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<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno"> 1</span> # Crash Course: runtime reflection system</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> * [Names and identifiers](#names-and-identifiers)</div><div class="line"><a name="l00010"></a><span class="lineno"> 10</span> * [Reflection in a nutshell](#reflection-in-a-nutshell)</div><div class="line"><a name="l00011"></a><span class="lineno"> 11</span>  * [Any as in any type](#any-as-in-any-type)</div><div class="line"><a name="l00012"></a><span class="lineno"> 12</span>  * [Enjoy the runtime](#enjoy-the-runtime)</div><div class="line"><a name="l00013"></a><span class="lineno"> 13</span>  * [Policies: the more, the less](#policies-the-more-the-less)</div><div class="line"><a name="l00014"></a><span class="lineno"> 14</span>  * [Named constants and enums](#named-constants-and-enums)</div><div class="line"><a name="l00015"></a><span class="lineno"> 15</span>  * [Properties and meta objects](#properties-and-meta-objects)</div><div class="line"><a name="l00016"></a><span class="lineno"> 16</span>  * [Unregister types](#unregister-types)</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> @endcond TURN_OFF_DOXYGEN</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> </div><div class="line"><a name="l00021"></a><span class="lineno"> 21</span> # Introduction</div><div class="line"><a name="l00022"></a><span class="lineno"> 22</span> </div><div class="line"><a name="l00023"></a><span class="lineno"> 23</span> Reflection (or rather, its lack) is a trending topic in the C++ world and, in</div><div class="line"><a name="l00024"></a><span class="lineno"> 24</span> the specific case of `EnTT`, a tool that can unlock a lot of other features. I</div><div class="line"><a name="l00025"></a><span class="lineno"> 25</span> looked for a third-party library that met my needs on the subject, but I always</div><div class="line"><a name="l00026"></a><span class="lineno"> 26</span> came across some details that I didn't like: macros, being intrusive, too many</div><div class="line"><a name="l00027"></a><span class="lineno"> 27</span> allocations. In one word: unsatisfactory.<br/></div><div class="line"><a name="l00028"></a><span class="lineno"> 28</span> I finally decided to write a built-in, non-intrusive and macro-free runtime</div><div class="line"><a name="l00029"></a><span class="lineno"> 29</span> reflection system for `EnTT`. Maybe I didn't do better than others or maybe yes,</div><div class="line"><a name="l00030"></a><span class="lineno"> 30</span> time will tell me, but at least I can model this tool around the library to</div><div class="line"><a name="l00031"></a><span class="lineno"> 31</span> which it belongs and not vice versa.</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> # Names and identifiers</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> The meta system doesn't force the user to use the tools provided by the library</div><div class="line"><a name="l00036"></a><span class="lineno"> 36</span> when it comes to working with names and identifiers. It does this by offering an</div><div class="line"><a name="l00037"></a><span class="lineno"> 37</span> API that works with opaque identifiers that may or may not be generated by means</div><div class="line"><a name="l00038"></a><span class="lineno"> 38</span> of a hashed string.<br/></div><div class="line"><a name="l00039"></a><span class="lineno"> 39</span> This means that users can assign any type of identifier to the meta objects, as</div><div class="line"><a name="l00040"></a><span class="lineno"> 40</span> long as they are numeric. It doesn't matter if they are generated at runtime, at</div><div class="line"><a name="l00041"></a><span class="lineno"> 41</span> compile-time or with custom functions.</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> However, the examples in the following sections are all based on the</div><div class="line"><a name="l00044"></a><span class="lineno"> 44</span> `hashed_string` class as provided by this library. Therefore, where an</div><div class="line"><a name="l00045"></a><span class="lineno"> 45</span> identifier is required, it's likely that a user defined literal is used as</div><div class="line"><a name="l00046"></a><span class="lineno"> 46</span> follows:</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> ```cpp</div><div class="line"><a name="l00049"></a><span class="lineno"> 49</span> auto factory = entt::reflect<my_type>("reflected_type"_hs);</div><div class="line"><a name="l00050"></a><span class="lineno"> 50</span> ```</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 what it's worth, this is likely completely equivalent to:</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> ```cpp</div><div class="line"><a name="l00055"></a><span class="lineno"> 55</span> auto factory = entt::reflect<my_type>(42);</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> Obviously, human-readable identifiers are more convenient to use and highly</div><div class="line"><a name="l00059"></a><span class="lineno"> 59</span> recommended.</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> # Reflection in a nutshell</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> Reflection always starts from real types (users cannot reflect imaginary types</div><div class="line"><a name="l00064"></a><span class="lineno"> 64</span> and it would not make much sense, we wouldn't be talking about reflection</div><div class="line"><a name="l00065"></a><span class="lineno"> 65</span> anymore).<br/></div><div class="line"><a name="l00066"></a><span class="lineno"> 66</span> To _reflect_ a type, the library provides the `reflect` function:</div><div class="line"><a name="l00067"></a><span class="lineno"> 67</span> </div><div class="line"><a name="l00068"></a><span class="lineno"> 68</span> ```cpp</div><div class="line"><a name="l00069"></a><span class="lineno"> 69</span> auto factory = entt::reflect<my_type>("reflected_type"_hs);</div><div class="line"><a name="l00070"></a><span class="lineno"> 70</span> ```</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> It accepts the type to reflect as a template parameter and an optional</div><div class="line"><a name="l00073"></a><span class="lineno"> 73</span> identifier as an argument. Identifiers are important because users can retrieve</div><div class="line"><a name="l00074"></a><span class="lineno"> 74</span> meta types at runtime by searching for them by _name_. However, there are cases</div><div class="line"><a name="l00075"></a><span class="lineno"> 75</span> in which users can be interested in adding features to a reflected type so that</div><div class="line"><a name="l00076"></a><span class="lineno"> 76</span> the reflection system can use it correctly under the hood, but they don't want</div><div class="line"><a name="l00077"></a><span class="lineno"> 77</span> to allow searching the type by _name_.<br/></div><div class="line"><a name="l00078"></a><span class="lineno"> 78</span> In both cases, the returned value is a factory object to use to continue</div><div class="line"><a name="l00079"></a><span class="lineno"> 79</span> building the meta type.</div><div class="line"><a name="l00080"></a><span class="lineno"> 80</span> </div><div class="line"><a name="l00081"></a><span class="lineno"> 81</span> A factory is such that all its member functions returns the factory itself.</div><div class="line"><a name="l00082"></a><span class="lineno"> 82</span> It can be used to extend the reflected type and add the following:</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> * _Constructors_. Actual constructors can be assigned to a reflected type by</div><div class="line"><a name="l00085"></a><span class="lineno"> 85</span>  specifying their list of arguments. Free functions (namely, factories) can be</div><div class="line"><a name="l00086"></a><span class="lineno"> 86</span>  used as well, as long as the return type is the expected one. From a client's</div><div class="line"><a name="l00087"></a><span class="lineno"> 87</span>  point of view, nothing changes if a constructor is a free function or an</div><div class="line"><a name="l00088"></a><span class="lineno"> 88</span>  actual constructor.<br/></div><div class="line"><a name="l00089"></a><span class="lineno"> 89</span>  Use the `ctor` member function for this purpose:</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>  ```cpp</div><div class="line"><a name="l00092"></a><span class="lineno"> 92</span>  entt::reflect<my_type>("reflected"_hs).ctor<int, char>().ctor<&factory>();</div><div class="line"><a name="l00093"></a><span class="lineno"> 93</span>  ```</div><div class="line"><a name="l00094"></a><span class="lineno"> 94</span> </div><div class="line"><a name="l00095"></a><span class="lineno"> 95</span> * _Destructors_. Free functions can be set as destructors of reflected types.</div><div class="line"><a name="l00096"></a><span class="lineno"> 96</span>  The purpose is to give users the ability to free up resources that require</div><div class="line"><a name="l00097"></a><span class="lineno"> 97</span>  special treatment before an object is actually destroyed.<br/></div><div class="line"><a name="l00098"></a><span class="lineno"> 98</span>  Use the `dtor` member function for this purpose:</div><div class="line"><a name="l00099"></a><span class="lineno"> 99</span> </div><div class="line"><a name="l00100"></a><span class="lineno"> 100</span>  ```cpp</div><div class="line"><a name="l00101"></a><span class="lineno"> 101</span>  entt::reflect<my_type>("reflected"_hs).dtor<&destroy>();</div><div class="line"><a name="l00102"></a><span class="lineno"> 102</span>  ```</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>  A function should neither delete nor explicitly invoke the destructor of a</div><div class="line"><a name="l00105"></a><span class="lineno"> 105</span>  given instance.</div><div class="line"><a name="l00106"></a><span class="lineno"> 106</span> </div><div class="line"><a name="l00107"></a><span class="lineno"> 107</span> * _Data members_. Both real data members of the underlying type and static and</div><div class="line"><a name="l00108"></a><span class="lineno"> 108</span>  global variables, as well as constants of any kind, can be attached to a meta</div><div class="line"><a name="l00109"></a><span class="lineno"> 109</span>  type. From a client's point of view, all the variables associated with the</div><div class="line"><a name="l00110"></a><span class="lineno"> 110</span>  reflected type will appear as if they were part of the type itself.<br/></div><div class="line"><a name="l00111"></a><span class="lineno"> 111</span>  Use the `data` member function for this purpose:</div><div class="line"><a name="l00112"></a><span class="lineno"> 112</span> </div><div class="line"><a name="l00113"></a><span class="lineno"> 113</span>  ```cpp</div><div class="line"><a name="l00114"></a><span class="lineno"> 114</span>  entt::reflect<my_type>("reflected"_hs)</div><div class="line"><a name="l00115"></a><span class="lineno"> 115</span>  .data<&my_type::static_variable>("static"_hs)</div><div class="line"><a name="l00116"></a><span class="lineno"> 116</span>  .data<&my_type::data_member>("member"_hs)</div><div class="line"><a name="l00117"></a><span class="lineno"> 117</span>  .data<&global_variable>("global"_hs);</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> </div><div class="line"><a name="l00120"></a><span class="lineno"> 120</span>  This function requires as an argument the identifier to give to the meta data</div><div class="line"><a name="l00121"></a><span class="lineno"> 121</span>  once created. Users can then access meta data at runtime by searching for them</div><div class="line"><a name="l00122"></a><span class="lineno"> 122</span>  by _name_.<br/></div><div class="line"><a name="l00123"></a><span class="lineno"> 123</span>  Data members can be set also by means of a couple of functions, namely a</div><div class="line"><a name="l00124"></a><span class="lineno"> 124</span>  setter and a getter. Setters and getters can be either free functions, member</div><div class="line"><a name="l00125"></a><span class="lineno"> 125</span>  functions or mixed ones, as long as they respect the required signatures.<br/></div><div class="line"><a name="l00126"></a><span class="lineno"> 126</span>  Refer to the inline documentation for all the details.</div><div class="line"><a name="l00127"></a><span class="lineno"> 127</span> </div><div class="line"><a name="l00128"></a><span class="lineno"> 128</span> * _Member functions_. Both real member functions of the underlying type and free</div><div class="line"><a name="l00129"></a><span class="lineno"> 129</span>  functions can be attached to a meta type. From a client's point of view, all</div><div class="line"><a name="l00130"></a><span class="lineno"> 130</span>  the functions associated with the reflected type will appear as if they were</div><div class="line"><a name="l00131"></a><span class="lineno"> 131</span>  part of the type itself.<br/></div><div class="line"><a name="l00132"></a><span class="lineno"> 132</span>  Use the `func` member function for this purpose:</div><div class="line"><a name="l00133"></a><span class="lineno"> 133</span> </div><div class="line"><a name="l00134"></a><span class="lineno"> 134</span>  ```cpp</div><div class="line"><a name="l00135"></a><span class="lineno"> 135</span>  entt::reflect<my_type>("reflected"_hs)</div><div class="line"><a name="l00136"></a><span class="lineno"> 136</span>  .func<&my_type::static_function>("static"_hs)</div><div class="line"><a name="l00137"></a><span class="lineno"> 137</span>  .func<&my_type::member_function>("member"_hs)</div><div class="line"><a name="l00138"></a><span class="lineno"> 138</span>  .func<&free_function>("free"_hs);</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>  This function requires as an argument the identifier to give to the meta</div><div class="line"><a name="l00142"></a><span class="lineno"> 142</span>  function once created. Users can then access meta functions at runtime by</div><div class="line"><a name="l00143"></a><span class="lineno"> 143</span>  searching for them by _name_.</div><div class="line"><a name="l00144"></a><span class="lineno"> 144</span> </div><div class="line"><a name="l00145"></a><span class="lineno"> 145</span> * _Base classes_. A base class is such that the underlying type is actually</div><div class="line"><a name="l00146"></a><span class="lineno"> 146</span>  derived from it. In this case, the reflection system tracks the relationship</div><div class="line"><a name="l00147"></a><span class="lineno"> 147</span>  and allows for implicit casts at runtime when required.<br/></div><div class="line"><a name="l00148"></a><span class="lineno"> 148</span>  Use the `base` member function for this purpose:</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>  ```cpp</div><div class="line"><a name="l00151"></a><span class="lineno"> 151</span>  entt::reflect<derived_type>("derived"_hs).base<base_type>();</div><div class="line"><a name="l00152"></a><span class="lineno"> 152</span>  ```</div><div class="line"><a name="l00153"></a><span class="lineno"> 153</span> </div><div class="line"><a name="l00154"></a><span class="lineno"> 154</span>  From now on, wherever a `base_type` is required, an instance of `derived_type`</div><div class="line"><a name="l00155"></a><span class="lineno"> 155</span>  will also be accepted.</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> * _Conversion functions_. Actual types can be converted, this is a fact. Just</div><div class="line"><a name="l00158"></a><span class="lineno"> 158</span>  think of the relationship between a `double` and an `int` to see it. Similar</div><div class="line"><a name="l00159"></a><span class="lineno"> 159</span>  to bases, conversion functions allow users to define conversions that will be</div><div class="line"><a name="l00160"></a><span class="lineno"> 160</span>  implicitly performed by the reflection system when required.<br/></div><div class="line"><a name="l00161"></a><span class="lineno"> 161</span>  Use the `conv` member function for this purpose:</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>  ```cpp</div><div class="line"><a name="l00164"></a><span class="lineno"> 164</span>  entt::reflect<double>().conv<int>();</div><div class="line"><a name="l00165"></a><span class="lineno"> 165</span>  ```</div><div class="line"><a name="l00166"></a><span class="lineno"> 166</span> </div><div class="line"><a name="l00167"></a><span class="lineno"> 167</span> That's all, everything users need to create meta types and enjoy the reflection</div><div class="line"><a name="l00168"></a><span class="lineno"> 168</span> system. At first glance it may not seem that much, but users usually learn to</div><div class="line"><a name="l00169"></a><span class="lineno"> 169</span> appreciate it over time.<br/></div><div class="line"><a name="l00170"></a><span class="lineno"> 170</span> Also, do not forget what these few lines hide under the hood: a built-in,</div><div class="line"><a name="l00171"></a><span class="lineno"> 171</span> non-intrusive and macro-free system for reflection in C++. Features that are</div><div class="line"><a name="l00172"></a><span class="lineno"> 172</span> definitely worth the price, at least for me.</div><div class="line"><a name="l00173"></a><span class="lineno"> 173</span> </div><div class="line"><a name="l00174"></a><span class="lineno"> 174</span> ## Any as in any type</div><div class="line"><a name="l00175"></a><span class="lineno"> 175</span> </div><div class="line"><a name="l00176"></a><span class="lineno"> 176</span> The reflection system comes with its own meta any type. It may seem redundant</div><div class="line"><a name="l00177"></a><span class="lineno"> 177</span> since C++17 introduced `std::any`, but it is not.<br/></div><div class="line"><a name="l00178"></a><span class="lineno"> 178</span> In fact, the _type_ returned by an `std::any` is a const reference to an</div><div class="line"><a name="l00179"></a><span class="lineno"> 179</span> `std::type_info`, an implementation defined class that's not something everyone</div><div class="line"><a name="l00180"></a><span class="lineno"> 180</span> wants to see in a software. Furthermore, the class `std::type_info` suffers from</div><div class="line"><a name="l00181"></a><span class="lineno"> 181</span> some design flaws and there is even no way to _convert_ an `std::type_info` into</div><div class="line"><a name="l00182"></a><span class="lineno"> 182</span> a meta type, thus linking the two worlds.</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> A meta any object provides an API similar to that of its most famous counterpart</div><div class="line"><a name="l00185"></a><span class="lineno"> 185</span> and serves the same purpose of being an opaque container for any type of</div><div class="line"><a name="l00186"></a><span class="lineno"> 186</span> value.<br/></div><div class="line"><a name="l00187"></a><span class="lineno"> 187</span> It minimizes the allocations required, which are almost absent thanks to _SBO_</div><div class="line"><a name="l00188"></a><span class="lineno"> 188</span> techniques. In fact, unless users deal with _fat types_ and create instances of</div><div class="line"><a name="l00189"></a><span class="lineno"> 189</span> them though the reflection system, allocations are at zero.</div><div class="line"><a name="l00190"></a><span class="lineno"> 190</span> </div><div class="line"><a name="l00191"></a><span class="lineno"> 191</span> A meta any object can be created by any other object or as an empty container</div><div class="line"><a name="l00192"></a><span class="lineno"> 192</span> to initialize later:</div><div class="line"><a name="l00193"></a><span class="lineno"> 193</span> </div><div class="line"><a name="l00194"></a><span class="lineno"> 194</span> ```cpp</div><div class="line"><a name="l00195"></a><span class="lineno"> 195</span> // a meta any object that contains an int</div><div class="line"><a name="l00196"></a><span class="lineno"> 196</span> entt::meta_any any{0};</div><div class="line"><a name="l00197"></a><span class="lineno"> 197</span> </div><div class="line"><a name="l00198"></a><span class="lineno"> 198</span> // an empty meta any object</div><div class="line"><a name="l00199"></a><span class="lineno"> 199</span> entt::meta_any empty{};</div><div class="line"><a name="l00200"></a><span class="lineno"> 200</span> ```</div><div class="line"><a name="l00201"></a><span class="lineno"> 201</span> </div><div class="line"><a name="l00202"></a><span class="lineno"> 202</span> It takes the burden of destroying the contained instance when required.<br/></div><div class="line"><a name="l00203"></a><span class="lineno"> 203</span> Moreover, it can be used as an opaque container for unmanaged objects if needed:</div><div class="line"><a name="l00204"></a><span class="lineno"> 204</span> </div><div class="line"><a name="l00205"></a><span class="lineno"> 205</span> ```cpp</div><div class="line"><a name="l00206"></a><span class="lineno"> 206</span> int value;</div><div class="line"><a name="l00207"></a><span class="lineno"> 207</span> entt::meta_any any{entt::as_alias, value};</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> </div><div class="line"><a name="l00210"></a><span class="lineno"> 210</span> In this case, the contained instance is never destroyed and users must ensure</div><div class="line"><a name="l00211"></a><span class="lineno"> 211</span> that the lifetime of the object exceeds that of the container.</div><div class="line"><a name="l00212"></a><span class="lineno"> 212</span> </div><div class="line"><a name="l00213"></a><span class="lineno"> 213</span> A meta any object has a `type` member function that returns the meta type of the</div><div class="line"><a name="l00214"></a><span class="lineno"> 214</span> contained value, if any. The member functions `try_cast`, `cast` and `convert`</div><div class="line"><a name="l00215"></a><span class="lineno"> 215</span> are used to know if the underlying object has a given type as a base or if it</div><div class="line"><a name="l00216"></a><span class="lineno"> 216</span> can be converted implicitly to it.</div><div class="line"><a name="l00217"></a><span class="lineno"> 217</span> </div><div class="line"><a name="l00218"></a><span class="lineno"> 218</span> ## Enjoy the runtime</div><div class="line"><a name="l00219"></a><span class="lineno"> 219</span> </div><div class="line"><a name="l00220"></a><span class="lineno"> 220</span> Once the web of reflected types has been constructed, it's a matter of using it</div><div class="line"><a name="l00221"></a><span class="lineno"> 221</span> at runtime where required.<br/></div><div class="line"><a name="l00222"></a><span class="lineno"> 222</span> All this has the great merit that, unlike the vast majority of the things</div><div class="line"><a name="l00223"></a><span class="lineno"> 223</span> present in this library and closely linked to the compile-time, the reflection</div><div class="line"><a name="l00224"></a><span class="lineno"> 224</span> system stands in fact as a non-intrusive tool for the runtime.</div><div class="line"><a name="l00225"></a><span class="lineno"> 225</span> </div><div class="line"><a name="l00226"></a><span class="lineno"> 226</span> To search for a reflected type there are two options: by type or by _name_. In</div><div class="line"><a name="l00227"></a><span class="lineno"> 227</span> both cases, the search can be done by means of the `resolve` function:</div><div class="line"><a name="l00228"></a><span class="lineno"> 228</span> </div><div class="line"><a name="l00229"></a><span class="lineno"> 229</span> ```cpp</div><div class="line"><a name="l00230"></a><span class="lineno"> 230</span> // search for a reflected type by type</div><div class="line"><a name="l00231"></a><span class="lineno"> 231</span> auto by_type = entt::resolve<my_type>();</div><div class="line"><a name="l00232"></a><span class="lineno"> 232</span> </div><div class="line"><a name="l00233"></a><span class="lineno"> 233</span> // search for a reflected type by name</div><div class="line"><a name="l00234"></a><span class="lineno"> 234</span> auto by_name = entt::resolve("reflected_type"_hs);</div><div class="line"><a name="l00235"></a><span class="lineno"> 235</span> ```</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> There exits also a third overload of the `resolve` function to use to iterate</div><div class="line"><a name="l00238"></a><span class="lineno"> 238</span> all the reflected types at once:</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> ```cpp</div><div class="line"><a name="l00241"></a><span class="lineno"> 241</span> resolve([](auto type) {</div><div class="line"><a name="l00242"></a><span class="lineno"> 242</span>  // ...</div><div class="line"><a name="l00243"></a><span class="lineno"> 243</span> });</div><div class="line"><a name="l00244"></a><span class="lineno"> 244</span> ```</div><div class="line"><a name="l00245"></a><span class="lineno"> 245</span> </div><div class="line"><a name="l00246"></a><span class="lineno"> 246</span> In all cases, the returned value is an instance of `meta_type`. This type of</div><div class="line"><a name="l00247"></a><span class="lineno"> 247</span> objects offer an API to know the _runtime identifier_ of the type, to iterate</div><div class="line"><a name="l00248"></a><span class="lineno"> 248</span> all the meta objects associated with them and even to build or destroy instances</div><div class="line"><a name="l00249"></a><span class="lineno"> 249</span> of the underlying type.<br/></div><div class="line"><a name="l00250"></a><span class="lineno"> 250</span> Refer to the inline documentation for all the details.</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> The meta objects that compose a meta type are accessed in the following ways:</div><div class="line"><a name="l00253"></a><span class="lineno"> 253</span> </div><div class="line"><a name="l00254"></a><span class="lineno"> 254</span> * _Meta constructors_. They are accessed by types of arguments:</div><div class="line"><a name="l00255"></a><span class="lineno"> 255</span> </div><div class="line"><a name="l00256"></a><span class="lineno"> 256</span>  ```cpp</div><div class="line"><a name="l00257"></a><span class="lineno"> 257</span>  auto ctor = entt::resolve<my_type>().ctor<int, char>();</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> </div><div class="line"><a name="l00260"></a><span class="lineno"> 260</span>  The returned type is `meta_ctor` and may be invalid if there is no constructor</div><div class="line"><a name="l00261"></a><span class="lineno"> 261</span>  that accepts the supplied arguments or at least some types from which they are</div><div class="line"><a name="l00262"></a><span class="lineno"> 262</span>  derived or to which they can be converted.<br/></div><div class="line"><a name="l00263"></a><span class="lineno"> 263</span>  A meta constructor offers an API to know the number of arguments, the expected</div><div class="line"><a name="l00264"></a><span class="lineno"> 264</span>  meta types and to invoke it, therefore to construct a new instance of the</div><div class="line"><a name="l00265"></a><span class="lineno"> 265</span>  underlying type.</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> * _Meta destructor_. It's returned by a dedicated function:</div><div class="line"><a name="l00268"></a><span class="lineno"> 268</span> </div><div class="line"><a name="l00269"></a><span class="lineno"> 269</span>  ```cpp</div><div class="line"><a name="l00270"></a><span class="lineno"> 270</span>  auto dtor = entt::resolve<my_type>().dtor();</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> </div><div class="line"><a name="l00273"></a><span class="lineno"> 273</span>  The returned type is `meta_dtor` and may be invalid if there is no custom</div><div class="line"><a name="l00274"></a><span class="lineno"> 274</span>  destructor set for the given meta type.<br/></div><div class="line"><a name="l00275"></a><span class="lineno"> 275</span>  All what a meta destructor has to offer is a way to invoke it on a given</div><div class="line"><a name="l00276"></a><span class="lineno"> 276</span>  instance. Be aware that the result may not be what is expected.</div><div class="line"><a name="l00277"></a><span class="lineno"> 277</span> </div><div class="line"><a name="l00278"></a><span class="lineno"> 278</span> * _Meta data_. They are accessed by _name_:</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>  auto data = entt::resolve<my_type>().data("member"_hs);</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>  The returned type is `meta_data` and may be invalid if there is no meta data</div><div class="line"><a name="l00285"></a><span class="lineno"> 285</span>  object associated with the given identifier.<br/></div><div class="line"><a name="l00286"></a><span class="lineno"> 286</span>  A meta data object offers an API to query the underlying type (ie to know if</div><div class="line"><a name="l00287"></a><span class="lineno"> 287</span>  it's a const or a static one), to get the meta type of the variable and to set</div><div class="line"><a name="l00288"></a><span class="lineno"> 288</span>  or get the contained value.</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> * _Meta functions_. They are accessed by _name_:</div><div class="line"><a name="l00291"></a><span class="lineno"> 291</span> </div><div class="line"><a name="l00292"></a><span class="lineno"> 292</span>  ```cpp</div><div class="line"><a name="l00293"></a><span class="lineno"> 293</span>  auto func = entt::resolve<my_type>().func("member"_hs);</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> </div><div class="line"><a name="l00296"></a><span class="lineno"> 296</span>  The returned type is `meta_func` and may be invalid if there is no meta</div><div class="line"><a name="l00297"></a><span class="lineno"> 297</span>  function object associated with the given identifier.<br/></div><div class="line"><a name="l00298"></a><span class="lineno"> 298</span>  A meta function object offers an API to query the underlying type (ie to know</div><div class="line"><a name="l00299"></a><span class="lineno"> 299</span>  if it's a const or a static function), to know the number of arguments, the</div><div class="line"><a name="l00300"></a><span class="lineno"> 300</span>  meta return type and the meta types of the parameters. In addition, a meta</div><div class="line"><a name="l00301"></a><span class="lineno"> 301</span>  function object can be used to invoke the underlying function and then get the</div><div class="line"><a name="l00302"></a><span class="lineno"> 302</span>  return value in the form of meta any object.</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> * _Meta bases_. They are accessed through the _name_ of the base types:</div><div class="line"><a name="l00305"></a><span class="lineno"> 305</span> </div><div class="line"><a name="l00306"></a><span class="lineno"> 306</span>  ```cpp</div><div class="line"><a name="l00307"></a><span class="lineno"> 307</span>  auto base = entt::resolve<derived_type>().base("base"_hs);</div><div class="line"><a name="l00308"></a><span class="lineno"> 308</span>  ```</div><div class="line"><a name="l00309"></a><span class="lineno"> 309</span> </div><div class="line"><a name="l00310"></a><span class="lineno"> 310</span>  The returned type is `meta_base` and may be invalid if there is no meta base</div><div class="line"><a name="l00311"></a><span class="lineno"> 311</span>  object associated with the given identifier.<br/></div><div class="line"><a name="l00312"></a><span class="lineno"> 312</span>  Meta bases aren't meant to be used directly, even though they are freely</div><div class="line"><a name="l00313"></a><span class="lineno"> 313</span>  accessible. They expose only a few methods to use to know the meta type of the</div><div class="line"><a name="l00314"></a><span class="lineno"> 314</span>  base class and to convert a raw pointer between types.</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> * _Meta conversion functions_. They are accessed by type:</div><div class="line"><a name="l00317"></a><span class="lineno"> 317</span> </div><div class="line"><a name="l00318"></a><span class="lineno"> 318</span>  ```cpp</div><div class="line"><a name="l00319"></a><span class="lineno"> 319</span>  auto conv = entt::resolve<double>().conv<int>();</div><div class="line"><a name="l00320"></a><span class="lineno"> 320</span>  ```</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>  The returned type is `meta_conv` and may be invalid if there is no meta</div><div class="line"><a name="l00323"></a><span class="lineno"> 323</span>  conversion function associated with the given type.<br/></div><div class="line"><a name="l00324"></a><span class="lineno"> 324</span>  The meta conversion functions are as thin as the meta bases and with a very</div><div class="line"><a name="l00325"></a><span class="lineno"> 325</span>  similar interface. The sole difference is that they return a newly created</div><div class="line"><a name="l00326"></a><span class="lineno"> 326</span>  instance wrapped in a meta any object when they convert between different</div><div class="line"><a name="l00327"></a><span class="lineno"> 327</span>  types.</div><div class="line"><a name="l00328"></a><span class="lineno"> 328</span> </div><div class="line"><a name="l00329"></a><span class="lineno"> 329</span> All the objects thus obtained as well as the meta types can be explicitly</div><div class="line"><a name="l00330"></a><span class="lineno"> 330</span> converted to a boolean value to check if they are valid:</div><div class="line"><a name="l00331"></a><span class="lineno"> 331</span> </div><div class="line"><a name="l00332"></a><span class="lineno"> 332</span> ```cpp</div><div class="line"><a name="l00333"></a><span class="lineno"> 333</span> auto func = entt::resolve<my_type>().func("member"_hs);</div><div class="line"><a name="l00334"></a><span class="lineno"> 334</span> </div><div class="line"><a name="l00335"></a><span class="lineno"> 335</span> if(func) {</div><div class="line"><a name="l00336"></a><span class="lineno"> 336</span>  // ...</div><div class="line"><a name="l00337"></a><span class="lineno"> 337</span> }</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> Furthermore, all meta objects with the exception of meta destructors can be</div><div class="line"><a name="l00341"></a><span class="lineno"> 341</span> iterated through an overload that accepts a callback through which to return</div><div class="line"><a name="l00342"></a><span class="lineno"> 342</span> them. As an example:</div><div class="line"><a name="l00343"></a><span class="lineno"> 343</span> </div><div class="line"><a name="l00344"></a><span class="lineno"> 344</span> ```cpp</div><div class="line"><a name="l00345"></a><span class="lineno"> 345</span> entt::resolve<my_type>().data([](auto data) {</div><div class="line"><a name="l00346"></a><span class="lineno"> 346</span>  // ...</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> </div><div class="line"><a name="l00350"></a><span class="lineno"> 350</span> A meta type can also be used to `construct` or `destroy` actual instances of the</div><div class="line"><a name="l00351"></a><span class="lineno"> 351</span> underlying type.<br/></div><div class="line"><a name="l00352"></a><span class="lineno"> 352</span> In particular, the `construct` member function accepts a variable number of</div><div class="line"><a name="l00353"></a><span class="lineno"> 353</span> arguments and searches for a match. It returns a `meta_any` object that may or</div><div class="line"><a name="l00354"></a><span class="lineno"> 354</span> may not be initialized, depending on whether a suitable constructor has been</div><div class="line"><a name="l00355"></a><span class="lineno"> 355</span> found or not. On the other side, the `destroy` member function accepts instances</div><div class="line"><a name="l00356"></a><span class="lineno"> 356</span> of `meta_any` as well as actual objects by reference and invokes the registered</div><div class="line"><a name="l00357"></a><span class="lineno"> 357</span> destructor if any.<br/></div><div class="line"><a name="l00358"></a><span class="lineno"> 358</span> Be aware that the result of a call to `destroy` may not be what is expected. The</div><div class="line"><a name="l00359"></a><span class="lineno"> 359</span> purpose is to give users the ability to free up resources that require special</div><div class="line"><a name="l00360"></a><span class="lineno"> 360</span> treatment and **not** to actually destroy instances.</div><div class="line"><a name="l00361"></a><span class="lineno"> 361</span> </div><div class="line"><a name="l00362"></a><span class="lineno"> 362</span> Meta types and meta objects in general contain much more than what is said: a</div><div class="line"><a name="l00363"></a><span class="lineno"> 363</span> plethora of functions in addition to those listed whose purposes and uses go</div><div class="line"><a name="l00364"></a><span class="lineno"> 364</span> unfortunately beyond the scope of this document.<br/></div><div class="line"><a name="l00365"></a><span class="lineno"> 365</span> I invite anyone interested in the subject to look at the code, experiment and</div><div class="line"><a name="l00366"></a><span class="lineno"> 366</span> read the official documentation to get the best out of this powerful tool.</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> ## Policies: the more, the less</div><div class="line"><a name="l00369"></a><span class="lineno"> 369</span> </div><div class="line"><a name="l00370"></a><span class="lineno"> 370</span> Policies are a kind of compile-time directives that can be used when recording</div><div class="line"><a name="l00371"></a><span class="lineno"> 371</span> reflection information.<br/></div><div class="line"><a name="l00372"></a><span class="lineno"> 372</span> Their purpose is to require slightly different behavior than the default in some</div><div class="line"><a name="l00373"></a><span class="lineno"> 373</span> specific cases. For example, when reading a given data member, its value is</div><div class="line"><a name="l00374"></a><span class="lineno"> 374</span> returned wrapped in a `meta_any` object which, by default, makes a copy of it.</div><div class="line"><a name="l00375"></a><span class="lineno"> 375</span> For large objects or if the caller wants to access the original instance, this</div><div class="line"><a name="l00376"></a><span class="lineno"> 376</span> behavior isn't desirable. Policies are there to offer a solution to this and</div><div class="line"><a name="l00377"></a><span class="lineno"> 377</span> other problems.</div><div class="line"><a name="l00378"></a><span class="lineno"> 378</span> </div><div class="line"><a name="l00379"></a><span class="lineno"> 379</span> There are a few alternatives available at the moment:</div><div class="line"><a name="l00380"></a><span class="lineno"> 380</span> </div><div class="line"><a name="l00381"></a><span class="lineno"> 381</span> * The _as-is_ policy, associated with the type `entt::as_is_t`.<br/></div><div class="line"><a name="l00382"></a><span class="lineno"> 382</span>  This is the default policy. In general, it should never be used explicitly,</div><div class="line"><a name="l00383"></a><span class="lineno"> 383</span>  since it's implicitly selected if no other policy is specified.<br/></div><div class="line"><a name="l00384"></a><span class="lineno"> 384</span>  In this case, the return values of the functions as well as the properties</div><div class="line"><a name="l00385"></a><span class="lineno"> 385</span>  exposed as data members are always returned by copy in a dedicated wrapper and</div><div class="line"><a name="l00386"></a><span class="lineno"> 386</span>  therefore associated with their original meta types.</div><div class="line"><a name="l00387"></a><span class="lineno"> 387</span> </div><div class="line"><a name="l00388"></a><span class="lineno"> 388</span> * The _as-void_ policy, associated with the type `entt::as_void_t`.<br/></div><div class="line"><a name="l00389"></a><span class="lineno"> 389</span>  Its purpose is to discard the return value of a meta object, whatever it is,</div><div class="line"><a name="l00390"></a><span class="lineno"> 390</span>  thus making it appear as if its type were `void`.<br/></div><div class="line"><a name="l00391"></a><span class="lineno"> 391</span>  If the use with functions is obvious, it must be said that it's also possible</div><div class="line"><a name="l00392"></a><span class="lineno"> 392</span>  to use this policy with constructors and data members. In the first case, the</div><div class="line"><a name="l00393"></a><span class="lineno"> 393</span>  constructor will be invoked but the returned wrapper will actually be empty.</div><div class="line"><a name="l00394"></a><span class="lineno"> 394</span>  In the second case, instead, the property will not be accessible for</div><div class="line"><a name="l00395"></a><span class="lineno"> 395</span>  reading.</div><div class="line"><a name="l00396"></a><span class="lineno"> 396</span> </div><div class="line"><a name="l00397"></a><span class="lineno"> 397</span>  As an example of use:</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>  ```cpp</div><div class="line"><a name="l00400"></a><span class="lineno"> 400</span>  entt::reflect<my_type>("reflected"_hs)</div><div class="line"><a name="l00401"></a><span class="lineno"> 401</span>  .func<&my_type::member_function, entt::as_void_t>("member"_hs);</div><div class="line"><a name="l00402"></a><span class="lineno"> 402</span>  ```</div><div class="line"><a name="l00403"></a><span class="lineno"> 403</span> </div><div class="line"><a name="l00404"></a><span class="lineno"> 404</span> * The _as-alias_ policy, associated with the type `entt::as_alias_t`.<br/></div><div class="line"><a name="l00405"></a><span class="lineno"> 405</span>  It allows to build wrappers that act as aliases for the objects used to</div><div class="line"><a name="l00406"></a><span class="lineno"> 406</span>  initialize them. Modifying the object contained in the wrapper for which the</div><div class="line"><a name="l00407"></a><span class="lineno"> 407</span>  _aliasing_ was requested will make it possible to directly modify the instance</div><div class="line"><a name="l00408"></a><span class="lineno"> 408</span>  used to initialize the wrapper itself.<br/></div><div class="line"><a name="l00409"></a><span class="lineno"> 409</span>  This policy works with constructors (for example, when objects are taken from</div><div class="line"><a name="l00410"></a><span class="lineno"> 410</span>  an external container rather than created on demand), data members and</div><div class="line"><a name="l00411"></a><span class="lineno"> 411</span>  functions in general (as long as their return types are lvalue references).</div><div class="line"><a name="l00412"></a><span class="lineno"> 412</span> </div><div class="line"><a name="l00413"></a><span class="lineno"> 413</span>  As an example of use:</div><div class="line"><a name="l00414"></a><span class="lineno"> 414</span> </div><div class="line"><a name="l00415"></a><span class="lineno"> 415</span>  ```cpp</div><div class="line"><a name="l00416"></a><span class="lineno"> 416</span>  entt::reflect<my_type>("reflected"_hs)</div><div class="line"><a name="l00417"></a><span class="lineno"> 417</span>  .data<&my_type::data_member, entt::as_alias_t>("member"_hs);</div><div class="line"><a name="l00418"></a><span class="lineno"> 418</span>  ```</div><div class="line"><a name="l00419"></a><span class="lineno"> 419</span> </div><div class="line"><a name="l00420"></a><span class="lineno"> 420</span> Some uses are rather trivial, but it's useful to note that there are some less</div><div class="line"><a name="l00421"></a><span class="lineno"> 421</span> obvious corner cases that can in turn be solved with the use of policies.</div><div class="line"><a name="l00422"></a><span class="lineno"> 422</span> </div><div class="line"><a name="l00423"></a><span class="lineno"> 423</span> ## Named constants and enums</div><div class="line"><a name="l00424"></a><span class="lineno"> 424</span> </div><div class="line"><a name="l00425"></a><span class="lineno"> 425</span> A special mention should be made for constant values and enums. It wouldn't be</div><div class="line"><a name="l00426"></a><span class="lineno"> 426</span> necessary, but it will help distracted readers.</div><div class="line"><a name="l00427"></a><span class="lineno"> 427</span> </div><div class="line"><a name="l00428"></a><span class="lineno"> 428</span> As mentioned, the `data` member function can be used to reflect constants of any</div><div class="line"><a name="l00429"></a><span class="lineno"> 429</span> type among the other things.<br/></div><div class="line"><a name="l00430"></a><span class="lineno"> 430</span> This allows users to create meta types for enums that will work exactly like any</div><div class="line"><a name="l00431"></a><span class="lineno"> 431</span> other meta type built from a class. Similarly, arithmetic types can be enriched</div><div class="line"><a name="l00432"></a><span class="lineno"> 432</span> with constants of special meaning where required.<br/></div><div class="line"><a name="l00433"></a><span class="lineno"> 433</span> Personally, I find it very useful not to export what is the difference between</div><div class="line"><a name="l00434"></a><span class="lineno"> 434</span> enums and classes in C++ directly in the space of the reflected types.</div><div class="line"><a name="l00435"></a><span class="lineno"> 435</span> </div><div class="line"><a name="l00436"></a><span class="lineno"> 436</span> All the values thus exported will appear to users as if they were constant data</div><div class="line"><a name="l00437"></a><span class="lineno"> 437</span> members of the reflected types.</div><div class="line"><a name="l00438"></a><span class="lineno"> 438</span> </div><div class="line"><a name="l00439"></a><span class="lineno"> 439</span> Exporting constant values or elements from an enum is as simple as ever:</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> ```cpp</div><div class="line"><a name="l00442"></a><span class="lineno"> 442</span> entt::reflect<my_enum>()</div><div class="line"><a name="l00443"></a><span class="lineno"> 443</span>  .data<my_enum::a_value>("a_value"_hs)</div><div class="line"><a name="l00444"></a><span class="lineno"> 444</span>  .data<my_enum::another_value>("another_value"_hs);</div><div class="line"><a name="l00445"></a><span class="lineno"> 445</span> </div><div class="line"><a name="l00446"></a><span class="lineno"> 446</span> entt::reflect<int>().data<2048>("max_int"_hs);</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> </div><div class="line"><a name="l00449"></a><span class="lineno"> 449</span> It goes without saying that accessing them is trivial as well. It's a matter of</div><div class="line"><a name="l00450"></a><span class="lineno"> 450</span> doing the following, as with any other data member of a meta type:</div><div class="line"><a name="l00451"></a><span class="lineno"> 451</span> </div><div class="line"><a name="l00452"></a><span class="lineno"> 452</span> ```cpp</div><div class="line"><a name="l00453"></a><span class="lineno"> 453</span> auto value = entt::resolve<my_enum>().data("a_value"_hs).get({}).cast<my_enum>();</div><div class="line"><a name="l00454"></a><span class="lineno"> 454</span> auto max = entt::resolve<int>().data("max_int"_hs).get({}).cast<int>();</div><div class="line"><a name="l00455"></a><span class="lineno"> 455</span> ```</div><div class="line"><a name="l00456"></a><span class="lineno"> 456</span> </div><div class="line"><a name="l00457"></a><span class="lineno"> 457</span> As a side note, remember that all this happens behind the scenes without any</div><div class="line"><a name="l00458"></a><span class="lineno"> 458</span> allocation because of the small object optimization performed by the meta any</div><div class="line"><a name="l00459"></a><span class="lineno"> 459</span> class.</div><div class="line"><a name="l00460"></a><span class="lineno"> 460</span> </div><div class="line"><a name="l00461"></a><span class="lineno"> 461</span> ## Properties and meta objects</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> Sometimes (for example, when it comes to creating an editor) it might be useful</div><div class="line"><a name="l00464"></a><span class="lineno"> 464</span> to be able to attach properties to the meta objects created. Fortunately, this</div><div class="line"><a name="l00465"></a><span class="lineno"> 465</span> is possible for most of them.<br/></div><div class="line"><a name="l00466"></a><span class="lineno"> 466</span> To attach a property to a meta object, no matter what as long as it supports</div><div class="line"><a name="l00467"></a><span class="lineno"> 467</span> properties, it is sufficient to provide an object at the time of construction</div><div class="line"><a name="l00468"></a><span class="lineno"> 468</span> such that `std::get<0>` and `std::get<1>` are valid for it. In other terms, the</div><div class="line"><a name="l00469"></a><span class="lineno"> 469</span> properties are nothing more than key/value pairs users can put in an</div><div class="line"><a name="l00470"></a><span class="lineno"> 470</span> `std::pair`. As an example:</div><div class="line"><a name="l00471"></a><span class="lineno"> 471</span> </div><div class="line"><a name="l00472"></a><span class="lineno"> 472</span> ```cpp</div><div class="line"><a name="l00473"></a><span class="lineno"> 473</span> entt::reflect<my_type>("reflected"_hs, std::make_pair("tooltip"_hs, "message"));</div><div class="line"><a name="l00474"></a><span class="lineno"> 474</span> ```</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> The meta objects that support properties offer then a couple of member functions</div><div class="line"><a name="l00477"></a><span class="lineno"> 477</span> named `prop` to iterate them at once and to search a specific property by key:</div><div class="line"><a name="l00478"></a><span class="lineno"> 478</span> </div><div class="line"><a name="l00479"></a><span class="lineno"> 479</span> ```cpp</div><div class="line"><a name="l00480"></a><span class="lineno"> 480</span> // iterate all the properties of a meta type</div><div class="line"><a name="l00481"></a><span class="lineno"> 481</span> entt::resolve<my_type>().prop([](auto prop) {</div><div class="line"><a name="l00482"></a><span class="lineno"> 482</span>  // ...</div><div class="line"><a name="l00483"></a><span class="lineno"> 483</span> });</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> // search for a given property by name</div><div class="line"><a name="l00486"></a><span class="lineno"> 486</span> auto prop = entt::resolve<my_type>().prop("tooltip"_hs);</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> Meta properties are objects having a fairly poor interface, all in all. They</div><div class="line"><a name="l00490"></a><span class="lineno"> 490</span> only provide the `key` and the `value` member functions to be used to retrieve</div><div class="line"><a name="l00491"></a><span class="lineno"> 491</span> the key and the value contained in the form of meta any objects, respectively.</div><div class="line"><a name="l00492"></a><span class="lineno"> 492</span> </div><div class="line"><a name="l00493"></a><span class="lineno"> 493</span> ## Unregister types</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> A type registered with the reflection system can also be unregistered. This</div><div class="line"><a name="l00496"></a><span class="lineno"> 496</span> means unregistering all its data members, member functions, conversion functions</div><div class="line"><a name="l00497"></a><span class="lineno"> 497</span> and so on. However, the base classes won't be unregistered, since they don't</div><div class="line"><a name="l00498"></a><span class="lineno"> 498</span> necessarily depend on it. Similarly, implicitly generated types (as an example,</div><div class="line"><a name="l00499"></a><span class="lineno"> 499</span> the meta types implicitly generated for function parameters when needed) won't</div><div class="line"><a name="l00500"></a><span class="lineno"> 500</span> be unregistered.</div><div class="line"><a name="l00501"></a><span class="lineno"> 501</span> </div><div class="line"><a name="l00502"></a><span class="lineno"> 502</span> To unregister a type, users can use the `unregister` function from the global</div><div class="line"><a name="l00503"></a><span class="lineno"> 503</span> namespace:</div><div class="line"><a name="l00504"></a><span class="lineno"> 504</span> </div><div class="line"><a name="l00505"></a><span class="lineno"> 505</span> ```cpp</div><div class="line"><a name="l00506"></a><span class="lineno"> 506</span> entt::unregister<my_type>();</div><div class="line"><a name="l00507"></a><span class="lineno"> 507</span> ```</div><div class="line"><a name="l00508"></a><span class="lineno"> 508</span> </div><div class="line"><a name="l00509"></a><span class="lineno"> 509</span> This function returns a boolean value that is true if the type is actually</div><div class="line"><a name="l00510"></a><span class="lineno"> 510</span> registered with the reflection system, false otherwise.<br/></div><div class="line"><a name="l00511"></a><span class="lineno"> 511</span> The type can be re-registered later with a completely different name and form.</div></div><!-- fragment --></div><!-- contents -->
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