Files
entt/meta_8md_source.html
2019-09-05 15:42:26 +02:00

75 lines
65 KiB
HTML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/xhtml;charset=UTF-8"/>
<meta http-equiv="X-UA-Compatible" content="IE=9"/>
<meta name="generator" content="Doxygen 1.8.13"/>
<meta name="viewport" content="width=device-width, initial-scale=1"/>
<title>EnTT: docs/md/meta.md Source File</title>
<link href="tabs.css" rel="stylesheet" type="text/css"/>
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript" src="dynsections.js"></script>
<link href="search/search.css" rel="stylesheet" type="text/css"/>
<script type="text/javascript" src="search/searchdata.js"></script>
<script type="text/javascript" src="search/search.js"></script>
<link href="doxygen.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div id="top"><!-- do not remove this div, it is closed by doxygen! -->
<div id="titlearea">
<table cellspacing="0" cellpadding="0">
<tbody>
<tr style="height: 56px;">
<td id="projectalign" style="padding-left: 0.5em;">
<div id="projectname">EnTT
&#160;<span id="projectnumber">3.1.1</span>
</div>
</td>
</tr>
</tbody>
</table>
</div>
<!-- end header part -->
<!-- Generated by Doxygen 1.8.13 -->
<script type="text/javascript">
var searchBox = new SearchBox("searchBox", "search",false,'Search');
</script>
<script type="text/javascript" src="menudata.js"></script>
<script type="text/javascript" src="menu.js"></script>
<script type="text/javascript">
$(function() {
initMenu('',true,false,'search.php','Search');
$(document).ready(function() { init_search(); });
});
</script>
<div id="main-nav"></div>
</div><!-- top -->
<!-- window showing the filter options -->
<div id="MSearchSelectWindow"
onmouseover="return searchBox.OnSearchSelectShow()"
onmouseout="return searchBox.OnSearchSelectHide()"
onkeydown="return searchBox.OnSearchSelectKey(event)">
</div>
<!-- iframe showing the search results (closed by default) -->
<div id="MSearchResultsWindow">
<iframe src="javascript:void(0)" frameborder="0"
name="MSearchResults" id="MSearchResults">
</iframe>
</div>
<div class="header">
<div class="headertitle">
<div class="title">docs/md/meta.md</div> </div>
</div><!--header-->
<div class="contents">
<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno"> 1</span>&#160;# Crash Course: runtime reflection system</div><div class="line"><a name="l00002"></a><span class="lineno"> 2</span>&#160;</div><div class="line"><a name="l00003"></a><span class="lineno"> 3</span>&#160;&lt;!--</div><div class="line"><a name="l00004"></a><span class="lineno"> 4</span>&#160;@cond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00005"></a><span class="lineno"> 5</span>&#160;--&gt;</div><div class="line"><a name="l00006"></a><span class="lineno"> 6</span>&#160;# Table of Contents</div><div class="line"><a name="l00007"></a><span class="lineno"> 7</span>&#160;</div><div class="line"><a name="l00008"></a><span class="lineno"> 8</span>&#160;* [Introduction](#introduction)</div><div class="line"><a name="l00009"></a><span class="lineno"> 9</span>&#160;* [Names and identifiers](#names-and-identifiers)</div><div class="line"><a name="l00010"></a><span class="lineno"> 10</span>&#160;* [Reflection in a nutshell](#reflection-in-a-nutshell)</div><div class="line"><a name="l00011"></a><span class="lineno"> 11</span>&#160; * [Any as in any type](#any-as-in-any-type)</div><div class="line"><a name="l00012"></a><span class="lineno"> 12</span>&#160; * [Enjoy the runtime](#enjoy-the-runtime)</div><div class="line"><a name="l00013"></a><span class="lineno"> 13</span>&#160; * [Policies: the more, the less](#policies-the-more-the-less)</div><div class="line"><a name="l00014"></a><span class="lineno"> 14</span>&#160; * [Named constants and enums](#named-constants-and-enums)</div><div class="line"><a name="l00015"></a><span class="lineno"> 15</span>&#160; * [Properties and meta objects](#properties-and-meta-objects)</div><div class="line"><a name="l00016"></a><span class="lineno"> 16</span>&#160; * [Unregister types](#unregister-types)</div><div class="line"><a name="l00017"></a><span class="lineno"> 17</span>&#160;&lt;!--</div><div class="line"><a name="l00018"></a><span class="lineno"> 18</span>&#160;@endcond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00019"></a><span class="lineno"> 19</span>&#160;--&gt;</div><div class="line"><a name="l00020"></a><span class="lineno"> 20</span>&#160;</div><div class="line"><a name="l00021"></a><span class="lineno"> 21</span>&#160;# Introduction</div><div class="line"><a name="l00022"></a><span class="lineno"> 22</span>&#160;</div><div class="line"><a name="l00023"></a><span class="lineno"> 23</span>&#160;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>&#160;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>&#160;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>&#160;came across some details that I didn&#39;t like: macros, being intrusive, too many</div><div class="line"><a name="l00027"></a><span class="lineno"> 27</span>&#160;allocations. In one word: unsatisfactory.&lt;br/&gt;</div><div class="line"><a name="l00028"></a><span class="lineno"> 28</span>&#160;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>&#160;reflection system for `EnTT`. Maybe I didn&#39;t do better than others or maybe yes,</div><div class="line"><a name="l00030"></a><span class="lineno"> 30</span>&#160;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>&#160;which it belongs and not vice versa.</div><div class="line"><a name="l00032"></a><span class="lineno"> 32</span>&#160;</div><div class="line"><a name="l00033"></a><span class="lineno"> 33</span>&#160;# Names and identifiers</div><div class="line"><a name="l00034"></a><span class="lineno"> 34</span>&#160;</div><div class="line"><a name="l00035"></a><span class="lineno"> 35</span>&#160;The meta system doesn&#39;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>&#160;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>&#160;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>&#160;of a hashed string.&lt;br/&gt;</div><div class="line"><a name="l00039"></a><span class="lineno"> 39</span>&#160;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>&#160;long as they are numeric. It doesn&#39;t matter if they are generated at runtime, at</div><div class="line"><a name="l00041"></a><span class="lineno"> 41</span>&#160;compile-time or with custom functions.</div><div class="line"><a name="l00042"></a><span class="lineno"> 42</span>&#160;</div><div class="line"><a name="l00043"></a><span class="lineno"> 43</span>&#160;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>&#160;`hashed_string` class as provided by this library. Therefore, where an</div><div class="line"><a name="l00045"></a><span class="lineno"> 45</span>&#160;identifier is required, it&#39;s likely that a user defined literal is used as</div><div class="line"><a name="l00046"></a><span class="lineno"> 46</span>&#160;follows:</div><div class="line"><a name="l00047"></a><span class="lineno"> 47</span>&#160;</div><div class="line"><a name="l00048"></a><span class="lineno"> 48</span>&#160;```cpp</div><div class="line"><a name="l00049"></a><span class="lineno"> 49</span>&#160;auto factory = entt::reflect&lt;my_type&gt;(&quot;reflected_type&quot;_hs);</div><div class="line"><a name="l00050"></a><span class="lineno"> 50</span>&#160;```</div><div class="line"><a name="l00051"></a><span class="lineno"> 51</span>&#160;</div><div class="line"><a name="l00052"></a><span class="lineno"> 52</span>&#160;For what it&#39;s worth, this is likely completely equivalent to:</div><div class="line"><a name="l00053"></a><span class="lineno"> 53</span>&#160;</div><div class="line"><a name="l00054"></a><span class="lineno"> 54</span>&#160;```cpp</div><div class="line"><a name="l00055"></a><span class="lineno"> 55</span>&#160;auto factory = entt::reflect&lt;my_type&gt;(42);</div><div class="line"><a name="l00056"></a><span class="lineno"> 56</span>&#160;```</div><div class="line"><a name="l00057"></a><span class="lineno"> 57</span>&#160;</div><div class="line"><a name="l00058"></a><span class="lineno"> 58</span>&#160;Obviously, human-readable identifiers are more convenient to use and highly</div><div class="line"><a name="l00059"></a><span class="lineno"> 59</span>&#160;recommended.</div><div class="line"><a name="l00060"></a><span class="lineno"> 60</span>&#160;</div><div class="line"><a name="l00061"></a><span class="lineno"> 61</span>&#160;# Reflection in a nutshell</div><div class="line"><a name="l00062"></a><span class="lineno"> 62</span>&#160;</div><div class="line"><a name="l00063"></a><span class="lineno"> 63</span>&#160;Reflection always starts from real types (users cannot reflect imaginary types</div><div class="line"><a name="l00064"></a><span class="lineno"> 64</span>&#160;and it would not make much sense, we wouldn&#39;t be talking about reflection</div><div class="line"><a name="l00065"></a><span class="lineno"> 65</span>&#160;anymore).&lt;br/&gt;</div><div class="line"><a name="l00066"></a><span class="lineno"> 66</span>&#160;To _reflect_ a type, the library provides the `reflect` function:</div><div class="line"><a name="l00067"></a><span class="lineno"> 67</span>&#160;</div><div class="line"><a name="l00068"></a><span class="lineno"> 68</span>&#160;```cpp</div><div class="line"><a name="l00069"></a><span class="lineno"> 69</span>&#160;auto factory = entt::reflect&lt;my_type&gt;(&quot;reflected_type&quot;_hs);</div><div class="line"><a name="l00070"></a><span class="lineno"> 70</span>&#160;```</div><div class="line"><a name="l00071"></a><span class="lineno"> 71</span>&#160;</div><div class="line"><a name="l00072"></a><span class="lineno"> 72</span>&#160;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>&#160;identifier as an argument. Identifiers are important because users can retrieve</div><div class="line"><a name="l00074"></a><span class="lineno"> 74</span>&#160;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>&#160;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>&#160;the reflection system can use it correctly under the hood, but they don&#39;t want</div><div class="line"><a name="l00077"></a><span class="lineno"> 77</span>&#160;to allow searching the type by _name_.&lt;br/&gt;</div><div class="line"><a name="l00078"></a><span class="lineno"> 78</span>&#160;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>&#160;building the meta type.</div><div class="line"><a name="l00080"></a><span class="lineno"> 80</span>&#160;</div><div class="line"><a name="l00081"></a><span class="lineno"> 81</span>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00084"></a><span class="lineno"> 84</span>&#160;* _Constructors_. Actual constructors can be assigned to a reflected type by</div><div class="line"><a name="l00085"></a><span class="lineno"> 85</span>&#160; specifying their list of arguments. Free functions (namely, factories) can be</div><div class="line"><a name="l00086"></a><span class="lineno"> 86</span>&#160; used as well, as long as the return type is the expected one. From a client&#39;s</div><div class="line"><a name="l00087"></a><span class="lineno"> 87</span>&#160; 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>&#160; actual constructor.&lt;br/&gt;</div><div class="line"><a name="l00089"></a><span class="lineno"> 89</span>&#160; Use the `ctor` member function for this purpose:</div><div class="line"><a name="l00090"></a><span class="lineno"> 90</span>&#160;</div><div class="line"><a name="l00091"></a><span class="lineno"> 91</span>&#160; ```cpp</div><div class="line"><a name="l00092"></a><span class="lineno"> 92</span>&#160; entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs).ctor&lt;int, char&gt;().ctor&lt;&amp;factory&gt;();</div><div class="line"><a name="l00093"></a><span class="lineno"> 93</span>&#160; ```</div><div class="line"><a name="l00094"></a><span class="lineno"> 94</span>&#160;</div><div class="line"><a name="l00095"></a><span class="lineno"> 95</span>&#160;* _Destructors_. Free functions can be set as destructors of reflected types.</div><div class="line"><a name="l00096"></a><span class="lineno"> 96</span>&#160; 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>&#160; special treatment before an object is actually destroyed.&lt;br/&gt;</div><div class="line"><a name="l00098"></a><span class="lineno"> 98</span>&#160; Use the `dtor` member function for this purpose:</div><div class="line"><a name="l00099"></a><span class="lineno"> 99</span>&#160;</div><div class="line"><a name="l00100"></a><span class="lineno"> 100</span>&#160; ```cpp</div><div class="line"><a name="l00101"></a><span class="lineno"> 101</span>&#160; entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs).dtor&lt;&amp;destroy&gt;();</div><div class="line"><a name="l00102"></a><span class="lineno"> 102</span>&#160; ```</div><div class="line"><a name="l00103"></a><span class="lineno"> 103</span>&#160;</div><div class="line"><a name="l00104"></a><span class="lineno"> 104</span>&#160; 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>&#160; given instance.</div><div class="line"><a name="l00106"></a><span class="lineno"> 106</span>&#160;</div><div class="line"><a name="l00107"></a><span class="lineno"> 107</span>&#160;* _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>&#160; 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>&#160; type. From a client&#39;s point of view, all the variables associated with the</div><div class="line"><a name="l00110"></a><span class="lineno"> 110</span>&#160; reflected type will appear as if they were part of the type itself.&lt;br/&gt;</div><div class="line"><a name="l00111"></a><span class="lineno"> 111</span>&#160; Use the `data` member function for this purpose:</div><div class="line"><a name="l00112"></a><span class="lineno"> 112</span>&#160;</div><div class="line"><a name="l00113"></a><span class="lineno"> 113</span>&#160; ```cpp</div><div class="line"><a name="l00114"></a><span class="lineno"> 114</span>&#160; entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs)</div><div class="line"><a name="l00115"></a><span class="lineno"> 115</span>&#160; .data&lt;&amp;my_type::static_variable&gt;(&quot;static&quot;_hs)</div><div class="line"><a name="l00116"></a><span class="lineno"> 116</span>&#160; .data&lt;&amp;my_type::data_member&gt;(&quot;member&quot;_hs)</div><div class="line"><a name="l00117"></a><span class="lineno"> 117</span>&#160; .data&lt;&amp;global_variable&gt;(&quot;global&quot;_hs);</div><div class="line"><a name="l00118"></a><span class="lineno"> 118</span>&#160; ```</div><div class="line"><a name="l00119"></a><span class="lineno"> 119</span>&#160;</div><div class="line"><a name="l00120"></a><span class="lineno"> 120</span>&#160; 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>&#160; 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>&#160; by _name_.&lt;br/&gt;</div><div class="line"><a name="l00123"></a><span class="lineno"> 123</span>&#160; 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>&#160; 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>&#160; functions or mixed ones, as long as they respect the required signatures.&lt;br/&gt;</div><div class="line"><a name="l00126"></a><span class="lineno"> 126</span>&#160; Refer to the inline documentation for all the details.</div><div class="line"><a name="l00127"></a><span class="lineno"> 127</span>&#160;</div><div class="line"><a name="l00128"></a><span class="lineno"> 128</span>&#160;* _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>&#160; functions can be attached to a meta type. From a client&#39;s point of view, all</div><div class="line"><a name="l00130"></a><span class="lineno"> 130</span>&#160; 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>&#160; part of the type itself.&lt;br/&gt;</div><div class="line"><a name="l00132"></a><span class="lineno"> 132</span>&#160; Use the `func` member function for this purpose:</div><div class="line"><a name="l00133"></a><span class="lineno"> 133</span>&#160;</div><div class="line"><a name="l00134"></a><span class="lineno"> 134</span>&#160; ```cpp</div><div class="line"><a name="l00135"></a><span class="lineno"> 135</span>&#160; entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs)</div><div class="line"><a name="l00136"></a><span class="lineno"> 136</span>&#160; .func&lt;&amp;my_type::static_function&gt;(&quot;static&quot;_hs)</div><div class="line"><a name="l00137"></a><span class="lineno"> 137</span>&#160; .func&lt;&amp;my_type::member_function&gt;(&quot;member&quot;_hs)</div><div class="line"><a name="l00138"></a><span class="lineno"> 138</span>&#160; .func&lt;&amp;free_function&gt;(&quot;free&quot;_hs);</div><div class="line"><a name="l00139"></a><span class="lineno"> 139</span>&#160; ```</div><div class="line"><a name="l00140"></a><span class="lineno"> 140</span>&#160;</div><div class="line"><a name="l00141"></a><span class="lineno"> 141</span>&#160; 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>&#160; 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>&#160; searching for them by _name_.</div><div class="line"><a name="l00144"></a><span class="lineno"> 144</span>&#160;</div><div class="line"><a name="l00145"></a><span class="lineno"> 145</span>&#160;* _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>&#160; 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>&#160; and allows for implicit casts at runtime when required.&lt;br/&gt;</div><div class="line"><a name="l00148"></a><span class="lineno"> 148</span>&#160; Use the `base` member function for this purpose:</div><div class="line"><a name="l00149"></a><span class="lineno"> 149</span>&#160;</div><div class="line"><a name="l00150"></a><span class="lineno"> 150</span>&#160; ```cpp</div><div class="line"><a name="l00151"></a><span class="lineno"> 151</span>&#160; entt::reflect&lt;derived_type&gt;(&quot;derived&quot;_hs).base&lt;base_type&gt;();</div><div class="line"><a name="l00152"></a><span class="lineno"> 152</span>&#160; ```</div><div class="line"><a name="l00153"></a><span class="lineno"> 153</span>&#160;</div><div class="line"><a name="l00154"></a><span class="lineno"> 154</span>&#160; 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>&#160; will also be accepted.</div><div class="line"><a name="l00156"></a><span class="lineno"> 156</span>&#160;</div><div class="line"><a name="l00157"></a><span class="lineno"> 157</span>&#160;* _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>&#160; 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>&#160; 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>&#160; implicitly performed by the reflection system when required.&lt;br/&gt;</div><div class="line"><a name="l00161"></a><span class="lineno"> 161</span>&#160; Use the `conv` member function for this purpose:</div><div class="line"><a name="l00162"></a><span class="lineno"> 162</span>&#160;</div><div class="line"><a name="l00163"></a><span class="lineno"> 163</span>&#160; ```cpp</div><div class="line"><a name="l00164"></a><span class="lineno"> 164</span>&#160; entt::reflect&lt;double&gt;().conv&lt;int&gt;();</div><div class="line"><a name="l00165"></a><span class="lineno"> 165</span>&#160; ```</div><div class="line"><a name="l00166"></a><span class="lineno"> 166</span>&#160;</div><div class="line"><a name="l00167"></a><span class="lineno"> 167</span>&#160;That&#39;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>&#160;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>&#160;appreciate it over time.&lt;br/&gt;</div><div class="line"><a name="l00170"></a><span class="lineno"> 170</span>&#160;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>&#160;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>&#160;definitely worth the price, at least for me.</div><div class="line"><a name="l00173"></a><span class="lineno"> 173</span>&#160;</div><div class="line"><a name="l00174"></a><span class="lineno"> 174</span>&#160;## Any as in any type</div><div class="line"><a name="l00175"></a><span class="lineno"> 175</span>&#160;</div><div class="line"><a name="l00176"></a><span class="lineno"> 176</span>&#160;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>&#160;since C++17 introduced `std::any`, but it is not.&lt;br/&gt;</div><div class="line"><a name="l00178"></a><span class="lineno"> 178</span>&#160;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>&#160;`std::type_info`, an implementation defined class that&#39;s not something everyone</div><div class="line"><a name="l00180"></a><span class="lineno"> 180</span>&#160;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>&#160;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>&#160;a meta type, thus linking the two worlds.</div><div class="line"><a name="l00183"></a><span class="lineno"> 183</span>&#160;</div><div class="line"><a name="l00184"></a><span class="lineno"> 184</span>&#160;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>&#160;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>&#160;value.&lt;br/&gt;</div><div class="line"><a name="l00187"></a><span class="lineno"> 187</span>&#160;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>&#160;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>&#160;them though the reflection system, allocations are at zero.</div><div class="line"><a name="l00190"></a><span class="lineno"> 190</span>&#160;</div><div class="line"><a name="l00191"></a><span class="lineno"> 191</span>&#160;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>&#160;to initialize later:</div><div class="line"><a name="l00193"></a><span class="lineno"> 193</span>&#160;</div><div class="line"><a name="l00194"></a><span class="lineno"> 194</span>&#160;```cpp</div><div class="line"><a name="l00195"></a><span class="lineno"> 195</span>&#160;// a meta any object that contains an int</div><div class="line"><a name="l00196"></a><span class="lineno"> 196</span>&#160;entt::meta_any any{0};</div><div class="line"><a name="l00197"></a><span class="lineno"> 197</span>&#160;</div><div class="line"><a name="l00198"></a><span class="lineno"> 198</span>&#160;// an empty meta any object</div><div class="line"><a name="l00199"></a><span class="lineno"> 199</span>&#160;entt::meta_any empty{};</div><div class="line"><a name="l00200"></a><span class="lineno"> 200</span>&#160;```</div><div class="line"><a name="l00201"></a><span class="lineno"> 201</span>&#160;</div><div class="line"><a name="l00202"></a><span class="lineno"> 202</span>&#160;It takes the burden of destroying the contained instance when required.&lt;br/&gt;</div><div class="line"><a name="l00203"></a><span class="lineno"> 203</span>&#160;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>&#160;</div><div class="line"><a name="l00205"></a><span class="lineno"> 205</span>&#160;```cpp</div><div class="line"><a name="l00206"></a><span class="lineno"> 206</span>&#160;int value;</div><div class="line"><a name="l00207"></a><span class="lineno"> 207</span>&#160;entt::meta_any any{entt::as_alias, value};</div><div class="line"><a name="l00208"></a><span class="lineno"> 208</span>&#160;```</div><div class="line"><a name="l00209"></a><span class="lineno"> 209</span>&#160;</div><div class="line"><a name="l00210"></a><span class="lineno"> 210</span>&#160;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>&#160;that the lifetime of the object exceeds that of the container.</div><div class="line"><a name="l00212"></a><span class="lineno"> 212</span>&#160;</div><div class="line"><a name="l00213"></a><span class="lineno"> 213</span>&#160;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>&#160;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>&#160;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>&#160;can be converted implicitly to it.</div><div class="line"><a name="l00217"></a><span class="lineno"> 217</span>&#160;</div><div class="line"><a name="l00218"></a><span class="lineno"> 218</span>&#160;## Enjoy the runtime</div><div class="line"><a name="l00219"></a><span class="lineno"> 219</span>&#160;</div><div class="line"><a name="l00220"></a><span class="lineno"> 220</span>&#160;Once the web of reflected types has been constructed, it&#39;s a matter of using it</div><div class="line"><a name="l00221"></a><span class="lineno"> 221</span>&#160;at runtime where required.&lt;br/&gt;</div><div class="line"><a name="l00222"></a><span class="lineno"> 222</span>&#160;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>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00226"></a><span class="lineno"> 226</span>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00229"></a><span class="lineno"> 229</span>&#160;```cpp</div><div class="line"><a name="l00230"></a><span class="lineno"> 230</span>&#160;// search for a reflected type by type</div><div class="line"><a name="l00231"></a><span class="lineno"> 231</span>&#160;auto by_type = entt::resolve&lt;my_type&gt;();</div><div class="line"><a name="l00232"></a><span class="lineno"> 232</span>&#160;</div><div class="line"><a name="l00233"></a><span class="lineno"> 233</span>&#160;// search for a reflected type by name</div><div class="line"><a name="l00234"></a><span class="lineno"> 234</span>&#160;auto by_name = entt::resolve(&quot;reflected_type&quot;_hs);</div><div class="line"><a name="l00235"></a><span class="lineno"> 235</span>&#160;```</div><div class="line"><a name="l00236"></a><span class="lineno"> 236</span>&#160;</div><div class="line"><a name="l00237"></a><span class="lineno"> 237</span>&#160;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>&#160;all the reflected types at once:</div><div class="line"><a name="l00239"></a><span class="lineno"> 239</span>&#160;</div><div class="line"><a name="l00240"></a><span class="lineno"> 240</span>&#160;```cpp</div><div class="line"><a name="l00241"></a><span class="lineno"> 241</span>&#160;resolve([](auto type) {</div><div class="line"><a name="l00242"></a><span class="lineno"> 242</span>&#160; // ...</div><div class="line"><a name="l00243"></a><span class="lineno"> 243</span>&#160;});</div><div class="line"><a name="l00244"></a><span class="lineno"> 244</span>&#160;```</div><div class="line"><a name="l00245"></a><span class="lineno"> 245</span>&#160;</div><div class="line"><a name="l00246"></a><span class="lineno"> 246</span>&#160;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>&#160;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>&#160;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>&#160;of the underlying type.&lt;br/&gt;</div><div class="line"><a name="l00250"></a><span class="lineno"> 250</span>&#160;Refer to the inline documentation for all the details.</div><div class="line"><a name="l00251"></a><span class="lineno"> 251</span>&#160;</div><div class="line"><a name="l00252"></a><span class="lineno"> 252</span>&#160;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>&#160;</div><div class="line"><a name="l00254"></a><span class="lineno"> 254</span>&#160;* _Meta constructors_. They are accessed by types of arguments:</div><div class="line"><a name="l00255"></a><span class="lineno"> 255</span>&#160;</div><div class="line"><a name="l00256"></a><span class="lineno"> 256</span>&#160; ```cpp</div><div class="line"><a name="l00257"></a><span class="lineno"> 257</span>&#160; auto ctor = entt::resolve&lt;my_type&gt;().ctor&lt;int, char&gt;();</div><div class="line"><a name="l00258"></a><span class="lineno"> 258</span>&#160; ```</div><div class="line"><a name="l00259"></a><span class="lineno"> 259</span>&#160;</div><div class="line"><a name="l00260"></a><span class="lineno"> 260</span>&#160; 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>&#160; 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>&#160; derived or to which they can be converted.&lt;br/&gt;</div><div class="line"><a name="l00263"></a><span class="lineno"> 263</span>&#160; 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>&#160; 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>&#160; underlying type.</div><div class="line"><a name="l00266"></a><span class="lineno"> 266</span>&#160;</div><div class="line"><a name="l00267"></a><span class="lineno"> 267</span>&#160;* _Meta destructor_. It&#39;s returned by a dedicated function:</div><div class="line"><a name="l00268"></a><span class="lineno"> 268</span>&#160;</div><div class="line"><a name="l00269"></a><span class="lineno"> 269</span>&#160; ```cpp</div><div class="line"><a name="l00270"></a><span class="lineno"> 270</span>&#160; auto dtor = entt::resolve&lt;my_type&gt;().dtor();</div><div class="line"><a name="l00271"></a><span class="lineno"> 271</span>&#160; ```</div><div class="line"><a name="l00272"></a><span class="lineno"> 272</span>&#160;</div><div class="line"><a name="l00273"></a><span class="lineno"> 273</span>&#160; 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>&#160; destructor set for the given meta type.&lt;br/&gt;</div><div class="line"><a name="l00275"></a><span class="lineno"> 275</span>&#160; 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>&#160; 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>&#160;</div><div class="line"><a name="l00278"></a><span class="lineno"> 278</span>&#160;* _Meta data_. They are accessed by _name_:</div><div class="line"><a name="l00279"></a><span class="lineno"> 279</span>&#160;</div><div class="line"><a name="l00280"></a><span class="lineno"> 280</span>&#160; ```cpp</div><div class="line"><a name="l00281"></a><span class="lineno"> 281</span>&#160; auto data = entt::resolve&lt;my_type&gt;().data(&quot;member&quot;_hs);</div><div class="line"><a name="l00282"></a><span class="lineno"> 282</span>&#160; ```</div><div class="line"><a name="l00283"></a><span class="lineno"> 283</span>&#160;</div><div class="line"><a name="l00284"></a><span class="lineno"> 284</span>&#160; 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>&#160; object associated with the given identifier.&lt;br/&gt;</div><div class="line"><a name="l00286"></a><span class="lineno"> 286</span>&#160; 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>&#160; it&#39;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>&#160; or get the contained value.</div><div class="line"><a name="l00289"></a><span class="lineno"> 289</span>&#160;</div><div class="line"><a name="l00290"></a><span class="lineno"> 290</span>&#160;* _Meta functions_. They are accessed by _name_:</div><div class="line"><a name="l00291"></a><span class="lineno"> 291</span>&#160;</div><div class="line"><a name="l00292"></a><span class="lineno"> 292</span>&#160; ```cpp</div><div class="line"><a name="l00293"></a><span class="lineno"> 293</span>&#160; auto func = entt::resolve&lt;my_type&gt;().func(&quot;member&quot;_hs);</div><div class="line"><a name="l00294"></a><span class="lineno"> 294</span>&#160; ```</div><div class="line"><a name="l00295"></a><span class="lineno"> 295</span>&#160;</div><div class="line"><a name="l00296"></a><span class="lineno"> 296</span>&#160; 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>&#160; function object associated with the given identifier.&lt;br/&gt;</div><div class="line"><a name="l00298"></a><span class="lineno"> 298</span>&#160; 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>&#160; if it&#39;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>&#160; 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>&#160; 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>&#160; return value in the form of meta any object.</div><div class="line"><a name="l00303"></a><span class="lineno"> 303</span>&#160;</div><div class="line"><a name="l00304"></a><span class="lineno"> 304</span>&#160;* _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>&#160;</div><div class="line"><a name="l00306"></a><span class="lineno"> 306</span>&#160; ```cpp</div><div class="line"><a name="l00307"></a><span class="lineno"> 307</span>&#160; auto base = entt::resolve&lt;derived_type&gt;().base(&quot;base&quot;_hs);</div><div class="line"><a name="l00308"></a><span class="lineno"> 308</span>&#160; ```</div><div class="line"><a name="l00309"></a><span class="lineno"> 309</span>&#160;</div><div class="line"><a name="l00310"></a><span class="lineno"> 310</span>&#160; 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>&#160; object associated with the given identifier.&lt;br/&gt;</div><div class="line"><a name="l00312"></a><span class="lineno"> 312</span>&#160; Meta bases aren&#39;t meant to be used directly, even though they are freely</div><div class="line"><a name="l00313"></a><span class="lineno"> 313</span>&#160; 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>&#160; base class and to convert a raw pointer between types.</div><div class="line"><a name="l00315"></a><span class="lineno"> 315</span>&#160;</div><div class="line"><a name="l00316"></a><span class="lineno"> 316</span>&#160;* _Meta conversion functions_. They are accessed by type:</div><div class="line"><a name="l00317"></a><span class="lineno"> 317</span>&#160;</div><div class="line"><a name="l00318"></a><span class="lineno"> 318</span>&#160; ```cpp</div><div class="line"><a name="l00319"></a><span class="lineno"> 319</span>&#160; auto conv = entt::resolve&lt;double&gt;().conv&lt;int&gt;();</div><div class="line"><a name="l00320"></a><span class="lineno"> 320</span>&#160; ```</div><div class="line"><a name="l00321"></a><span class="lineno"> 321</span>&#160;</div><div class="line"><a name="l00322"></a><span class="lineno"> 322</span>&#160; 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>&#160; conversion function associated with the given type.&lt;br/&gt;</div><div class="line"><a name="l00324"></a><span class="lineno"> 324</span>&#160; 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>&#160; 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>&#160; 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>&#160; types.</div><div class="line"><a name="l00328"></a><span class="lineno"> 328</span>&#160;</div><div class="line"><a name="l00329"></a><span class="lineno"> 329</span>&#160;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>&#160;converted to a boolean value to check if they are valid:</div><div class="line"><a name="l00331"></a><span class="lineno"> 331</span>&#160;</div><div class="line"><a name="l00332"></a><span class="lineno"> 332</span>&#160;```cpp</div><div class="line"><a name="l00333"></a><span class="lineno"> 333</span>&#160;auto func = entt::resolve&lt;my_type&gt;().func(&quot;member&quot;_hs);</div><div class="line"><a name="l00334"></a><span class="lineno"> 334</span>&#160;</div><div class="line"><a name="l00335"></a><span class="lineno"> 335</span>&#160;if(func) {</div><div class="line"><a name="l00336"></a><span class="lineno"> 336</span>&#160; // ...</div><div class="line"><a name="l00337"></a><span class="lineno"> 337</span>&#160;}</div><div class="line"><a name="l00338"></a><span class="lineno"> 338</span>&#160;```</div><div class="line"><a name="l00339"></a><span class="lineno"> 339</span>&#160;</div><div class="line"><a name="l00340"></a><span class="lineno"> 340</span>&#160;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>&#160;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>&#160;them. As an example:</div><div class="line"><a name="l00343"></a><span class="lineno"> 343</span>&#160;</div><div class="line"><a name="l00344"></a><span class="lineno"> 344</span>&#160;```cpp</div><div class="line"><a name="l00345"></a><span class="lineno"> 345</span>&#160;entt::resolve&lt;my_type&gt;().data([](auto data) {</div><div class="line"><a name="l00346"></a><span class="lineno"> 346</span>&#160; // ...</div><div class="line"><a name="l00347"></a><span class="lineno"> 347</span>&#160;});</div><div class="line"><a name="l00348"></a><span class="lineno"> 348</span>&#160;```</div><div class="line"><a name="l00349"></a><span class="lineno"> 349</span>&#160;</div><div class="line"><a name="l00350"></a><span class="lineno"> 350</span>&#160;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>&#160;underlying type.&lt;br/&gt;</div><div class="line"><a name="l00352"></a><span class="lineno"> 352</span>&#160;In particular, the `construct` member function accepts a variable number of</div><div class="line"><a name="l00353"></a><span class="lineno"> 353</span>&#160;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>&#160;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>&#160;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>&#160;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>&#160;destructor if any.&lt;br/&gt;</div><div class="line"><a name="l00358"></a><span class="lineno"> 358</span>&#160;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>&#160;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>&#160;treatment and **not** to actually destroy instances.</div><div class="line"><a name="l00361"></a><span class="lineno"> 361</span>&#160;</div><div class="line"><a name="l00362"></a><span class="lineno"> 362</span>&#160;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>&#160;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>&#160;unfortunately beyond the scope of this document.&lt;br/&gt;</div><div class="line"><a name="l00365"></a><span class="lineno"> 365</span>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00368"></a><span class="lineno"> 368</span>&#160;## Policies: the more, the less</div><div class="line"><a name="l00369"></a><span class="lineno"> 369</span>&#160;</div><div class="line"><a name="l00370"></a><span class="lineno"> 370</span>&#160;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>&#160;reflection information.&lt;br/&gt;</div><div class="line"><a name="l00372"></a><span class="lineno"> 372</span>&#160;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>&#160;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>&#160;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>&#160;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>&#160;behavior isn&#39;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>&#160;other problems.</div><div class="line"><a name="l00378"></a><span class="lineno"> 378</span>&#160;</div><div class="line"><a name="l00379"></a><span class="lineno"> 379</span>&#160;There are a few alternatives available at the moment:</div><div class="line"><a name="l00380"></a><span class="lineno"> 380</span>&#160;</div><div class="line"><a name="l00381"></a><span class="lineno"> 381</span>&#160;* The _as-is_ policy, associated with the type `entt::as_is_t`.&lt;br/&gt;</div><div class="line"><a name="l00382"></a><span class="lineno"> 382</span>&#160; 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>&#160; since it&#39;s implicitly selected if no other policy is specified.&lt;br/&gt;</div><div class="line"><a name="l00384"></a><span class="lineno"> 384</span>&#160; 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>&#160; 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>&#160; therefore associated with their original meta types.</div><div class="line"><a name="l00387"></a><span class="lineno"> 387</span>&#160;</div><div class="line"><a name="l00388"></a><span class="lineno"> 388</span>&#160;* The _as-void_ policy, associated with the type `entt::as_void_t`.&lt;br/&gt;</div><div class="line"><a name="l00389"></a><span class="lineno"> 389</span>&#160; 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>&#160; thus making it appear as if its type were `void`.&lt;br/&gt;</div><div class="line"><a name="l00391"></a><span class="lineno"> 391</span>&#160; If the use with functions is obvious, it must be said that it&#39;s also possible</div><div class="line"><a name="l00392"></a><span class="lineno"> 392</span>&#160; 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>&#160; 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>&#160; 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>&#160; reading.</div><div class="line"><a name="l00396"></a><span class="lineno"> 396</span>&#160;</div><div class="line"><a name="l00397"></a><span class="lineno"> 397</span>&#160; As an example of use:</div><div class="line"><a name="l00398"></a><span class="lineno"> 398</span>&#160;</div><div class="line"><a name="l00399"></a><span class="lineno"> 399</span>&#160; ```cpp</div><div class="line"><a name="l00400"></a><span class="lineno"> 400</span>&#160; entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs)</div><div class="line"><a name="l00401"></a><span class="lineno"> 401</span>&#160; .func&lt;&amp;my_type::member_function, entt::as_void_t&gt;(&quot;member&quot;_hs);</div><div class="line"><a name="l00402"></a><span class="lineno"> 402</span>&#160; ```</div><div class="line"><a name="l00403"></a><span class="lineno"> 403</span>&#160;</div><div class="line"><a name="l00404"></a><span class="lineno"> 404</span>&#160;* The _as-alias_ policy, associated with the type `entt::as_alias_t`.&lt;br/&gt;</div><div class="line"><a name="l00405"></a><span class="lineno"> 405</span>&#160; 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>&#160; 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>&#160; _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>&#160; used to initialize the wrapper itself.&lt;br/&gt;</div><div class="line"><a name="l00409"></a><span class="lineno"> 409</span>&#160; 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>&#160; an external container rather than created on demand), data members and</div><div class="line"><a name="l00411"></a><span class="lineno"> 411</span>&#160; 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>&#160;</div><div class="line"><a name="l00413"></a><span class="lineno"> 413</span>&#160; As an example of use:</div><div class="line"><a name="l00414"></a><span class="lineno"> 414</span>&#160;</div><div class="line"><a name="l00415"></a><span class="lineno"> 415</span>&#160; ```cpp</div><div class="line"><a name="l00416"></a><span class="lineno"> 416</span>&#160; entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs)</div><div class="line"><a name="l00417"></a><span class="lineno"> 417</span>&#160; .data&lt;&amp;my_type::data_member, entt::as_alias_t&gt;(&quot;member&quot;_hs);</div><div class="line"><a name="l00418"></a><span class="lineno"> 418</span>&#160; ```</div><div class="line"><a name="l00419"></a><span class="lineno"> 419</span>&#160;</div><div class="line"><a name="l00420"></a><span class="lineno"> 420</span>&#160;Some uses are rather trivial, but it&#39;s useful to note that there are some less</div><div class="line"><a name="l00421"></a><span class="lineno"> 421</span>&#160;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>&#160;</div><div class="line"><a name="l00423"></a><span class="lineno"> 423</span>&#160;## Named constants and enums</div><div class="line"><a name="l00424"></a><span class="lineno"> 424</span>&#160;</div><div class="line"><a name="l00425"></a><span class="lineno"> 425</span>&#160;A special mention should be made for constant values and enums. It wouldn&#39;t be</div><div class="line"><a name="l00426"></a><span class="lineno"> 426</span>&#160;necessary, but it will help distracted readers.</div><div class="line"><a name="l00427"></a><span class="lineno"> 427</span>&#160;</div><div class="line"><a name="l00428"></a><span class="lineno"> 428</span>&#160;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>&#160;type among the other things.&lt;br/&gt;</div><div class="line"><a name="l00430"></a><span class="lineno"> 430</span>&#160;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>&#160;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>&#160;with constants of special meaning where required.&lt;br/&gt;</div><div class="line"><a name="l00433"></a><span class="lineno"> 433</span>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00436"></a><span class="lineno"> 436</span>&#160;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>&#160;members of the reflected types.</div><div class="line"><a name="l00438"></a><span class="lineno"> 438</span>&#160;</div><div class="line"><a name="l00439"></a><span class="lineno"> 439</span>&#160;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>&#160;</div><div class="line"><a name="l00441"></a><span class="lineno"> 441</span>&#160;```cpp</div><div class="line"><a name="l00442"></a><span class="lineno"> 442</span>&#160;entt::reflect&lt;my_enum&gt;()</div><div class="line"><a name="l00443"></a><span class="lineno"> 443</span>&#160; .data&lt;my_enum::a_value&gt;(&quot;a_value&quot;_hs)</div><div class="line"><a name="l00444"></a><span class="lineno"> 444</span>&#160; .data&lt;my_enum::another_value&gt;(&quot;another_value&quot;_hs);</div><div class="line"><a name="l00445"></a><span class="lineno"> 445</span>&#160;</div><div class="line"><a name="l00446"></a><span class="lineno"> 446</span>&#160;entt::reflect&lt;int&gt;().data&lt;2048&gt;(&quot;max_int&quot;_hs);</div><div class="line"><a name="l00447"></a><span class="lineno"> 447</span>&#160;```</div><div class="line"><a name="l00448"></a><span class="lineno"> 448</span>&#160;</div><div class="line"><a name="l00449"></a><span class="lineno"> 449</span>&#160;It goes without saying that accessing them is trivial as well. It&#39;s a matter of</div><div class="line"><a name="l00450"></a><span class="lineno"> 450</span>&#160;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>&#160;</div><div class="line"><a name="l00452"></a><span class="lineno"> 452</span>&#160;```cpp</div><div class="line"><a name="l00453"></a><span class="lineno"> 453</span>&#160;auto value = entt::resolve&lt;my_enum&gt;().data(&quot;a_value&quot;_hs).get({}).cast&lt;my_enum&gt;();</div><div class="line"><a name="l00454"></a><span class="lineno"> 454</span>&#160;auto max = entt::resolve&lt;int&gt;().data(&quot;max_int&quot;_hs).get({}).cast&lt;int&gt;();</div><div class="line"><a name="l00455"></a><span class="lineno"> 455</span>&#160;```</div><div class="line"><a name="l00456"></a><span class="lineno"> 456</span>&#160;</div><div class="line"><a name="l00457"></a><span class="lineno"> 457</span>&#160;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>&#160;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>&#160;class.</div><div class="line"><a name="l00460"></a><span class="lineno"> 460</span>&#160;</div><div class="line"><a name="l00461"></a><span class="lineno"> 461</span>&#160;## Properties and meta objects</div><div class="line"><a name="l00462"></a><span class="lineno"> 462</span>&#160;</div><div class="line"><a name="l00463"></a><span class="lineno"> 463</span>&#160;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>&#160;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>&#160;is possible for most of them.&lt;br/&gt;</div><div class="line"><a name="l00466"></a><span class="lineno"> 466</span>&#160;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>&#160;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>&#160;such that `std::get&lt;0&gt;` and `std::get&lt;1&gt;` are valid for it. In other terms, the</div><div class="line"><a name="l00469"></a><span class="lineno"> 469</span>&#160;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>&#160;`std::pair`. As an example:</div><div class="line"><a name="l00471"></a><span class="lineno"> 471</span>&#160;</div><div class="line"><a name="l00472"></a><span class="lineno"> 472</span>&#160;```cpp</div><div class="line"><a name="l00473"></a><span class="lineno"> 473</span>&#160;entt::reflect&lt;my_type&gt;(&quot;reflected&quot;_hs, std::make_pair(&quot;tooltip&quot;_hs, &quot;message&quot;));</div><div class="line"><a name="l00474"></a><span class="lineno"> 474</span>&#160;```</div><div class="line"><a name="l00475"></a><span class="lineno"> 475</span>&#160;</div><div class="line"><a name="l00476"></a><span class="lineno"> 476</span>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00479"></a><span class="lineno"> 479</span>&#160;```cpp</div><div class="line"><a name="l00480"></a><span class="lineno"> 480</span>&#160;// iterate all the properties of a meta type</div><div class="line"><a name="l00481"></a><span class="lineno"> 481</span>&#160;entt::resolve&lt;my_type&gt;().prop([](auto prop) {</div><div class="line"><a name="l00482"></a><span class="lineno"> 482</span>&#160; // ...</div><div class="line"><a name="l00483"></a><span class="lineno"> 483</span>&#160;});</div><div class="line"><a name="l00484"></a><span class="lineno"> 484</span>&#160;</div><div class="line"><a name="l00485"></a><span class="lineno"> 485</span>&#160;// search for a given property by name</div><div class="line"><a name="l00486"></a><span class="lineno"> 486</span>&#160;auto prop = entt::resolve&lt;my_type&gt;().prop(&quot;tooltip&quot;_hs);</div><div class="line"><a name="l00487"></a><span class="lineno"> 487</span>&#160;```</div><div class="line"><a name="l00488"></a><span class="lineno"> 488</span>&#160;</div><div class="line"><a name="l00489"></a><span class="lineno"> 489</span>&#160;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>&#160;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>&#160;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>&#160;</div><div class="line"><a name="l00493"></a><span class="lineno"> 493</span>&#160;## Unregister types</div><div class="line"><a name="l00494"></a><span class="lineno"> 494</span>&#160;</div><div class="line"><a name="l00495"></a><span class="lineno"> 495</span>&#160;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>&#160;means unregistering all its data members, member functions, conversion functions</div><div class="line"><a name="l00497"></a><span class="lineno"> 497</span>&#160;and so on. However, the base classes won&#39;t be unregistered, since they don&#39;t</div><div class="line"><a name="l00498"></a><span class="lineno"> 498</span>&#160;necessarily depend on it. Similarly, implicitly generated types (as an example,</div><div class="line"><a name="l00499"></a><span class="lineno"> 499</span>&#160;the meta types implicitly generated for function parameters when needed) won&#39;t</div><div class="line"><a name="l00500"></a><span class="lineno"> 500</span>&#160;be unregistered.</div><div class="line"><a name="l00501"></a><span class="lineno"> 501</span>&#160;</div><div class="line"><a name="l00502"></a><span class="lineno"> 502</span>&#160;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>&#160;namespace:</div><div class="line"><a name="l00504"></a><span class="lineno"> 504</span>&#160;</div><div class="line"><a name="l00505"></a><span class="lineno"> 505</span>&#160;```cpp</div><div class="line"><a name="l00506"></a><span class="lineno"> 506</span>&#160;entt::unregister&lt;my_type&gt;();</div><div class="line"><a name="l00507"></a><span class="lineno"> 507</span>&#160;```</div><div class="line"><a name="l00508"></a><span class="lineno"> 508</span>&#160;</div><div class="line"><a name="l00509"></a><span class="lineno"> 509</span>&#160;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>&#160;registered with the reflection system, false otherwise.&lt;br/&gt;</div><div class="line"><a name="l00511"></a><span class="lineno"> 511</span>&#160;The type can be re-registered later with a completely different name and form.</div></div><!-- fragment --></div><!-- contents -->
<!-- start footer part -->
<hr class="footer"/><address class="footer"><small>
Generated by &#160;<a href="http://www.doxygen.org/index.html">
<img class="footer" src="doxygen.png" alt="doxygen"/>
</a> 1.8.13
</small></address>
</body>
</html>