</div><div class="line"><a name="l00051"></a><span class="lineno"> 51</span> </div><div class="line"><a name="l00052"></a><span class="lineno"> 52</span>  for(auto i = 0; i < 10; ++i) {</div><div class="line"><a name="l00053"></a><span class="lineno"> 53</span>  auto entity = registry.create();</div><div class="line"><a name="l00054"></a><span class="lineno"> 54</span>  registry.assign<Position>(entity, i * 1.f, i * 1.f);</div><div class="line"><a name="l00055"></a><span class="lineno"> 55</span>  if(i % 2 == 0) { registry.assign<Velocity>(entity, i * .1f, i * .1f); }</div><div class="line"><a name="l00056"></a><span class="lineno"> 56</span>  }</div><div class="line"><a name="l00057"></a><span class="lineno"> 57</span> </div><div class="line"><a name="l00058"></a><span class="lineno"> 58</span>  update(registry);</div><div class="line"><a name="l00059"></a><span class="lineno"> 59</span>  // ...</div><div class="line"><a name="l00060"></a><span class="lineno"> 60</span> }</div><div class="line"><a name="l00061"></a><span class="lineno"> 61</span> ```</div><div class="line"><a name="l00062"></a><span class="lineno"> 62</span> </div><div class="line"><a name="l00063"></a><span class="lineno"> 63</span> ## Motivation</div><div class="line"><a name="l00064"></a><span class="lineno"> 64</span> </div><div class="line"><a name="l00065"></a><span class="lineno"> 65</span> I started working on `EnTT` because of the wrong reason: my goal was to design</div><div class="line"><a name="l00066"></a><span class="lineno"> 66</span> an entity-component system that beated another well known open source solution</div><div class="line"><a name="l00067"></a><span class="lineno"> 67</span> in terms of performance.<br/></div><div class="line"><a name="l00068"></a><span class="lineno"> 68</span> I did it, of course, but it wasn't much satisfying. Actually it wasn't</div><div class="line"><a name="l00069"></a><span class="lineno"> 69</span> satisfying at all. The fastest and nothing more, fairly little indeed. When I</div><div class="line"><a name="l00070"></a><span class="lineno"> 70</span> realized it, I tried hard to keep intact the great performance of `EnTT` and to</div><div class="line"><a name="l00071"></a><span class="lineno"> 71</span> add all the features I wanted to see in *my* entity-component system at the same</div><div class="line"><a name="l00072"></a><span class="lineno"> 72</span> time.</div><div class="line"><a name="l00073"></a><span class="lineno"> 73</span> </div><div class="line"><a name="l00074"></a><span class="lineno"> 74</span> Today `EnTT` is finally what I was looking for: still faster than its _rivals_,</div><div class="line"><a name="l00075"></a><span class="lineno"> 75</span> a really good API and an amazing set of features. And even more, of course.</div><div class="line"><a name="l00076"></a><span class="lineno"> 76</span> </div><div class="line"><a name="l00077"></a><span class="lineno"> 77</span> ## Performance</div><div class="line"><a name="l00078"></a><span class="lineno"> 78</span> </div><div class="line"><a name="l00079"></a><span class="lineno"> 79</span> As it stands right now, `EnTT` is just fast enough for my requirements if</div><div class="line"><a name="l00080"></a><span class="lineno"> 80</span> compared to my first choice (that was already amazingly fast indeed).<br/></div><div class="line"><a name="l00081"></a><span class="lineno"> 81</span> Here is a comparision between the two (both of them compiled with GCC 7.2.0 on a</div><div class="line"><a name="l00082"></a><span class="lineno"> 82</span> Dell XPS 13 out of the mid 2014):</div><div class="line"><a name="l00083"></a><span class="lineno"> 83</span> </div><div class="line"><a name="l00084"></a><span class="lineno"> 84</span> | Benchmark | EntityX (experimental/compile_time) | EnTT |</div><div class="line"><a name="l00085"></a><span class="lineno"> 85</span> |-----------|-------------|-------------|</div><div class="line"><a name="l00086"></a><span class="lineno"> 86</span> | Creating 10M entities | 0.128881s | **0.0408754s** |</div><div class="line"><a name="l00087"></a><span class="lineno"> 87</span> | Destroying 10M entities | **0.0531374s** | 0.0545839s |</div><div class="line"><a name="l00088"></a><span class="lineno"> 88</span> | Iterating over 10M entities, unpacking one component, standard view | 0.010661s | **1.58e-07s** |</div><div class="line"><a name="l00089"></a><span class="lineno"> 89</span> | Iterating over 10M entities, unpacking two components, standard view | **0.0112664s** | 0.0840068s |</div><div class="line"><a name="l00090"></a><span class="lineno"> 90</span> | Iterating over 10M entities, unpacking two components, standard view, half of the entities have all the components | **0.0077951s** | 0.042168s |</div><div class="line"><a name="l00091"></a><span class="lineno"> 91</span> | Iterating over 10M entities, unpacking two components, standard view, one of the entities has all the components | 0.00713398s | **8.93e-07s** |</div><div class="line"><a name="l00092"></a><span class="lineno"> 92</span> | Iterating over 10M entities, unpacking two components, persistent view | 0.0112664s | **5.68e-07s** |</div><div class="line"><a name="l00093"></a><span class="lineno"> 93</span> | Iterating over 10M entities, unpacking five components, standard view | **0.00905084s** | 0.137757s |</div><div class="line"><a name="l00094"></a><span class="lineno"> 94</span> | Iterating over 10M entities, unpacking five components, persistent view | 0.00905084s | **2.9e-07s** |</div><div class="line"><a name="l00095"></a><span class="lineno"> 95</span> | Iterating over 10M entities, unpacking ten components, standard view | **0.0104708s** | 0.388602s |</div><div class="line"><a name="l00096"></a><span class="lineno"> 96</span> | Iterating over 10M entities, unpacking ten components, standard view, half of the entities have all the components | **0.00899859s** | 0.200752s |</div><div class="line"><a name="l00097"></a><span class="lineno"> 97</span> | Iterating over 10M entities, unpacking ten components, standard view, one of the entities has all the components | 0.00700349s | **2.565e-06s** |</div><div class="line"><a name="l00098"></a><span class="lineno"> 98</span> | Iterating over 10M entities, unpacking ten components, persistent view | 0.0104708s | **6.23e-07s** |</div><div class="line"><a name="l00099"></a><span class="lineno"> 99</span> | Iterating over 50M entities, unpacking one component, standard view | 0.055194s | **2.87e-07s** |</div><div class="line"><a name="l00100"></a><span class="lineno"> 100</span> | Iterating over 50M entities, unpacking two components, standard view | **0.0533921s** | 0.243197s |</div><div class="line"><a name="l00101"></a><span class="lineno"> 101</span> | Iterating over 50M entities, unpacking two components, persistent view | 0.055194s | **4.47e-07s** |</div><div class="line"><a name="l00102"></a><span class="lineno"> 102</span> | Sort 150k entities, one component | - | **0.0080046s** |</div><div class="line"><a name="l00103"></a><span class="lineno"> 103</span> | Sort 150k entities, match two components | - | **0.00608322s** |</div><div class="line"><a name="l00104"></a><span class="lineno"> 104</span> </div><div class="line"><a name="l00105"></a><span class="lineno"> 105</span> `EnTT` includes its own tests and benchmarks. See</div><div class="line"><a name="l00106"></a><span class="lineno"> 106</span> [benchmark.cpp](https://github.com/skypjack/entt/blob/master/test/benchmark.cpp)</div><div class="line"><a name="l00107"></a><span class="lineno"> 107</span> for further details.<br/></div><div class="line"><a name="l00108"></a><span class="lineno"> 108</span> On Github users can find also a</div><div class="line"><a name="l00109"></a><span class="lineno"> 109</span> [benchmark suite](https://github.com/abeimler/ecs_benchmark) that compares a</div><div class="line"><a name="l00110"></a><span class="lineno"> 110</span> bunch of different projects, one of which is `EnTT`.</div><div class="line"><a name="l00111"></a><span class="lineno"> 111</span> </div><div class="line"><a name="l00112"></a><span class="lineno"> 112</span> Of course, probably I'll try to get out of `EnTT` more features and better</div><div class="line"><a name="l00113"></a><span class="lineno"> 113</span> performance in the future, mainly for fun.<br/></div><div class="line"><a name="l00114"></a><span class="lineno"> 114</span> If you want to contribute and/or have any suggestion, feel free to make a PR or</div><div class="line"><a name="l00115"></a><span class="lineno"> 115</span> open an issue to discuss your idea.</div><div class="line"><a name="l00116"></a><span class="lineno"> 116</span> </div><div class="line"><a name="l00117"></a><span class="lineno"> 117</span> # Build Instructions</div><div class="line"><a name="l00118"></a><span class="lineno"> 118</span> </div><div class="line"><a name="l00119"></a><span class="lineno"> 119</span> ## Requirements</div><div class="line"><a name="l00120"></a><span class="lineno"> 120</span> </div><div class="line"><a name="l00121"></a><span class="lineno"> 121</span> To be able to use `EnTT`, users must provide a full-featured compiler that</div><div class="line"><a name="l00122"></a><span class="lineno"> 122</span> supports at least C++14.<br/></div><div class="line"><a name="l00123"></a><span class="lineno"> 123</span> The requirements below are mandatory to compile the tests and to extract the</div><div class="line"><a name="l00124"></a><span class="lineno"> 124</span> documentation:</div><div class="line"><a name="l00125"></a><span class="lineno"> 125</span> </div><div class="line"><a name="l00126"></a><span class="lineno"> 126</span> * CMake version 3.2 or later.</div><div class="line"><a name="l00127"></a><span class="lineno"> 127</span> * Doxygen version 1.8 or later.</div><div class="line"><a name="l00128"></a><span class="lineno"> 128</span> </div><div class="line"><a name="l00129"></a><span class="lineno"> 129</span> ## Library</div><div class="line"><a name="l00130"></a><span class="lineno"> 130</span> </div><div class="line"><a name="l00131"></a><span class="lineno"> 131</span> `EnTT` is a header-only library. This means that including the `entt.hpp`</div><div class="line"><a name="l00132"></a><span class="lineno"> 132</span> header is enough to include the whole framework and use it. For those who are</div><div class="line"><a name="l00133"></a><span class="lineno"> 133</span> interested only in the entity-component system, consider to include the sole</div><div class="line"><a name="l00134"></a><span class="lineno"> 134</span> `entity/registry.hpp` header instead.<br/></div><div class="line"><a name="l00135"></a><span class="lineno"> 135</span> It's a matter of adding the following line to the top of a file:</div><div class="line"><a name="l00136"></a><span class="lineno"> 136</span> </div><div class="line"><a name="l00137"></a><span class="lineno"> 137</span> ```cpp</div><div class="line"><a name="l00138"></a><span class="lineno"> 138</span> #include <entt/entt.hpp></div><div class="line"><a name="l00139"></a><span class="lineno"> 139</span> ```</div><div class="line"><a name="l00140"></a><span class="lineno"> 140</span> </div><div class="line"><a name="l00141"></a><span class="lineno"> 141</span> Use the line below to include only the entity-component system instead:</div><div class="line"><a name="l00142"></a><span class="lineno"> 142</span> </div><div class="line"><a name="l00143"></a><span class="lineno"> 143</span> ```cpp</div><div class="line"><a name="l00144"></a><span class="lineno"> 144</span> #include <entt/entity/registry.hpp></div><div class="line"><a name="l00145"></a><span class="lineno"> 145</span> ```</div><div class="line"><a name="l00146"></a><span class="lineno"> 146</span> </div><div class="line"><a name="l00147"></a><span class="lineno"> 147</span> Then pass the proper `-I` argument to the compiler to add the `src` directory to</div><div class="line"><a name="l00148"></a><span class="lineno"> 148</span> the include paths.</div><div class="line"><a name="l00149"></a><span class="lineno"> 149</span> </div><div class="line"><a name="l00150"></a><span class="lineno"> 150</span> ## Documentation</div><div class="line"><a name="l00151"></a><span class="lineno"> 151</span> </div><div class="line"><a name="l00152"></a><span class="lineno"> 152</span> The documentation is based on [doxygen](http://www.stack.nl/~dimitri/doxygen/).</div><div class="line"><a name="l00153"></a><span class="lineno"> 153</span> To build it:</div><div class="line"><a name="l00154"></a><span class="lineno"> 154</span> </div><div class="line"><a name="l00155"></a><span class="lineno"> 155</span>  $ cd build</div><div class="line"><a name="l00156"></a><span class="lineno"> 156</span>  $ cmake ..</div><div class="line"><a name="l00157"></a><span class="lineno"> 157</span>  $ make docs</div><div class="line"><a name="l00158"></a><span class="lineno"> 158</span> </div><div class="line"><a name="l00159"></a><span class="lineno"> 159</span> The API reference will be created in HTML format within the directory</div><div class="line"><a name="l00160"></a><span class="lineno"> 160</span> `build/docs/html`. To navigate it with your favorite browser:</div><div class="line"><a name="l00161"></a><span class="lineno"> 161</span> </div><div class="line"><a name="l00162"></a><span class="lineno"> 162</span>  $ cd build</div><div class="line"><a name="l00163"></a><span class="lineno"> 163</span>  $ your_favorite_browser docs/html/index.html</div><div class="line"><a name="l00164"></a><span class="lineno"> 164</span> </div><div class="line"><a name="l00165"></a><span class="lineno"> 165</span> The API reference is also available [online](https://skypjack.github.io/entt/)</div><div class="line"><a name="l00166"></a><span class="lineno"> 166</span> for the latest version.</div><div class="line"><a name="l00167"></a><span class="lineno"> 167</span> </div><div class="line"><a name="l00168"></a><span class="lineno"> 168</span> ## Tests</div><div class="line"><a name="l00169"></a><span class="lineno"> 169</span> </div><div class="line"><a name="l00170"></a><span class="lineno"> 170</span> To compile and run the tests, `EnTT` requires *googletest*.<br/></div><div class="line"><a name="l00171"></a><span class="lineno"> 171</span> `cmake` will download and compile the library before to compile anything else.</div><div class="line"><a name="l00172"></a><span class="lineno"> 172</span> </div><div class="line"><a name="l00173"></a><span class="lineno"> 173</span> To build the tests:</div><div class="line"><a name="l00174"></a><span class="lineno"> 174</span> </div><div class="line"><a name="l00175"></a><span class="lineno"> 175</span> * `$ cd build`</div><div class="line"><a name="l00176"></a><span class="lineno"> 176</span> * `$ cmake ..`</div><div class="line"><a name="l00177"></a><span class="lineno"> 177</span> * `$ make`</div><div class="line"><a name="l00178"></a><span class="lineno"> 178</span> * `$ make test`</div><div class="line"><a name="l00179"></a><span class="lineno"> 179</span> </div><div class="line"><a name="l00180"></a><span class="lineno"> 180</span> To build the benchmarks, use the following line instead:</div><div class="line"><a name="l00181"></a><span class="lineno"> 181</span> </div><div class="line"><a name="l00182"></a><span class="lineno"> 182</span> * `$ cmake -DCMAKE_BUILD_TYPE=Release ..`</div><div class="line"><a name="l00183"></a><span class="lineno"> 183</span> </div><div class="line"><a name="l00184"></a><span class="lineno"> 184</span> Benchmarks are compiled only in release mode currently.</div><div class="line"><a name="l00185"></a><span class="lineno"> 185</span> </div><div class="line"><a name="l00186"></a><span class="lineno"> 186</span> # Crash Course</div><div class="line"><a name="l00187"></a><span class="lineno"> 187</span> </div><div class="line"><a name="l00188"></a><span class="lineno"> 188</span> ## Vademecum</div><div class="line"><a name="l00189"></a><span class="lineno"> 189</span> </div><div class="line"><a name="l00190"></a><span class="lineno"> 190</span> The `Registry` to store, the `View`s to iterate. That's all.</div><div class="line"><a name="l00191"></a><span class="lineno"> 191</span> </div><div class="line"><a name="l00192"></a><span class="lineno"> 192</span> An entity (the _E_ of an _ECS_) is an opaque identifier that users should just</div><div class="line"><a name="l00193"></a><span class="lineno"> 193</span> use as-is and store around if needed. Do not try to inspect an entity</div><div class="line"><a name="l00194"></a><span class="lineno"> 194</span> identifier, its type can change in future and a registry offers all the</div><div class="line"><a name="l00195"></a><span class="lineno"> 195</span> functionalities to query them out-of-the-box. The underlying type of an entity</div><div class="line"><a name="l00196"></a><span class="lineno"> 196</span> (either `std::uint16_t`, `std::uint32_t` or `std::uint64_t`) can be specified</div><div class="line"><a name="l00197"></a><span class="lineno"> 197</span> when defining a registry (actually the DefaultRegistry is nothing more than a</div><div class="line"><a name="l00198"></a><span class="lineno"> 198</span> Registry where the type of the entities is `std::uint32_t`).<br/></div><div class="line"><a name="l00199"></a><span class="lineno"> 199</span> Components (the _C_ of an _ECS_) should be plain old data structures or more</div><div class="line"><a name="l00200"></a><span class="lineno"> 200</span> complex and moveable data structures with a proper constructor. They are list</div><div class="line"><a name="l00201"></a><span class="lineno"> 201</span> initialized by using the parameters provided to construct the component. No need</div><div class="line"><a name="l00202"></a><span class="lineno"> 202</span> to register components or their types neither with the registry nor with the</div><div class="line"><a name="l00203"></a><span class="lineno"> 203</span> entity-component system at all.<br/></div><div class="line"><a name="l00204"></a><span class="lineno"> 204</span> Systems (the _S_ of an _ECS_) are just plain functions, functors, lambdas or</div><div class="line"><a name="l00205"></a><span class="lineno"> 205</span> whatever the users want. They can accept a Registry, a View or a PersistentView</div><div class="line"><a name="l00206"></a><span class="lineno"> 206</span> and use them the way they prefer. No need to register systems or their types</div><div class="line"><a name="l00207"></a><span class="lineno"> 207</span> neither with the registry nor with the entity-component system at all.</div><div class="line"><a name="l00208"></a><span class="lineno"> 208</span> </div><div class="line"><a name="l00209"></a><span class="lineno"> 209</span> The following sections will explain in short how to use the entity-component</div><div class="line"><a name="l00210"></a><span class="lineno"> 210</span> system, the core part of the `EnTT` framework.<br/></div><div class="line"><a name="l00211"></a><span class="lineno"> 211</span> In fact, the framework is composed of many other classes in addition to those</div><div class="line"><a name="l00212"></a><span class="lineno"> 212</span> describe below. For more details, please refer to the</div><div class="line"><a name="l00213"></a><span class="lineno"> 213</span> [online documentation](https://skypjack.github.io/entt/).</div><div class="line"><a name="l00214"></a><span class="lineno"> 214</span> </div><div class="line"><a name="l00215"></a><span class="lineno"> 215</span> ## The Registry, the Entity and the Component</div><div class="line"><a name="l00216"></a><span class="lineno"> 216</span> </div><div class="line"><a name="l00217"></a><span class="lineno"> 217</span> A registry is used to store and manage entities as well as to create views to</div><div class="line"><a name="l00218"></a><span class="lineno"> 218</span> iterate the underlying data structures.<br/></div><div class="line"><a name="l00219"></a><span class="lineno"> 219</span> Registry is a class template that lets the users decide what's the preferred</div><div class="line"><a name="l00220"></a><span class="lineno"> 220</span> type to represent an entity. Because `std::uint32_t` is large enough for almost</div><div class="line"><a name="l00221"></a><span class="lineno"> 221</span> all the cases, there exists also an alias named DefaultRegistry for</div><div class="line"><a name="l00222"></a><span class="lineno"> 222</span> `Registry<std::uint32_t>`.</div><div class="line"><a name="l00223"></a><span class="lineno"> 223</span> </div><div class="line"><a name="l00224"></a><span class="lineno"> 224</span> Entities are represented by _entitiy identifiers_. An entity identifier is an</div><div class="line"><a name="l00225"></a><span class="lineno"> 225</span> opaque type that users should not inspect or modify in any way. It carries</div><div class="line"><a name="l00226"></a><span class="lineno"> 226</span> information about the entity itself and its version.</div><div class="line"><a name="l00227"></a><span class="lineno"> 227</span> </div><div class="line"><a name="l00228"></a><span class="lineno"> 228</span> A registry can be used both to construct and to destroy entities:</div><div class="line"><a name="l00229"></a><span class="lineno"> 229</span> </div><div class="line"><a name="l00230"></a><span class="lineno"> 230</span> ```cpp</div><div class="line"><a name="l00231"></a><span class="lineno"> 231</span> // constructs a naked entity with no components ad returns its identifier</div><div class="line"><a name="l00232"></a><span class="lineno"> 232</span> auto entity = registry.create();</div><div class="line"><a name="l00233"></a><span class="lineno"> 233</span> </div><div class="line"><a name="l00234"></a><span class="lineno"> 234</span> // constructs an entity and assigns it default-initialized components</div><div class="line"><a name="l00235"></a><span class="lineno"> 235</span> auto another = registry.create<Position, Velocity>();</div><div class="line"><a name="l00236"></a><span class="lineno"> 236</span> </div><div class="line"><a name="l00237"></a><span class="lineno"> 237</span> // destroys an entity and all its components</div><div class="line"><a name="l00238"></a><span class="lineno"> 238</span> registry.destroy(entity);</div><div class="line"><a name="l00239"></a><span class="lineno"> 239</span> ```</div><div class="line"><a name="l00240"></a><span class="lineno"> 240</span> </div><div class="line"><a name="l00241"></a><span class="lineno"> 241</span> Once an entity is deleted, the registry can freely reuse it internally with a</div><div class="line"><a name="l00242"></a><span class="lineno"> 242</span> slightly different identifier. In particular, the version of an entity is</div><div class="line"><a name="l00243"></a><span class="lineno"> 243</span> increased each and every time it's destroyed.<br/></div><div class="line"><a name="l00244"></a><span class="lineno"> 244</span> In case entity identifiers are stored around, the registry offers all the</div><div class="line"><a name="l00245"></a><span class="lineno"> 245</span> functionalities required to test them and get out of the them all the</div><div class="line"><a name="l00246"></a><span class="lineno"> 246</span> information they carry:</div><div class="line"><a name="l00247"></a><span class="lineno"> 247</span> </div><div class="line"><a name="l00248"></a><span class="lineno"> 248</span> ```cpp</div><div class="line"><a name="l00249"></a><span class="lineno"> 249</span> // returns true if the entity is still valid, false otherwise</div><div class="line"><a name="l00250"></a><span class="lineno"> 250</span> bool b = registry.valid(entity);</div><div class="line"><a name="l00251"></a><span class="lineno"> 251</span> </div><div class="line"><a name="l00252"></a><span class="lineno"> 252</span> // gets the version contained in the entity identifier</div><div class="line"><a name="l00253"></a><span class="lineno"> 253</span> auto version = registry.version(entity);</div><div class="line"><a name="l00254"></a><span class="lineno"> 254</span> </div><div class="line"><a name="l00255"></a><span class="lineno"> 255</span> // gets the actual version for the given entity</div><div class="line"><a name="l00256"></a><span class="lineno"> 256</span> auto curr = registry.current(entity);</div><div class="line"><a name="l00257"></a><span class="lineno"> 257</span> ```</div><div class="line"><a name="l00258"></a><span class="lineno"> 258</span> </div><div class="line"><a name="l00259"></a><span class="lineno"> 259</span> Components can be assigned to or removed from entities at any time with a few</div><div class="line"><a name="l00260"></a><span class="lineno"> 260</span> calls to member functions of the registry. As for the entities, the registry</div><div class="line"><a name="l00261"></a><span class="lineno"> 261</span> offers also a set of functionalities users can use to work with the components.</div><div class="line"><a name="l00262"></a><span class="lineno"> 262</span> </div><div class="line"><a name="l00263"></a><span class="lineno"> 263</span> The `assign` member function template creates, initializes and assigns to an</div><div class="line"><a name="l00264"></a><span class="lineno"> 264</span> entity the given component. It accepts a variable number of arguments that are</div><div class="line"><a name="l00265"></a><span class="lineno"> 265</span> used to construct the component itself if present:</div><div class="line"><a name="l00266"></a><span class="lineno"> 266</span> </div><div class="line"><a name="l00267"></a><span class="lineno"> 267</span> ```cpp</div><div class="line"><a name="l00268"></a><span class="lineno"> 268</span> registry.assign<Position>(entity, 0., 0.);</div><div class="line"><a name="l00269"></a><span class="lineno"> 269</span> </div><div class="line"><a name="l00270"></a><span class="lineno"> 270</span> // ...</div><div class="line"><a name="l00271"></a><span class="lineno"> 271</span> </div><div class="line"><a name="l00272"></a><span class="lineno"> 272</span> auto &velocity = registry.assign<Velocity>(entity);</div><div class="line"><a name="l00273"></a><span class="lineno"> 273</span> velocity.dx = 0.;</div><div class="line"><a name="l00274"></a><span class="lineno"> 274</span> velocity.dy = 0.;</div><div class="line"><a name="l00275"></a><span class="lineno"> 275</span> ```</div><div class="line"><a name="l00276"></a><span class="lineno"> 276</span> </div><div class="line"><a name="l00277"></a><span class="lineno"> 277</span> If the entity already has the given component, the `replace` member function</div><div class="line"><a name="l00278"></a><span class="lineno"> 278</span> template can be used to replace it:</div><div class="line"><a name="l00279"></a><span class="lineno"> 279</span> </div><div class="line"><a name="l00280"></a><span class="lineno"> 280</span> ```cpp</div><div class="line"><a name="l00281"></a><span class="lineno"> 281</span> registry.replace<Position>(entity, 0., 0.);</div><div class="line"><a name="l00282"></a><span class="lineno"> 282</span> </div><div class="line"><a name="l00283"></a><span class="lineno"> 283</span> // ...</div><div class="line"><a name="l00284"></a><span class="lineno"> 284</span> </div><div class="line"><a name="l00285"></a><span class="lineno"> 285</span> auto &velocity = registry.replace<Velocity>(entity);</div><div class="line"><a name="l00286"></a><span class="lineno"> 286</span> velocity.dx = 0.;</div><div class="line"><a name="l00287"></a><span class="lineno"> 287</span> velocity.dy = 0.;</div><div class="line"><a name="l00288"></a><span class="lineno"> 288</span> ```</div><div class="line"><a name="l00289"></a><span class="lineno"> 289</span> </div><div class="line"><a name="l00290"></a><span class="lineno"> 290</span> In case users want to assign a component to an entity, but it's unknown whether</div><div class="line"><a name="l00291"></a><span class="lineno"> 291</span> the entity already has it or not, `accomodate` does the work in a single call</div><div class="line"><a name="l00292"></a><span class="lineno"> 292</span> (of course, there is a performance penalty to pay for that mainly due to the</div><div class="line"><a name="l00293"></a><span class="lineno"> 293</span> fact that it must check if `entity` already has the given component or not):</div><div class="line"><a name="l00294"></a><span class="lineno"> 294</span> </div><div class="line"><a name="l00295"></a><span class="lineno"> 295</span> ```cpp</div><div class="line"><a name="l00296"></a><span class="lineno"> 296</span> registry.accomodate<Position>(entity, 0., 0.);</div><div class="line"><a name="l00297"></a><span class="lineno"> 297</span> </div><div class="line"><a name="l00298"></a><span class="lineno"> 298</span> // ...</div><div class="line"><a name="l00299"></a><span class="lineno"> 299</span> </div><div class="line"><a name="l00300"></a><span class="lineno"> 300</span> auto &velocity = registry.accomodate<Velocity>(entity);</div><div class="line"><a name="l00301"></a><span class="lineno"> 301</span> velocity.dx = 0.;</div><div class="line"><a name="l00302"></a><span class="lineno"> 302</span> velocity.dy = 0.;</div><div class="line"><a name="l00303"></a><span class="lineno"> 303</span> ```</div><div class="line"><a name="l00304"></a><span class="lineno"> 304</span> </div><div class="line"><a name="l00305"></a><span class="lineno"> 305</span> Note that `accomodate` is a sliglhty faster alternative for the following</div><div class="line"><a name="l00306"></a><span class="lineno"> 306</span> `if`/`else` statement and nothing more:</div><div class="line"><a name="l00307"></a><span class="lineno"> 307</span> </div><div class="line"><a name="l00308"></a><span class="lineno"> 308</span> ```cpp</div><div class="line"><a name="l00309"></a><span class="lineno"> 309</span> if(registry.has<Comp>(entity)) {</div><div class="line"><a name="l00310"></a><span class="lineno"> 310</span>  registry.replace<Comp>(entity, arg1, argN);</div><div class="line"><a name="l00311"></a><span class="lineno"> 311</span> } else {</div><div class="line"><a name="l00312"></a><span class="lineno"> 312</span>  registry.assign<Comp>(entity, arg1, argN);</div><div class="line"><a name="l00313"></a><span class="lineno"> 313</span> }</div><div class="line"><a name="l00314"></a><span class="lineno"> 314</span> ```</div><div class="line"><a name="l00315"></a><span class="lineno"> 315</span> </div><div class="line"><a name="l00316"></a><span class="lineno"> 316</span> As already shown, if in doubt about whether or not an entity has one or more</div><div class="line"><a name="l00317"></a><span class="lineno"> 317</span> components, the `has` member function template may be useful:</div><div class="line"><a name="l00318"></a><span class="lineno"> 318</span> </div><div class="line"><a name="l00319"></a><span class="lineno"> 319</span> ```cpp</div><div class="line"><a name="l00320"></a><span class="lineno"> 320</span> bool b = registry.has<Position, Velocity>(entity);</div><div class="line"><a name="l00321"></a><span class="lineno"> 321</span> ```</div><div class="line"><a name="l00322"></a><span class="lineno"> 322</span> </div><div class="line"><a name="l00323"></a><span class="lineno"> 323</span> On the other side, if the goal is to delete a single component, the `remove`</div><div class="line"><a name="l00324"></a><span class="lineno"> 324</span> member function template is the way to go when it's certain that the entity owns</div><div class="line"><a name="l00325"></a><span class="lineno"> 325</span> a copy of the component:</div><div class="line"><a name="l00326"></a><span class="lineno"> 326</span> </div><div class="line"><a name="l00327"></a><span class="lineno"> 327</span> ```cpp</div><div class="line"><a name="l00328"></a><span class="lineno"> 328</span> registry.remove<Position>(entity);</div><div class="line"><a name="l00329"></a><span class="lineno"> 329</span> ```</div><div class="line"><a name="l00330"></a><span class="lineno"> 330</span> </div><div class="line"><a name="l00331"></a><span class="lineno"> 331</span> Otherwise consider to use the `reset` member function. It behaves similarly to</div><div class="line"><a name="l00332"></a><span class="lineno"> 332</span> `remove` but with a strictly defined behaviour (and a performance penalty is the</div><div class="line"><a name="l00333"></a><span class="lineno"> 333</span> price to pay for that). In particular it removes the component if and only if it</div><div class="line"><a name="l00334"></a><span class="lineno"> 334</span> exists, otherwise it returns safely to the caller:</div><div class="line"><a name="l00335"></a><span class="lineno"> 335</span> </div><div class="line"><a name="l00336"></a><span class="lineno"> 336</span> ```cpp</div><div class="line"><a name="l00337"></a><span class="lineno"> 337</span> registry.reset<Position>(entity);</div><div class="line"><a name="l00338"></a><span class="lineno"> 338</span> ```</div><div class="line"><a name="l00339"></a><span class="lineno"> 339</span> </div><div class="line"><a name="l00340"></a><span class="lineno"> 340</span> There exist also two other _versions_ of the `reset` member function:</div><div class="line"><a name="l00341"></a><span class="lineno"> 341</span> </div><div class="line"><a name="l00342"></a><span class="lineno"> 342</span> * If no entity is passed to it, `reset` will remove the given component from</div><div class="line"><a name="l00343"></a><span class="lineno"> 343</span> each entity that has it:</div><div class="line"><a name="l00344"></a><span class="lineno"> 344</span> </div><div class="line"><a name="l00345"></a><span class="lineno"> 345</span>  ```cpp</div><div class="line"><a name="l00346"></a><span class="lineno"> 346</span>  registry.reset<Position>();</div><div class="line"><a name="l00347"></a><span class="lineno"> 347</span>  ```</div><div class="line"><a name="l00348"></a><span class="lineno"> 348</span> </div><div class="line"><a name="l00349"></a><span class="lineno"> 349</span> * If neither the entity nor the component are specified, all the entities and</div><div class="line"><a name="l00350"></a><span class="lineno"> 350</span> their components are destroyed:</div><div class="line"><a name="l00351"></a><span class="lineno"> 351</span> </div><div class="line"><a name="l00352"></a><span class="lineno"> 352</span>  ```cpp</div><div class="line"><a name="l00353"></a><span class="lineno"> 353</span>  registry.reset();</div><div class="line"><a name="l00354"></a><span class="lineno"> 354</span>  ```</div><div class="line"><a name="l00355"></a><span class="lineno"> 355</span> </div><div class="line"><a name="l00356"></a><span class="lineno"> 356</span> Finally, references to components can be retrieved by just doing this:</div><div class="line"><a name="l00357"></a><span class="lineno"> 357</span> </div><div class="line"><a name="l00358"></a><span class="lineno"> 358</span> ```cpp</div><div class="line"><a name="l00359"></a><span class="lineno"> 359</span> // either a non-const reference ...</div><div class="line"><a name="l00360"></a><span class="lineno"> 360</span> DefaultRegistry registry;</div><div class="line"><a name="l00361"></a><span class="lineno"> 361</span> auto &position = registry.get<Position>(entity);</div><div class="line"><a name="l00362"></a><span class="lineno"> 362</span> </div><div class="line"><a name="l00363"></a><span class="lineno"> 363</span> // ... or a const one</div><div class="line"><a name="l00364"></a><span class="lineno"> 364</span> const auto &cregistry = registry;</div><div class="line"><a name="l00365"></a><span class="lineno"> 365</span> const auto &position = cregistry.get<Position>(entity);</div><div class="line"><a name="l00366"></a><span class="lineno"> 366</span> ```</div><div class="line"><a name="l00367"></a><span class="lineno"> 367</span> </div><div class="line"><a name="l00368"></a><span class="lineno"> 368</span> The `get` member function template gives direct access to the component of an</div><div class="line"><a name="l00369"></a><span class="lineno"> 369</span> entity stored in the underlying data structures of the registry.</div><div class="line"><a name="l00370"></a><span class="lineno"> 370</span> </div><div class="line"><a name="l00371"></a><span class="lineno"> 371</span> ### Sorting: is it possible?</div><div class="line"><a name="l00372"></a><span class="lineno"> 372</span> </div><div class="line"><a name="l00373"></a><span class="lineno"> 373</span> Of course, sorting entities and components is possible with `EnTT`.<br/></div><div class="line"><a name="l00374"></a><span class="lineno"> 374</span> In fact, there are two functions that respond to slightly different needs:</div><div class="line"><a name="l00375"></a><span class="lineno"> 375</span> </div><div class="line"><a name="l00376"></a><span class="lineno"> 376</span> * Components can be sorted directly:</div><div class="line"><a name="l00377"></a><span class="lineno"> 377</span> </div><div class="line"><a name="l00378"></a><span class="lineno"> 378</span>  ```cpp</div><div class="line"><a name="l00379"></a><span class="lineno"> 379</span>  registry.sort<Renderable>([](const auto &lhs, const auto &rhs) {</div><div class="line"><a name="l00380"></a><span class="lineno"> 380</span>  return lhs.z < rhs.z;</div><div class="line"><a name="l00381"></a><span class="lineno"> 381</span>  });</div><div class="line"><a name="l00382"></a><span class="lineno"> 382</span>  ```</div><div class="line"><a name="l00383"></a><span class="lineno"> 383</span> </div><div class="line"><a name="l00384"></a><span class="lineno"> 384</span> * Components can be sorted according to the order imposed by another component:</div><div class="line"><a name="l00385"></a><span class="lineno"> 385</span> </div><div class="line"><a name="l00386"></a><span class="lineno"> 386</span>  ```cpp</div><div class="line"><a name="l00387"></a><span class="lineno"> 387</span>  registry.sort<Movement, Physics>();</div><div class="line"><a name="l00388"></a><span class="lineno"> 388</span>  ```</div><div class="line"><a name="l00389"></a><span class="lineno"> 389</span> </div><div class="line"><a name="l00390"></a><span class="lineno"> 390</span>  In this case, instances of `Movement` are arranged in memory so that cache</div><div class="line"><a name="l00391"></a><span class="lineno"> 391</span>  misses are minimized when the two components are iterated together.</div><div class="line"><a name="l00392"></a><span class="lineno"> 392</span> </div><div class="line"><a name="l00393"></a><span class="lineno"> 393</span> ## View: to persist or not to persist?</div><div class="line"><a name="l00394"></a><span class="lineno"> 394</span> </div><div class="line"><a name="l00395"></a><span class="lineno"> 395</span> There are mainly two kinds of views: standard (also known as View) and</div><div class="line"><a name="l00396"></a><span class="lineno"> 396</span> persistent (alsa known as PersistentView).<br/></div><div class="line"><a name="l00397"></a><span class="lineno"> 397</span> Both of them have pros and cons to take in consideration. In particular:</div><div class="line"><a name="l00398"></a><span class="lineno"> 398</span> </div><div class="line"><a name="l00399"></a><span class="lineno"> 399</span> * Standard views:</div><div class="line"><a name="l00400"></a><span class="lineno"> 400</span> </div><div class="line"><a name="l00401"></a><span class="lineno"> 401</span>  Pros:</div><div class="line"><a name="l00402"></a><span class="lineno"> 402</span>  * They work out-of-the-box and don't require any dedicated data</div><div class="line"><a name="l00403"></a><span class="lineno"> 403</span>  structure.</div><div class="line"><a name="l00404"></a><span class="lineno"> 404</span>  * Creating and destroying them isn't expensive at all because they don't</div><div class="line"><a name="l00405"></a><span class="lineno"> 405</span>  have any type of initialization.</div><div class="line"><a name="l00406"></a><span class="lineno"> 406</span>  * They are the best tool to iterate single components.</div><div class="line"><a name="l00407"></a><span class="lineno"> 407</span>  * They are the best tool to iterate multiple components at once when</div><div class="line"><a name="l00408"></a><span class="lineno"> 408</span>  tags are involved or one of the component is assigned to a</div><div class="line"><a name="l00409"></a><span class="lineno"> 409</span>  significantly low number of entities.</div><div class="line"><a name="l00410"></a><span class="lineno"> 410</span>  * They don't affect any other operations of the registry.</div><div class="line"><a name="l00411"></a><span class="lineno"> 411</span> </div><div class="line"><a name="l00412"></a><span class="lineno"> 412</span>  Cons:</div><div class="line"><a name="l00413"></a><span class="lineno"> 413</span>  * Their performance tend to degenerate when the number of components</div><div class="line"><a name="l00414"></a><span class="lineno"> 414</span>  to iterate grows up and the most of the entities have all of them.</div><div class="line"><a name="l00415"></a><span class="lineno"> 415</span> </div><div class="line"><a name="l00416"></a><span class="lineno"> 416</span> * Persistent views:</div><div class="line"><a name="l00417"></a><span class="lineno"> 417</span> </div><div class="line"><a name="l00418"></a><span class="lineno"> 418</span>  Pros:</div><div class="line"><a name="l00419"></a><span class="lineno"> 419</span>  * Once prepared, creating and destroying them isn't expensive at all</div><div class="line"><a name="l00420"></a><span class="lineno"> 420</span>  because they don't have any type of initialization.</div><div class="line"><a name="l00421"></a><span class="lineno"> 421</span>  * They are the best tool to iterate multiple components at once when</div><div class="line"><a name="l00422"></a><span class="lineno"> 422</span>  the most of the entities have all of them.</div><div class="line"><a name="l00423"></a><span class="lineno"> 423</span> </div><div class="line"><a name="l00424"></a><span class="lineno"> 424</span>  Cons:</div><div class="line"><a name="l00425"></a><span class="lineno"> 425</span>  * They have dedicated data structures and thus affect the memory</div><div class="line"><a name="l00426"></a><span class="lineno"> 426</span>  pressure to a minimal extent.</div><div class="line"><a name="l00427"></a><span class="lineno"> 427</span>  * If not previously prepared, the first time they are used they go</div><div class="line"><a name="l00428"></a><span class="lineno"> 428</span>  through an initialization step that could take a while.</div><div class="line"><a name="l00429"></a><span class="lineno"> 429</span>  * They affect to a minimum the creation and destruction of entities and</div><div class="line"><a name="l00430"></a><span class="lineno"> 430</span>  components. In other terms: the more persistent views there will be,</div><div class="line"><a name="l00431"></a><span class="lineno"> 431</span>  the less performing will be creating and destroying entities and</div><div class="line"><a name="l00432"></a><span class="lineno"> 432</span>  components.</div><div class="line"><a name="l00433"></a><span class="lineno"> 433</span> </div><div class="line"><a name="l00434"></a><span class="lineno"> 434</span> To sum up and as a rule of thumb, use a standard view:</div><div class="line"><a name="l00435"></a><span class="lineno"> 435</span> * To iterate entities for a single component.</div><div class="line"><a name="l00436"></a><span class="lineno"> 436</span> * To iterate entities for multiple components when a significantly low</div><div class="line"><a name="l00437"></a><span class="lineno"> 437</span>  number of entities have one of the components.</div><div class="line"><a name="l00438"></a><span class="lineno"> 438</span> * In all those cases where a persistent view would give a boost to</div><div class="line"><a name="l00439"></a><span class="lineno"> 439</span>  performance but the iteration isn't performed frequently.</div><div class="line"><a name="l00440"></a><span class="lineno"> 440</span> </div><div class="line"><a name="l00441"></a><span class="lineno"> 441</span> Use a persistent view in all the other cases.</div><div class="line"><a name="l00442"></a><span class="lineno"> 442</span> </div><div class="line"><a name="l00443"></a><span class="lineno"> 443</span> To easily iterate entities, all the views offer _C++-ish_ `begin` and `end`</div><div class="line"><a name="l00444"></a><span class="lineno"> 444</span> member functions that allow users to use them in a typical range-for loop.<br/></div><div class="line"><a name="l00445"></a><span class="lineno"> 445</span> Continue reading for more details or refer to the</div><div class="line"><a name="l00446"></a><span class="lineno"> 446</span> [official documentation](https://skypjack.github.io/entt/).</div><div class="line"><a name="l00447"></a><span class="lineno"> 447</span> </div><div class="line"><a name="l00448"></a><span class="lineno"> 448</span> ### Standard View</div><div class="line"><a name="l00449"></a><span class="lineno"> 449</span> </div><div class="line"><a name="l00450"></a><span class="lineno"> 450</span> A standard view behaves differently if it's constructed for a single component</div><div class="line"><a name="l00451"></a><span class="lineno"> 451</span> or if it has been requested to iterate multiple components. Even the API is</div><div class="line"><a name="l00452"></a><span class="lineno"> 452</span> different in the two cases.<br/></div><div class="line"><a name="l00453"></a><span class="lineno"> 453</span> All that they share is the way they are created by means of a registry:</div><div class="line"><a name="l00454"></a><span class="lineno"> 454</span> </div><div class="line"><a name="l00455"></a><span class="lineno"> 455</span> ```cpp</div><div class="line"><a name="l00456"></a><span class="lineno"> 456</span> // single component standard view</div><div class="line"><a name="l00457"></a><span class="lineno"> 457</span> auto single = registry.view<Position>();</div><div class="line"><a name="l00458"></a><span class="lineno"> 458</span> </div><div class="line"><a name="l00459"></a><span class="lineno"> 459</span> // multi component standard view</div><div class="line"><a name="l00460"></a><span class="lineno"> 460</span> auto multi = registry.view<Position, Velocity>();</div><div class="line"><a name="l00461"></a><span class="lineno"> 461</span> ```</div><div class="line"><a name="l00462"></a><span class="lineno"> 462</span> </div><div class="line"><a name="l00463"></a><span class="lineno"> 463</span> For all that remains, it's worth discussing them separately.<br/></div><div class="line"><a name="l00464"></a><span class="lineno"> 464</span> </div><div class="line"><a name="l00465"></a><span class="lineno"> 465</span> #### Single component standard view</div><div class="line"><a name="l00466"></a><span class="lineno"> 466</span> </div><div class="line"><a name="l00467"></a><span class="lineno"> 467</span> Single component standard views are specialized in order to give a boost in</div><div class="line"><a name="l00468"></a><span class="lineno"> 468</span> terms of performance in all the situation. This kind of views can access the</div><div class="line"><a name="l00469"></a><span class="lineno"> 469</span> underlying data structures directly and avoid superflous checks.<br/></div><div class="line"><a name="l00470"></a><span class="lineno"> 470</span> They offer a bunch of functionalities to get the number of entities they are</div><div class="line"><a name="l00471"></a><span class="lineno"> 471</span> going to return and a raw access to the entity list as well as to the component</div><div class="line"><a name="l00472"></a><span class="lineno"> 472</span> list.<br/></div><div class="line"><a name="l00473"></a><span class="lineno"> 473</span> Refer to the [official documentation](https://skypjack.github.io/entt/) for all</div><div class="line"><a name="l00474"></a><span class="lineno"> 474</span> the details.</div><div class="line"><a name="l00475"></a><span class="lineno"> 475</span> </div><div class="line"><a name="l00476"></a><span class="lineno"> 476</span> There is no need to store views around for they are extremely cheap to</div><div class="line"><a name="l00477"></a><span class="lineno"> 477</span> construct, even though they can be copied without problems and reused</div><div class="line"><a name="l00478"></a><span class="lineno"> 478</span> freely. In fact, they return newly created and correctly initialized iterators</div><div class="line"><a name="l00479"></a><span class="lineno"> 479</span> whenever `begin` or `end` are invoked.<br/></div><div class="line"><a name="l00480"></a><span class="lineno"> 480</span> To iterate a single component standard view, just use it in range-for:</div><div class="line"><a name="l00481"></a><span class="lineno"> 481</span> </div><div class="line"><a name="l00482"></a><span class="lineno"> 482</span> ```cpp</div><div class="line"><a name="l00483"></a><span class="lineno"> 483</span> auto view = registry.view<Renderable>();</div><div class="line"><a name="l00484"></a><span class="lineno"> 484</span> </div><div class="line"><a name="l00485"></a><span class="lineno"> 485</span> for(auto entity: view) {</div><div class="line"><a name="l00486"></a><span class="lineno"> 486</span>  auto &renderable = view.get(entity);</div><div class="line"><a name="l00487"></a><span class="lineno"> 487</span> </div><div class="line"><a name="l00488"></a><span class="lineno"> 488</span>  // ...</div><div class="line"><a name="l00489"></a><span class="lineno"> 489</span> }</div><div class="line"><a name="l00490"></a><span class="lineno"> 490</span> ```</div><div class="line"><a name="l00491"></a><span class="lineno"> 491</span> </div><div class="line"><a name="l00492"></a><span class="lineno"> 492</span> **Note**: prefer the `get` member function of the view instead of the `get`</div><div class="line"><a name="l00493"></a><span class="lineno"> 493</span> member function template of the registry during iterations.</div><div class="line"><a name="l00494"></a><span class="lineno"> 494</span> </div><div class="line"><a name="l00495"></a><span class="lineno"> 495</span> #### Multi component standard view</div><div class="line"><a name="l00496"></a><span class="lineno"> 496</span> </div><div class="line"><a name="l00497"></a><span class="lineno"> 497</span> Multi component standard views iterate entities that have at least all the given</div><div class="line"><a name="l00498"></a><span class="lineno"> 498</span> components in their bags. During construction, these views look at the number</div><div class="line"><a name="l00499"></a><span class="lineno"> 499</span> of entities available for each component and pick up a reference to the smallest</div><div class="line"><a name="l00500"></a><span class="lineno"> 500</span> set of candidates in order to speed up iterations.<br/></div><div class="line"><a name="l00501"></a><span class="lineno"> 501</span> They offer fewer functionalities than their companion views for single</div><div class="line"><a name="l00502"></a><span class="lineno"> 502</span> component, the most important of which can be used to reset the view and refresh</div><div class="line"><a name="l00503"></a><span class="lineno"> 503</span> the reference to the set of candidate entities to iterate.<br/></div><div class="line"><a name="l00504"></a><span class="lineno"> 504</span> Refer to the [official documentation](https://skypjack.github.io/entt/) for all</div><div class="line"><a name="l00505"></a><span class="lineno"> 505</span> the details.</div><div class="line"><a name="l00506"></a><span class="lineno"> 506</span> </div><div class="line"><a name="l00507"></a><span class="lineno"> 507</span> There is no need to store views around for they are extremely cheap to</div><div class="line"><a name="l00508"></a><span class="lineno"> 508</span> construct, even though they can be copied without problems and reused</div><div class="line"><a name="l00509"></a><span class="lineno"> 509</span> freely. In fact, they return newly created and correctly initialized iterators</div><div class="line"><a name="l00510"></a><span class="lineno"> 510</span> whenever `begin` or `end` are invoked.<br/></div><div class="line"><a name="l00511"></a><span class="lineno"> 511</span> To iterate a multi component standard view, just use it in range-for:</div><div class="line"><a name="l00512"></a><span class="lineno"> 512</span> </div><div class="line"><a name="l00513"></a><span class="lineno"> 513</span> ```cpp</div><div class="line"><a name="l00514"></a><span class="lineno"> 514</span> auto view = registry.view<Position, Velocity>();</div><div class="line"><a name="l00515"></a><span class="lineno"> 515</span> </div><div class="line"><a name="l00516"></a><span class="lineno"> 516</span> for(auto entity: view) {</div><div class="line"><a name="l00517"></a><span class="lineno"> 517</span>  auto &position = view.get<Position>(entity);</div><div class="line"><a name="l00518"></a><span class="lineno"> 518</span>  auto &velocity = view.get<Velocity>(entity);</div><div class="line"><a name="l00519"></a><span class="lineno"> 519</span> </div><div class="line"><a name="l00520"></a><span class="lineno"> 520</span>  // ...</div><div class="line"><a name="l00521"></a><span class="lineno"> 521</span> }</div><div class="line"><a name="l00522"></a><span class="lineno"> 522</span> ```</div><div class="line"><a name="l00523"></a><span class="lineno"> 523</span> </div><div class="line"><a name="l00524"></a><span class="lineno"> 524</span> **Note**: prefer the `get` member function template of the view instead of the</div><div class="line"><a name="l00525"></a><span class="lineno"> 525</span> `get` member function template of the registry during iterations.</div><div class="line"><a name="l00526"></a><span class="lineno"> 526</span> </div><div class="line"><a name="l00527"></a><span class="lineno"> 527</span> ### Persistent View</div><div class="line"><a name="l00528"></a><span class="lineno"> 528</span> </div><div class="line"><a name="l00529"></a><span class="lineno"> 529</span> A persistent view returns all the entities and only the entities that have at</div><div class="line"><a name="l00530"></a><span class="lineno"> 530</span> least the given components. Moreover, it's guaranteed that the entity list is</div><div class="line"><a name="l00531"></a><span class="lineno"> 531</span> thightly packed in memory for fast iterations.<br/></div><div class="line"><a name="l00532"></a><span class="lineno"> 532</span> In general, persistent views don't stay true to the order of any set of</div><div class="line"><a name="l00533"></a><span class="lineno"> 533</span> components unless users explicitly sort them.</div><div class="line"><a name="l00534"></a><span class="lineno"> 534</span> </div><div class="line"><a name="l00535"></a><span class="lineno"> 535</span> Persistent views can be used only to iterate multiple components. Create them</div><div class="line"><a name="l00536"></a><span class="lineno"> 536</span> as it follows:</div><div class="line"><a name="l00537"></a><span class="lineno"> 537</span> </div><div class="line"><a name="l00538"></a><span class="lineno"> 538</span> ```cpp</div><div class="line"><a name="l00539"></a><span class="lineno"> 539</span> auto view = registry.persistent<Position, Velocity>();</div><div class="line"><a name="l00540"></a><span class="lineno"> 540</span> ```</div><div class="line"><a name="l00541"></a><span class="lineno"> 541</span> </div><div class="line"><a name="l00542"></a><span class="lineno"> 542</span> There is no need to store views around for they are extremely cheap to</div><div class="line"><a name="l00543"></a><span class="lineno"> 543</span> construct, even though they can be copied without problems and reused</div><div class="line"><a name="l00544"></a><span class="lineno"> 544</span> freely. In fact, they return newly created and correctly initialized iterators</div><div class="line"><a name="l00545"></a><span class="lineno"> 545</span> whenever `begin` or `end` are invoked.<br/></div><div class="line"><a name="l00546"></a><span class="lineno"> 546</span> That being said, persistent views perform an initialization step the very first</div><div class="line"><a name="l00547"></a><span class="lineno"> 547</span> time they are constructed and this could be quite costly. To avoid it, consider</div><div class="line"><a name="l00548"></a><span class="lineno"> 548</span> asking to the registry to _prepare_ them when no entities have been created yet:</div><div class="line"><a name="l00549"></a><span class="lineno"> 549</span> </div><div class="line"><a name="l00550"></a><span class="lineno"> 550</span> ```cpp</div><div class="line"><a name="l00551"></a><span class="lineno"> 551</span> registry.prepare<Position, Velocity>();</div><div class="line"><a name="l00552"></a><span class="lineno"> 552</span> ```</div><div class="line"><a name="l00553"></a><span class="lineno"> 553</span> </div><div class="line"><a name="l00554"></a><span class="lineno"> 554</span> If the registry is empty, preparation is extremely fast. Moreover the `prepare`</div><div class="line"><a name="l00555"></a><span class="lineno"> 555</span> member function template is idempotent. Feel free to invoke it even more than</div><div class="line"><a name="l00556"></a><span class="lineno"> 556</span> once: if the view has been alreadt prepared before, the function returns</div><div class="line"><a name="l00557"></a><span class="lineno"> 557</span> immediately and does nothing.</div><div class="line"><a name="l00558"></a><span class="lineno"> 558</span> </div><div class="line"><a name="l00559"></a><span class="lineno"> 559</span> A persistent view offers a bunch of functionalities to get the number of</div><div class="line"><a name="l00560"></a><span class="lineno"> 560</span> entities it's going to return, a raw access to the entity list and the</div><div class="line"><a name="l00561"></a><span class="lineno"> 561</span> possibility to sort the underlying data structures according to the order of one</div><div class="line"><a name="l00562"></a><span class="lineno"> 562</span> of the components for which it has been constructed.<br/></div><div class="line"><a name="l00563"></a><span class="lineno"> 563</span> Refer to the [official documentation](https://skypjack.github.io/entt/) for all</div><div class="line"><a name="l00564"></a><span class="lineno"> 564</span> the details.</div><div class="line"><a name="l00565"></a><span class="lineno"> 565</span> </div><div class="line"><a name="l00566"></a><span class="lineno"> 566</span> To iterate a persistent view, just use it in range-for:</div><div class="line"><a name="l00567"></a><span class="lineno"> 567</span> </div><div class="line"><a name="l00568"></a><span class="lineno"> 568</span> ```cpp</div><div class="line"><a name="l00569"></a><span class="lineno"> 569</span> auto view = registry.persistent<Position, Velocity>();</div><div class="line"><a name="l00570"></a><span class="lineno"> 570</span> </div><div class="line"><a name="l00571"></a><span class="lineno"> 571</span> for(auto entity: view) {</div><div class="line"><a name="l00572"></a><span class="lineno"> 572</span>  auto &position = view.get<Position>(entity);</div><div class="line"><a name="l00573"></a><span class="lineno"> 573</span>  auto &velocity = view.get<Velocity>(entity);</div><div class="line"><a name="l00574"></a><span class="lineno"> 574</span> </div><div class="line"><a name="l00575"></a><span class="lineno"> 575</span>  // ...</div><div class="line"><a name="l00576"></a><span class="lineno"> 576</span> }</div><div class="line"><a name="l00577"></a><span class="lineno"> 577</span> ```</div><div class="line"><a name="l00578"></a><span class="lineno"> 578</span> </div><div class="line"><a name="l00579"></a><span class="lineno"> 579</span> **Note**: prefer the `get` member function template of the view instead of the</div><div class="line"><a name="l00580"></a><span class="lineno"> 580</span> `get` member function template of the registry during iterations.</div><div class="line"><a name="l00581"></a><span class="lineno"> 581</span> </div><div class="line"><a name="l00582"></a><span class="lineno"> 582</span> ## Side notes</div><div class="line"><a name="l00583"></a><span class="lineno"> 583</span> </div><div class="line"><a name="l00584"></a><span class="lineno"> 584</span> * Entity identifiers are numbers and nothing more. They are not classes and they</div><div class="line"><a name="l00585"></a><span class="lineno"> 585</span>  have no member functions at all. As already mentioned, do no try to inspect or</div><div class="line"><a name="l00586"></a><span class="lineno"> 586</span>  modify an entity descriptor in any way.</div><div class="line"><a name="l00587"></a><span class="lineno"> 587</span> </div><div class="line"><a name="l00588"></a><span class="lineno"> 588</span> * As shown in the examples above, the preferred way to get references to the</div><div class="line"><a name="l00589"></a><span class="lineno"> 589</span>  components while iterating a view is by using the view itself. It's a faster</div><div class="line"><a name="l00590"></a><span class="lineno"> 590</span>  alternative to the `get` member function template that is part of the API of</div><div class="line"><a name="l00591"></a><span class="lineno"> 591</span>  the Registry. That's because the registry must ensure that a pool for the</div><div class="line"><a name="l00592"></a><span class="lineno"> 592</span>  given component exists before to use it; on the other side, views force the</div><div class="line"><a name="l00593"></a><span class="lineno"> 593</span>  construction of the pools for all their components and access them directly,</div><div class="line"><a name="l00594"></a><span class="lineno"> 594</span>  thus avoiding all the checks.</div><div class="line"><a name="l00595"></a><span class="lineno"> 595</span> </div><div class="line"><a name="l00596"></a><span class="lineno"> 596</span> * Most of the _ECS_ available out there have an annoying limitation (at least</div><div class="line"><a name="l00597"></a><span class="lineno"> 597</span>  from my point of view): entities and components cannot be created and/or</div><div class="line"><a name="l00598"></a><span class="lineno"> 598</span>  deleted during iterations.<br/></div><div class="line"><a name="l00599"></a><span class="lineno"> 599</span>  `EnTT` partially solves the problem with a few limitations:</div><div class="line"><a name="l00600"></a><span class="lineno"> 600</span> </div><div class="line"><a name="l00601"></a><span class="lineno"> 601</span>  * Creating entities and components is allowed during iterations.</div><div class="line"><a name="l00602"></a><span class="lineno"> 602</span>  * Deleting an entity or removing its components is allowed during</div><div class="line"><a name="l00603"></a><span class="lineno"> 603</span>  iterations if it's the one currently returned by a view. For all the</div><div class="line"><a name="l00604"></a><span class="lineno"> 604</span>  other entities, destroying them or removing their components isn't</div><div class="line"><a name="l00605"></a><span class="lineno"> 605</span>  allowed and it can result in undefined behavior.</div><div class="line"><a name="l00606"></a><span class="lineno"> 606</span> </div><div class="line"><a name="l00607"></a><span class="lineno"> 607</span>  Iterators are invalidated and the behaviour is undefined if an entity is</div><div class="line"><a name="l00608"></a><span class="lineno"> 608</span>  modified or destroyed and it's not the one currently returned by the</div><div class="line"><a name="l00609"></a><span class="lineno"> 609</span>  view.<br/></div><div class="line"><a name="l00610"></a><span class="lineno"> 610</span>  To work around it, possible approaches are:</div><div class="line"><a name="l00611"></a><span class="lineno"> 611</span> </div><div class="line"><a name="l00612"></a><span class="lineno"> 612</span>  * Store aside the entities and the components to be removed and perform the</div><div class="line"><a name="l00613"></a><span class="lineno"> 613</span>  operations at the end of the iteration.</div><div class="line"><a name="l00614"></a><span class="lineno"> 614</span>  * Mark entities and components with a proper tag component that indicates</div><div class="line"><a name="l00615"></a><span class="lineno"> 615</span>  they must be purged, then perform a second iteration to clean them up one</div><div class="line"><a name="l00616"></a><span class="lineno"> 616</span>  by one.</div><div class="line"><a name="l00617"></a><span class="lineno"> 617</span> </div><div class="line"><a name="l00618"></a><span class="lineno"> 618</span> * Views and thus their iterators aren't thread safe. Do no try to iterate a set</div><div class="line"><a name="l00619"></a><span class="lineno"> 619</span>  of components and modify the same set concurrently.<br/></div><div class="line"><a name="l00620"></a><span class="lineno"> 620</span>  That being said, as long as a thread iterates the entities that have the</div><div class="line"><a name="l00621"></a><span class="lineno"> 621</span>  component `X` or assign and removes that component from a set of entities,</div><div class="line"><a name="l00622"></a><span class="lineno"> 622</span>  another thread can safely do the same with components `Y` and `Z` and</div><div class="line"><a name="l00623"></a><span class="lineno"> 623</span>  everything will work like a charm.<br/></div><div class="line"><a name="l00624"></a><span class="lineno"> 624</span>  As an example, users can freely execute the rendering system and iterate the</div><div class="line"><a name="l00625"></a><span class="lineno"> 625</span>  renderable entities while updating a physic component concurrently on a</div><div class="line"><a name="l00626"></a><span class="lineno"> 626</span>  separate thread if needed.</div><div class="line"><a name="l00627"></a><span class="lineno"> 627</span> </div><div class="line"><a name="l00628"></a><span class="lineno"> 628</span> ## What else?</div><div class="line"><a name="l00629"></a><span class="lineno"> 629</span> </div><div class="line"><a name="l00630"></a><span class="lineno"> 630</span> The `EnTT` framework is moving its first steps. More and more will come in the</div><div class="line"><a name="l00631"></a><span class="lineno"> 631</span> future and hopefully I'm going to work on it for a long time.<br/></div><div class="line"><a name="l00632"></a><span class="lineno"> 632</span> Here is a brief list of what it offers today:</div><div class="line"><a name="l00633"></a><span class="lineno"> 633</span> </div><div class="line"><a name="l00634"></a><span class="lineno"> 634</span> * Statically generated integer identifiers for types.</div><div class="line"><a name="l00635"></a><span class="lineno"> 635</span> * An entity-component system based on sparse sets.</div><div class="line"><a name="l00636"></a><span class="lineno"> 636</span> * Signal handlers and event emitters of any type.</div><div class="line"><a name="l00637"></a><span class="lineno"> 637</span> * ...</div><div class="line"><a name="l00638"></a><span class="lineno"> 638</span> * Any other business.</div><div class="line"><a name="l00639"></a><span class="lineno"> 639</span> </div><div class="line"><a name="l00640"></a><span class="lineno"> 640</span> Consider it a work in progress. For more details and an updated list, please</div><div class="line"><a name="l00641"></a><span class="lineno"> 641</span> refer to the [online documentation](https://skypjack.github.io/entt/).</div><div class="line"><a name="l00642"></a><span class="lineno"> 642</span> </div><div class="line"><a name="l00643"></a><span class="lineno"> 643</span> # Contributors</div><div class="line"><a name="l00644"></a><span class="lineno"> 644</span> </div><div class="line"><a name="l00645"></a><span class="lineno"> 645</span> If you want to contribute, please send patches as pull requests against the</div><div class="line"><a name="l00646"></a><span class="lineno"> 646</span> branch `master`.<br/></div><div class="line"><a name="l00647"></a><span class="lineno"> 647</span> Check the</div><div class="line"><a name="l00648"></a><span class="lineno"> 648</span> [contributors list](https://github.com/skypjack/entt/blob/master/AUTHORS) to see</div><div class="line"><a name="l00649"></a><span class="lineno"> 649</span> who has partecipated so far.</div><div class="line"><a name="l00650"></a><span class="lineno"> 650</span> </div><div class="line"><a name="l00651"></a><span class="lineno"> 651</span> # License</div><div class="line"><a name="l00652"></a><span class="lineno"> 652</span> </div><div class="line"><a name="l00653"></a><span class="lineno"> 653</span> Code and documentation Copyright (c) 2017 Michele Caini.<br/></div><div class="line"><a name="l00654"></a><span class="lineno"> 654</span> Code released under</div><div class="line"><a name="l00655"></a><span class="lineno"> 655</span> [the MIT license](https://github.com/skypjack/entt/blob/master/LICENSE).</div><div class="line"><a name="l00656"></a><span class="lineno"> 656</span> Docs released under</div><div class="line"><a name="l00657"></a><span class="lineno"> 657</span> [Creative Commons](https://github.com/skypjack/entt/blob/master/docs/LICENSE).</div><div class="line"><a name="l00658"></a><span class="lineno"> 658</span> </div><div class="line"><a name="l00659"></a><span class="lineno"> 659</span> # Donation</div><div class="line"><a name="l00660"></a><span class="lineno"> 660</span> </div><div class="line"><a name="l00661"></a><span class="lineno"> 661</span> Developing and maintaining `EnTT` takes some time and lots of coffee. I'd like</div><div class="line"><a name="l00662"></a><span class="lineno"> 662</span> to add more and more functionalities in future and turn it in a full-featured</div><div class="line"><a name="l00663"></a><span class="lineno"> 663</span> framework.<br/></div><div class="line"><a name="l00664"></a><span class="lineno"> 664</span> If you want to support this project, you can offer me an espresso. I'm from</div><div class="line"><a name="l00665"></a><span class="lineno"> 665</span> Italy, we're used to turning the best coffee ever in code. If you find that</div><div class="line"><a name="l00666"></a><span class="lineno"> 666</span> it's not enough, feel free to support me the way you prefer.<br/></div><div class="line"><a name="l00667"></a><span class="lineno"> 667</span> Take a look at the donation button at the top of the page for more details or</div><div class="line"><a name="l00668"></a><span class="lineno"> 668</span> just click [here](https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=W2HF9FESD5LJY&lc=IT&item_name=Michele%20Caini&currency_code=EUR&bn=PP%2dDonationsBF%3abtn_donateCC_LG%2egif%3aNonHosted).</div></div><!-- fragment --></div><!-- contents -->
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