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<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno"> 1</span> # Crash Course: core functionalities</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> <!--</div><div class="line"><a name="l00004"></a><span class="lineno"> 4</span> @cond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00005"></a><span class="lineno"> 5</span> --></div><div class="line"><a name="l00006"></a><span class="lineno"> 6</span> # Table of Contents</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](#introduction)</div><div class="line"><a name="l00009"></a><span class="lineno"> 9</span> * [Compile-time identifiers](#compile-time-identifiers)</div><div class="line"><a name="l00010"></a><span class="lineno"> 10</span> * [Runtime identifiers](#runtime-identifiers)</div><div class="line"><a name="l00011"></a><span class="lineno"> 11</span> * [Hashed strings](#hashed-strings)</div><div class="line"><a name="l00012"></a><span class="lineno"> 12</span>  * [Conflicts](#conflicts)</div><div class="line"><a name="l00013"></a><span class="lineno"> 13</span> * [Monostate](#monostate)</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> @endcond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00016"></a><span class="lineno"> 16</span> --></div><div class="line"><a name="l00017"></a><span class="lineno"> 17</span> </div><div class="line"><a name="l00018"></a><span class="lineno"> 18</span> # Introduction</div><div class="line"><a name="l00019"></a><span class="lineno"> 19</span> </div><div class="line"><a name="l00020"></a><span class="lineno"> 20</span> `EnTT` comes with a bunch of core functionalities mostly used by the other parts</div><div class="line"><a name="l00021"></a><span class="lineno"> 21</span> of the library itself.<br/></div><div class="line"><a name="l00022"></a><span class="lineno"> 22</span> Hardly users will include these features in their code, but it's worth</div><div class="line"><a name="l00023"></a><span class="lineno"> 23</span> describing what `EnTT` offers so as not to reinvent the wheel in case of need.</div><div class="line"><a name="l00024"></a><span class="lineno"> 24</span> </div><div class="line"><a name="l00025"></a><span class="lineno"> 25</span> # Compile-time identifiers</div><div class="line"><a name="l00026"></a><span class="lineno"> 26</span> </div><div class="line"><a name="l00027"></a><span class="lineno"> 27</span> Sometimes it's useful to be able to give unique identifiers to types at</div><div class="line"><a name="l00028"></a><span class="lineno"> 28</span> compile-time.<br/></div><div class="line"><a name="l00029"></a><span class="lineno"> 29</span> There are plenty of different solutions out there and I could have used one of</div><div class="line"><a name="l00030"></a><span class="lineno"> 30</span> them. However, I decided to spend my time to define a compact and versatile tool</div><div class="line"><a name="l00031"></a><span class="lineno"> 31</span> that fully embraces what the modern C++ has to offer.</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> The _result of my efforts_ is the `Identifier` class template:</div><div class="line"><a name="l00034"></a><span class="lineno"> 34</span> </div><div class="line"><a name="l00035"></a><span class="lineno"> 35</span> ```cpp</div><div class="line"><a name="l00036"></a><span class="lineno"> 36</span> #include <ident.hpp></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> // defines the identifiers for the given types</div><div class="line"><a name="l00039"></a><span class="lineno"> 39</span> using ID = entt::Identifier<AType, AnotherType>;</div><div class="line"><a name="l00040"></a><span class="lineno"> 40</span> </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> </div><div class="line"><a name="l00043"></a><span class="lineno"> 43</span> switch(aTypeIdentifier) {</div><div class="line"><a name="l00044"></a><span class="lineno"> 44</span> case ID::get<AType>():</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>  break;</div><div class="line"><a name="l00047"></a><span class="lineno"> 47</span> case ID::get<AnotherType>():</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>  break;</div><div class="line"><a name="l00050"></a><span class="lineno"> 50</span> default:</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> }</div><div class="line"><a name="l00053"></a><span class="lineno"> 53</span> ```</div><div class="line"><a name="l00054"></a><span class="lineno"> 54</span> </div><div class="line"><a name="l00055"></a><span class="lineno"> 55</span> This is all what the class template has to offer: a static `get` member function</div><div class="line"><a name="l00056"></a><span class="lineno"> 56</span> that returns a numerical identifier for the given type. It can be used in any</div><div class="line"><a name="l00057"></a><span class="lineno"> 57</span> context where constant expressions are required.</div><div class="line"><a name="l00058"></a><span class="lineno"> 58</span> </div><div class="line"><a name="l00059"></a><span class="lineno"> 59</span> As long as the list remains unchanged, identifiers are also guaranteed to be the</div><div class="line"><a name="l00060"></a><span class="lineno"> 60</span> same for every run. In case they have been used in a production environment and</div><div class="line"><a name="l00061"></a><span class="lineno"> 61</span> a type has to be removed, one can just use a placeholder to left the other</div><div class="line"><a name="l00062"></a><span class="lineno"> 62</span> identifiers unchanged:</div><div class="line"><a name="l00063"></a><span class="lineno"> 63</span> </div><div class="line"><a name="l00064"></a><span class="lineno"> 64</span> ```cpp</div><div class="line"><a name="l00065"></a><span class="lineno"> 65</span> template<typename> struct IgnoreType {};</div><div class="line"><a name="l00066"></a><span class="lineno"> 66</span> </div><div class="line"><a name="l00067"></a><span class="lineno"> 67</span> using ID = entt::Identifier<</div><div class="line"><a name="l00068"></a><span class="lineno"> 68</span>  ATypeStillValid,</div><div class="line"><a name="l00069"></a><span class="lineno"> 69</span>  IgnoreType<ATypeNoLongerValid>,</div><div class="line"><a name="l00070"></a><span class="lineno"> 70</span>  AnotherTypeStillValid</div><div class="line"><a name="l00071"></a><span class="lineno"> 71</span> >;</div><div class="line"><a name="l00072"></a><span class="lineno"> 72</span> ```</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> A bit ugly to see, but it works at least.</div><div class="line"><a name="l00075"></a><span class="lineno"> 75</span> </div><div class="line"><a name="l00076"></a><span class="lineno"> 76</span> # Runtime identifiers</div><div class="line"><a name="l00077"></a><span class="lineno"> 77</span> </div><div class="line"><a name="l00078"></a><span class="lineno"> 78</span> Sometimes it's useful to be able to give unique identifiers to types at</div><div class="line"><a name="l00079"></a><span class="lineno"> 79</span> runtime.<br/></div><div class="line"><a name="l00080"></a><span class="lineno"> 80</span> There are plenty of different solutions out there and I could have used one of</div><div class="line"><a name="l00081"></a><span class="lineno"> 81</span> them. In fact, I adapted the most common one to my requirements and used it</div><div class="line"><a name="l00082"></a><span class="lineno"> 82</span> extensively within the entire library.</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> It's the `Family` class. Here is an example of use directly from the</div><div class="line"><a name="l00085"></a><span class="lineno"> 85</span> entity-component system:</div><div class="line"><a name="l00086"></a><span class="lineno"> 86</span> </div><div class="line"><a name="l00087"></a><span class="lineno"> 87</span> ```cpp</div><div class="line"><a name="l00088"></a><span class="lineno"> 88</span> using component_family = entt::Family<struct InternalRegistryComponentFamily>;</div><div class="line"><a name="l00089"></a><span class="lineno"> 89</span> </div><div class="line"><a name="l00090"></a><span class="lineno"> 90</span> // ...</div><div class="line"><a name="l00091"></a><span class="lineno"> 91</span> </div><div class="line"><a name="l00092"></a><span class="lineno"> 92</span> template<typename Component></div><div class="line"><a name="l00093"></a><span class="lineno"> 93</span> component_type component() const noexcept {</div><div class="line"><a name="l00094"></a><span class="lineno"> 94</span>  return component_family::type<Component>();</div><div class="line"><a name="l00095"></a><span class="lineno"> 95</span> }</div><div class="line"><a name="l00096"></a><span class="lineno"> 96</span> ```</div><div class="line"><a name="l00097"></a><span class="lineno"> 97</span> </div><div class="line"><a name="l00098"></a><span class="lineno"> 98</span> This is all what a _family_ has to offer: a `type` member function that returns</div><div class="line"><a name="l00099"></a><span class="lineno"> 99</span> a numerical identifier for the given type.</div><div class="line"><a name="l00100"></a><span class="lineno"> 100</span> </div><div class="line"><a name="l00101"></a><span class="lineno"> 101</span> Please, note that identifiers aren't guaranteed to be the same for every run.</div><div class="line"><a name="l00102"></a><span class="lineno"> 102</span> Indeed it mostly depends on the flow of execution.</div><div class="line"><a name="l00103"></a><span class="lineno"> 103</span> </div><div class="line"><a name="l00104"></a><span class="lineno"> 104</span> # Hashed strings</div><div class="line"><a name="l00105"></a><span class="lineno"> 105</span> </div><div class="line"><a name="l00106"></a><span class="lineno"> 106</span> A hashed string is a zero overhead resource identifier. Users can use</div><div class="line"><a name="l00107"></a><span class="lineno"> 107</span> human-readable identifiers in the codebase while using their numeric</div><div class="line"><a name="l00108"></a><span class="lineno"> 108</span> counterparts at runtime, thus without affecting performance.<br/></div><div class="line"><a name="l00109"></a><span class="lineno"> 109</span> The class has an implicit `constexpr` constructor that chews a bunch of</div><div class="line"><a name="l00110"></a><span class="lineno"> 110</span> characters. Once created, all what one can do with it is getting back the</div><div class="line"><a name="l00111"></a><span class="lineno"> 111</span> original string or converting it into a number.<br/></div><div class="line"><a name="l00112"></a><span class="lineno"> 112</span> The good part is that a hashed string can be used wherever a constant expression</div><div class="line"><a name="l00113"></a><span class="lineno"> 113</span> is required and no _string-to-number_ conversion will take place at runtime if</div><div class="line"><a name="l00114"></a><span class="lineno"> 114</span> used carefully.</div><div class="line"><a name="l00115"></a><span class="lineno"> 115</span> </div><div class="line"><a name="l00116"></a><span class="lineno"> 116</span> Example of use:</div><div class="line"><a name="l00117"></a><span class="lineno"> 117</span> </div><div class="line"><a name="l00118"></a><span class="lineno"> 118</span> ```cpp</div><div class="line"><a name="l00119"></a><span class="lineno"> 119</span> auto load(entt::HashedString::hash_type resource) {</div><div class="line"><a name="l00120"></a><span class="lineno"> 120</span>  // uses the numeric representation of the resource to load and return it</div><div class="line"><a name="l00121"></a><span class="lineno"> 121</span> }</div><div class="line"><a name="l00122"></a><span class="lineno"> 122</span> </div><div class="line"><a name="l00123"></a><span class="lineno"> 123</span> auto resource = load(entt::HashedString{"gui/background"});</div><div class="line"><a name="l00124"></a><span class="lineno"> 124</span> ```</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> There is also a _user defined literal_ dedicated to hashed strings to make them</div><div class="line"><a name="l00127"></a><span class="lineno"> 127</span> more user-friendly:</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> ```cpp</div><div class="line"><a name="l00130"></a><span class="lineno"> 130</span> constexpr auto str = "text"_hs;</div><div class="line"><a name="l00131"></a><span class="lineno"> 131</span> ```</div><div class="line"><a name="l00132"></a><span class="lineno"> 132</span> </div><div class="line"><a name="l00133"></a><span class="lineno"> 133</span> ## Conflicts</div><div class="line"><a name="l00134"></a><span class="lineno"> 134</span> </div><div class="line"><a name="l00135"></a><span class="lineno"> 135</span> The hashed string class uses internally FNV-1a to compute the numeric</div><div class="line"><a name="l00136"></a><span class="lineno"> 136</span> counterpart of a string. Because of the _pigeonhole principle_, conflicts are</div><div class="line"><a name="l00137"></a><span class="lineno"> 137</span> possible. This is a fact.<br/></div><div class="line"><a name="l00138"></a><span class="lineno"> 138</span> There is no silver bullet to solve the problem of conflicts when dealing with</div><div class="line"><a name="l00139"></a><span class="lineno"> 139</span> hashing functions. In this case, the best solution seemed to be to give up.</div><div class="line"><a name="l00140"></a><span class="lineno"> 140</span> That's all.<br/></div><div class="line"><a name="l00141"></a><span class="lineno"> 141</span> After all, human-readable resource identifiers aren't something strictly defined</div><div class="line"><a name="l00142"></a><span class="lineno"> 142</span> and over which users have not the control. Choosing a slightly different</div><div class="line"><a name="l00143"></a><span class="lineno"> 143</span> identifier is probably the best solution to make the conflict disappear in this</div><div class="line"><a name="l00144"></a><span class="lineno"> 144</span> case.</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> # Monostate</div><div class="line"><a name="l00147"></a><span class="lineno"> 147</span> </div><div class="line"><a name="l00148"></a><span class="lineno"> 148</span> The monostate pattern is often presented as an alternative to a singleton based</div><div class="line"><a name="l00149"></a><span class="lineno"> 149</span> configuration system. This is exactly its purpose in `EnTT`. Moreover, this</div><div class="line"><a name="l00150"></a><span class="lineno"> 150</span> implementation is thread safe by design (hopefully).<br/></div><div class="line"><a name="l00151"></a><span class="lineno"> 151</span> Keys are represented by hashed strings, values are basic types like `int`s or</div><div class="line"><a name="l00152"></a><span class="lineno"> 152</span> `bool`s. Values of different types can be associated to each key, even more than</div><div class="line"><a name="l00153"></a><span class="lineno"> 153</span> one at a time. Because of this, users must pay attention to use the same type</div><div class="line"><a name="l00154"></a><span class="lineno"> 154</span> both during an assignment and when they try to read back their data. Otherwise,</div><div class="line"><a name="l00155"></a><span class="lineno"> 155</span> they will probably incur in unexpected results.</div><div class="line"><a name="l00156"></a><span class="lineno"> 156</span> </div><div class="line"><a name="l00157"></a><span class="lineno"> 157</span> Example of use:</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> ```cpp</div><div class="line"><a name="l00160"></a><span class="lineno"> 160</span> entt::Monostate<entt::HashedString{"mykey"}>{} = true;</div><div class="line"><a name="l00161"></a><span class="lineno"> 161</span> entt::Monostate<"mykey"_hs>{} = 42;</div><div class="line"><a name="l00162"></a><span class="lineno"> 162</span> </div><div class="line"><a name="l00163"></a><span class="lineno"> 163</span> // ...</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> const bool b = entt::Monostate<"mykey"_hs>{};</div><div class="line"><a name="l00166"></a><span class="lineno"> 166</span> const int i = entt::Monostate<entt::HashedString{"mykey"}>{};</div><div class="line"><a name="l00167"></a><span class="lineno"> 167</span> ```</div></div><!-- fragment --></div><!-- contents -->
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