75 lines
30 KiB
HTML
75 lines
30 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/resource.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
|
|
 <span id="projectnumber">3.1.0</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/resource.md</div> </div>
|
|
</div><!--header-->
|
|
<div class="contents">
|
|
<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno"> 1</span> # Crash Course: resource management</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> * [The resource, the loader and the cache](#the-resource-the-loader-and-the-cache)</div><div class="line"><a name="l00010"></a><span class="lineno"> 10</span> <!--</div><div class="line"><a name="l00011"></a><span class="lineno"> 11</span> @endcond TURN_OFF_DOXYGEN</div><div class="line"><a name="l00012"></a><span class="lineno"> 12</span> --></div><div class="line"><a name="l00013"></a><span class="lineno"> 13</span> </div><div class="line"><a name="l00014"></a><span class="lineno"> 14</span> # Introduction</div><div class="line"><a name="l00015"></a><span class="lineno"> 15</span> </div><div class="line"><a name="l00016"></a><span class="lineno"> 16</span> Resource management is usually one of the most critical part of a software like</div><div class="line"><a name="l00017"></a><span class="lineno"> 17</span> a game. Solutions are often tuned to the particular application. There exist</div><div class="line"><a name="l00018"></a><span class="lineno"> 18</span> several approaches and all of them are perfectly fine as long as they fit the</div><div class="line"><a name="l00019"></a><span class="lineno"> 19</span> requirements of the piece of software in which they are used.<br/></div><div class="line"><a name="l00020"></a><span class="lineno"> 20</span> Examples are loading everything on start, loading on request, predictive</div><div class="line"><a name="l00021"></a><span class="lineno"> 21</span> loading, and so on.</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> `EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different</div><div class="line"><a name="l00024"></a><span class="lineno"> 24</span> cases. Instead, it offers a minimal and perhaps trivial cache that can be useful</div><div class="line"><a name="l00025"></a><span class="lineno"> 25</span> most of the time during prototyping and sometimes even in a production</div><div class="line"><a name="l00026"></a><span class="lineno"> 26</span> environment.<br/></div><div class="line"><a name="l00027"></a><span class="lineno"> 27</span> For those interested in the subject, the plan is to improve it considerably over</div><div class="line"><a name="l00028"></a><span class="lineno"> 28</span> time in terms of performance, memory usage and functionalities. Hoping to make</div><div class="line"><a name="l00029"></a><span class="lineno"> 29</span> it, of course, one step at a time.</div><div class="line"><a name="l00030"></a><span class="lineno"> 30</span> </div><div class="line"><a name="l00031"></a><span class="lineno"> 31</span> # The resource, the loader and the cache</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> There are three main actors in the model: the resource, the loader and the</div><div class="line"><a name="l00034"></a><span class="lineno"> 34</span> cache.</div><div class="line"><a name="l00035"></a><span class="lineno"> 35</span> </div><div class="line"><a name="l00036"></a><span class="lineno"> 36</span> The _resource_ is whatever users want it to be. An image, a video, an audio,</div><div class="line"><a name="l00037"></a><span class="lineno"> 37</span> whatever. There are no limits.<br/></div><div class="line"><a name="l00038"></a><span class="lineno"> 38</span> As a minimal example:</div><div class="line"><a name="l00039"></a><span class="lineno"> 39</span> </div><div class="line"><a name="l00040"></a><span class="lineno"> 40</span> ```cpp</div><div class="line"><a name="l00041"></a><span class="lineno"> 41</span> struct my_resource { const int value; };</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> </div><div class="line"><a name="l00044"></a><span class="lineno"> 44</span> A _loader_ is a class the aim of which is to load a specific resource. It has to</div><div class="line"><a name="l00045"></a><span class="lineno"> 45</span> inherit directly from the dedicated base class as in the following example:</div><div class="line"><a name="l00046"></a><span class="lineno"> 46</span> </div><div class="line"><a name="l00047"></a><span class="lineno"> 47</span> ```cpp</div><div class="line"><a name="l00048"></a><span class="lineno"> 48</span> struct my_loader final: entt::resource_loader<my_loader, my_resource> {</div><div class="line"><a name="l00049"></a><span class="lineno"> 49</span>  // ...</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> </div><div class="line"><a name="l00053"></a><span class="lineno"> 53</span> Where `my_resource` is the type of resources it creates.<br/></div><div class="line"><a name="l00054"></a><span class="lineno"> 54</span> A resource loader must also expose a public const member function named `load`</div><div class="line"><a name="l00055"></a><span class="lineno"> 55</span> that accepts a variable number of arguments and returns a shared pointer to a</div><div class="line"><a name="l00056"></a><span class="lineno"> 56</span> resource.<br/></div><div class="line"><a name="l00057"></a><span class="lineno"> 57</span> As an example:</div><div class="line"><a name="l00058"></a><span class="lineno"> 58</span> </div><div class="line"><a name="l00059"></a><span class="lineno"> 59</span> ```cpp</div><div class="line"><a name="l00060"></a><span class="lineno"> 60</span> struct my_loader: entt::resource_loader<my_loader, my_resource> {</div><div class="line"><a name="l00061"></a><span class="lineno"> 61</span>  std::shared_ptr<my_resource> load(int value) const {</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>  return std::shared_ptr<my_resource>(new my_resource{ value });</div><div class="line"><a name="l00064"></a><span class="lineno"> 64</span>  }</div><div class="line"><a name="l00065"></a><span class="lineno"> 65</span> };</div><div class="line"><a name="l00066"></a><span class="lineno"> 66</span> ```</div><div class="line"><a name="l00067"></a><span class="lineno"> 67</span> </div><div class="line"><a name="l00068"></a><span class="lineno"> 68</span> In general, resource loaders should not have a state or retain data of any type.</div><div class="line"><a name="l00069"></a><span class="lineno"> 69</span> They should let the cache manage their resources instead.<br/></div><div class="line"><a name="l00070"></a><span class="lineno"> 70</span> As a side note, base class and CRTP idiom aren't strictly required with the</div><div class="line"><a name="l00071"></a><span class="lineno"> 71</span> current implementation. One could argue that a cache can easily work with</div><div class="line"><a name="l00072"></a><span class="lineno"> 72</span> loaders of any type. However, future changes won't be breaking ones by forcing</div><div class="line"><a name="l00073"></a><span class="lineno"> 73</span> the use of a base class today and that's why the model is already in its place.</div><div class="line"><a name="l00074"></a><span class="lineno"> 74</span> </div><div class="line"><a name="l00075"></a><span class="lineno"> 75</span> Finally, a cache is a specialization of a class template tailored to a specific</div><div class="line"><a name="l00076"></a><span class="lineno"> 76</span> resource:</div><div class="line"><a name="l00077"></a><span class="lineno"> 77</span> </div><div class="line"><a name="l00078"></a><span class="lineno"> 78</span> ```cpp</div><div class="line"><a name="l00079"></a><span class="lineno"> 79</span> using my_resource_cache = entt::resource_cache<my_resource>;</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> // ...</div><div class="line"><a name="l00082"></a><span class="lineno"> 82</span> </div><div class="line"><a name="l00083"></a><span class="lineno"> 83</span> my_resource_cache cache{};</div><div class="line"><a name="l00084"></a><span class="lineno"> 84</span> ```</div><div class="line"><a name="l00085"></a><span class="lineno"> 85</span> </div><div class="line"><a name="l00086"></a><span class="lineno"> 86</span> The idea is to create different caches for different types of resources and to</div><div class="line"><a name="l00087"></a><span class="lineno"> 87</span> manage each one independently in the most appropriate way.<br/></div><div class="line"><a name="l00088"></a><span class="lineno"> 88</span> As a (very) trivial example, audio tracks can survive in most of the scenes of</div><div class="line"><a name="l00089"></a><span class="lineno"> 89</span> an application while meshes can be associated with a single scene and then</div><div class="line"><a name="l00090"></a><span class="lineno"> 90</span> discarded when users leave it.</div><div class="line"><a name="l00091"></a><span class="lineno"> 91</span> </div><div class="line"><a name="l00092"></a><span class="lineno"> 92</span> A cache offers a set of basic functionalities to query its internal state and to</div><div class="line"><a name="l00093"></a><span class="lineno"> 93</span> _organize_ it:</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> ```cpp</div><div class="line"><a name="l00096"></a><span class="lineno"> 96</span> // gets the number of resources managed by a cache</div><div class="line"><a name="l00097"></a><span class="lineno"> 97</span> const auto size = cache.size();</div><div class="line"><a name="l00098"></a><span class="lineno"> 98</span> </div><div class="line"><a name="l00099"></a><span class="lineno"> 99</span> // checks if a cache contains at least a valid resource</div><div class="line"><a name="l00100"></a><span class="lineno"> 100</span> const auto empty = cache.empty();</div><div class="line"><a name="l00101"></a><span class="lineno"> 101</span> </div><div class="line"><a name="l00102"></a><span class="lineno"> 102</span> // clears a cache and discards its content</div><div class="line"><a name="l00103"></a><span class="lineno"> 103</span> cache.clear();</div><div class="line"><a name="l00104"></a><span class="lineno"> 104</span> ```</div><div class="line"><a name="l00105"></a><span class="lineno"> 105</span> </div><div class="line"><a name="l00106"></a><span class="lineno"> 106</span> Besides these member functions, a cache contains what is needed to load, use and</div><div class="line"><a name="l00107"></a><span class="lineno"> 107</span> discard resources of the given type.<br/></div><div class="line"><a name="l00108"></a><span class="lineno"> 108</span> Before to explore this part of the interface, it makes sense to mention how</div><div class="line"><a name="l00109"></a><span class="lineno"> 109</span> resources are identified. The type of the identifiers to use is defined as:</div><div class="line"><a name="l00110"></a><span class="lineno"> 110</span> </div><div class="line"><a name="l00111"></a><span class="lineno"> 111</span> ```cpp</div><div class="line"><a name="l00112"></a><span class="lineno"> 112</span> entt::resource_cache<resource>::resource_type</div><div class="line"><a name="l00113"></a><span class="lineno"> 113</span> ```</div><div class="line"><a name="l00114"></a><span class="lineno"> 114</span> </div><div class="line"><a name="l00115"></a><span class="lineno"> 115</span> Where `resource_type` is an alias for `entt::hashed_string::hash_type`.</div><div class="line"><a name="l00116"></a><span class="lineno"> 116</span> Therefore, resource identifiers are created explicitly as in the following</div><div class="line"><a name="l00117"></a><span class="lineno"> 117</span> example:</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> ```cpp</div><div class="line"><a name="l00120"></a><span class="lineno"> 120</span> constexpr auto identifier = entt::resource_cache<resource>::resource_type{"my/resource/identifier"_hs};</div><div class="line"><a name="l00121"></a><span class="lineno"> 121</span> // this is equivalent to the following</div><div class="line"><a name="l00122"></a><span class="lineno"> 122</span> constexpr auto hs = entt::hashed_string{"my/resource/identifier"};</div><div class="line"><a name="l00123"></a><span class="lineno"> 123</span> ```</div><div class="line"><a name="l00124"></a><span class="lineno"> 124</span> </div><div class="line"><a name="l00125"></a><span class="lineno"> 125</span> The class `hashed_string` is described in a dedicated section, so I won't go in</div><div class="line"><a name="l00126"></a><span class="lineno"> 126</span> details here.</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> Resources are loaded and thus stored in a cache through the `load` member</div><div class="line"><a name="l00129"></a><span class="lineno"> 129</span> function. It accepts the loader to use as a template parameter, the resource</div><div class="line"><a name="l00130"></a><span class="lineno"> 130</span> identifier and the parameters used to construct the resource as arguments:</div><div class="line"><a name="l00131"></a><span class="lineno"> 131</span> </div><div class="line"><a name="l00132"></a><span class="lineno"> 132</span> ```cpp</div><div class="line"><a name="l00133"></a><span class="lineno"> 133</span> // uses the identifier declared above</div><div class="line"><a name="l00134"></a><span class="lineno"> 134</span> cache.load<my_loader>(identifier, 0);</div><div class="line"><a name="l00135"></a><span class="lineno"> 135</span> </div><div class="line"><a name="l00136"></a><span class="lineno"> 136</span> // uses a const char * directly as an identifier</div><div class="line"><a name="l00137"></a><span class="lineno"> 137</span> cache.load<my_loader>("another/identifier"_hs, 42);</div><div class="line"><a name="l00138"></a><span class="lineno"> 138</span> ```</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> The function returns a handle to the resource, whether it already exists or is</div><div class="line"><a name="l00141"></a><span class="lineno"> 141</span> loaded. In case the loader returns an invalid pointer, the handle is invalid as</div><div class="line"><a name="l00142"></a><span class="lineno"> 142</span> well and therefore it can be easily used with an `if` statement:</div><div class="line"><a name="l00143"></a><span class="lineno"> 143</span> </div><div class="line"><a name="l00144"></a><span class="lineno"> 144</span> ```cpp</div><div class="line"><a name="l00145"></a><span class="lineno"> 145</span> if(auto handle = cache.load<my_loader>("another/identifier"_hs, 42); handle) {</div><div class="line"><a name="l00146"></a><span class="lineno"> 146</span>  // ...</div><div class="line"><a name="l00147"></a><span class="lineno"> 147</span> }</div><div class="line"><a name="l00148"></a><span class="lineno"> 148</span> ```</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> Before trying to load a resource, the `contains` member function can be used to</div><div class="line"><a name="l00151"></a><span class="lineno"> 151</span> know if a cache already contains a specific resource:</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> ```cpp</div><div class="line"><a name="l00154"></a><span class="lineno"> 154</span> auto exists = cache.contains("my/identifier"_hs);</div><div class="line"><a name="l00155"></a><span class="lineno"> 155</span> ```</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> There exists also a member function to use to force a reload of an already</div><div class="line"><a name="l00158"></a><span class="lineno"> 158</span> existing resource if needed:</div><div class="line"><a name="l00159"></a><span class="lineno"> 159</span> </div><div class="line"><a name="l00160"></a><span class="lineno"> 160</span> ```cpp</div><div class="line"><a name="l00161"></a><span class="lineno"> 161</span> auto handle = cache.reload<my_loader>("another/identifier"_hs, 42);</div><div class="line"><a name="l00162"></a><span class="lineno"> 162</span> ```</div><div class="line"><a name="l00163"></a><span class="lineno"> 163</span> </div><div class="line"><a name="l00164"></a><span class="lineno"> 164</span> As above, the function returns a handle to the resource that is invalid in case</div><div class="line"><a name="l00165"></a><span class="lineno"> 165</span> of errors. The `reload` member function is a kind of alias of the following</div><div class="line"><a name="l00166"></a><span class="lineno"> 166</span> snippet:</div><div class="line"><a name="l00167"></a><span class="lineno"> 167</span> </div><div class="line"><a name="l00168"></a><span class="lineno"> 168</span> ```cpp</div><div class="line"><a name="l00169"></a><span class="lineno"> 169</span> cache.discard(identifier);</div><div class="line"><a name="l00170"></a><span class="lineno"> 170</span> cache.load<my_loader>(identifier, 42);</div><div class="line"><a name="l00171"></a><span class="lineno"> 171</span> ```</div><div class="line"><a name="l00172"></a><span class="lineno"> 172</span> </div><div class="line"><a name="l00173"></a><span class="lineno"> 173</span> Where the `discard` member function is used to get rid of a resource if loaded.</div><div class="line"><a name="l00174"></a><span class="lineno"> 174</span> In case the cache doesn't contain a resource for the given identifier, `discard`</div><div class="line"><a name="l00175"></a><span class="lineno"> 175</span> does nothing and returns immediately.</div><div class="line"><a name="l00176"></a><span class="lineno"> 176</span> </div><div class="line"><a name="l00177"></a><span class="lineno"> 177</span> So far, so good. Resources are finally loaded and stored within the cache.<br/></div><div class="line"><a name="l00178"></a><span class="lineno"> 178</span> They are returned to users in the form of handles. To get one of them later on:</div><div class="line"><a name="l00179"></a><span class="lineno"> 179</span> </div><div class="line"><a name="l00180"></a><span class="lineno"> 180</span> ```cpp</div><div class="line"><a name="l00181"></a><span class="lineno"> 181</span> auto handle = cache.handle("my/identifier"_hs);</div><div class="line"><a name="l00182"></a><span class="lineno"> 182</span> ```</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> The idea behind a handle is the same of the flyweight pattern. In other terms,</div><div class="line"><a name="l00185"></a><span class="lineno"> 185</span> resources aren't copied around. Instead, instances are shared between handles.</div><div class="line"><a name="l00186"></a><span class="lineno"> 186</span> Users of a resource own a handle that guarantees that a resource isn't destroyed</div><div class="line"><a name="l00187"></a><span class="lineno"> 187</span> until all the handles are destroyed, even if the resource itself is removed from</div><div class="line"><a name="l00188"></a><span class="lineno"> 188</span> the cache.<br/></div><div class="line"><a name="l00189"></a><span class="lineno"> 189</span> Handles are tiny objects both movable and copyable. They return the contained</div><div class="line"><a name="l00190"></a><span class="lineno"> 190</span> resource as a const reference on request:</div><div class="line"><a name="l00191"></a><span class="lineno"> 191</span> </div><div class="line"><a name="l00192"></a><span class="lineno"> 192</span> * By means of the `get` member function:</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>  const auto &resource = handle.get();</div><div class="line"><a name="l00196"></a><span class="lineno"> 196</span>  ```</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> * Using the proper cast operator:</div><div class="line"><a name="l00199"></a><span class="lineno"> 199</span> </div><div class="line"><a name="l00200"></a><span class="lineno"> 200</span>  ```cpp</div><div class="line"><a name="l00201"></a><span class="lineno"> 201</span>  const auto &resource = handle;</div><div class="line"><a name="l00202"></a><span class="lineno"> 202</span>  ```</div><div class="line"><a name="l00203"></a><span class="lineno"> 203</span> </div><div class="line"><a name="l00204"></a><span class="lineno"> 204</span> * Through the dereference operator:</div><div class="line"><a name="l00205"></a><span class="lineno"> 205</span> </div><div class="line"><a name="l00206"></a><span class="lineno"> 206</span>  ```cpp</div><div class="line"><a name="l00207"></a><span class="lineno"> 207</span>  const auto &resource = *handle;</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> The resource can also be accessed directly using the arrow operator if required:</div><div class="line"><a name="l00211"></a><span class="lineno"> 211</span> </div><div class="line"><a name="l00212"></a><span class="lineno"> 212</span> ```cpp</div><div class="line"><a name="l00213"></a><span class="lineno"> 213</span> auto value = handle->value;</div><div class="line"><a name="l00214"></a><span class="lineno"> 214</span> ```</div><div class="line"><a name="l00215"></a><span class="lineno"> 215</span> </div><div class="line"><a name="l00216"></a><span class="lineno"> 216</span> To test if a handle is still valid, the cast operator to `bool` allows users to</div><div class="line"><a name="l00217"></a><span class="lineno"> 217</span> use it in a guard:</div><div class="line"><a name="l00218"></a><span class="lineno"> 218</span> </div><div class="line"><a name="l00219"></a><span class="lineno"> 219</span> ```cpp</div><div class="line"><a name="l00220"></a><span class="lineno"> 220</span> if(handle) {</div><div class="line"><a name="l00221"></a><span class="lineno"> 221</span>  // ...</div><div class="line"><a name="l00222"></a><span class="lineno"> 222</span> }</div><div class="line"><a name="l00223"></a><span class="lineno"> 223</span> ```</div><div class="line"><a name="l00224"></a><span class="lineno"> 224</span> </div><div class="line"><a name="l00225"></a><span class="lineno"> 225</span> Finally, in case there is the need to load a resource and thus to get a handle</div><div class="line"><a name="l00226"></a><span class="lineno"> 226</span> without storing the resource itself in the cache, users can rely on the `temp`</div><div class="line"><a name="l00227"></a><span class="lineno"> 227</span> member function template.<br/></div><div class="line"><a name="l00228"></a><span class="lineno"> 228</span> The declaration is similar to that of `load`, a (possibly invalid) handle for</div><div class="line"><a name="l00229"></a><span class="lineno"> 229</span> the resource is returned also in this case:</div><div class="line"><a name="l00230"></a><span class="lineno"> 230</span> </div><div class="line"><a name="l00231"></a><span class="lineno"> 231</span> ```cpp</div><div class="line"><a name="l00232"></a><span class="lineno"> 232</span> if(auto handle = cache.temp<my_loader>(42); handle) {</div><div class="line"><a name="l00233"></a><span class="lineno"> 233</span>  // ...</div><div class="line"><a name="l00234"></a><span class="lineno"> 234</span> }</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> Do not forget to test the handle for validity. Otherwise, getting a reference to</div><div class="line"><a name="l00238"></a><span class="lineno"> 238</span> the resource it points may result in undefined behavior.</div></div><!-- fragment --></div><!-- contents -->
|
|
<!-- start footer part -->
|
|
<hr class="footer"/><address class="footer"><small>
|
|
Generated by  <a href="http://www.doxygen.org/index.html">
|
|
<img class="footer" src="doxygen.png" alt="doxygen"/>
|
|
</a> 1.8.13
|
|
</small></address>
|
|
</body>
|
|
</html>
|