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type_requirements.html

type_requirements.html 12 KB
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  4. <title>Type requirements</title>
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    25. 

  25. <div class="section">
    26. 

  26. <div class="titlepage"><div><div><h3 class="title">
    27. 

  27. <a name="boost_optional.tutorial.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
    28. 

  28. </h3></div></div></div>
    29. 

  29. <p>
    30. 

  30.         The very minimum requirement of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
    31. 

  31.         is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type
    32. 

  32.         and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code>
    33. 

  33.         doesn't even need to be constructible. You can use a very minimum interface:
    34. 

  34.       </p>
    35. 

  35. <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span><span class="special">;</span>     <span class="comment">// uninitialized</span>
    36. 

  36. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// check if initialized</span>
    37. 

  37. <span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span>        <span class="comment">//</span>
    38. 

  38. <span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span>         <span class="comment">// always throws</span>
    39. 

  39. </pre>
    40. 

  40. <p>
    41. 

  41.         But this is practically useless. In order for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
    42. 

  42.         to be able to do anything useful and offer all the spectrum of ways of accessing
    43. 

  43.         the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs
    44. 

  44.         to have at least one accessible constructor. In that case you need to initialize
    45. 

  45.         the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>, or if your compiler does not support it,
    46. 

  46.         resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
    47. 

  47.         Factories</a>:
    48. 

  48.       </p>
    49. 

  49. <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span><span class="special">;</span>
    50. 

  50. <span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"T"</span><span class="special">,</span> <span class="string">"ctor"</span><span class="special">,</span> <span class="string">"params"</span><span class="special">);</span>
    51. 

  51. </pre>
    52. 

  52. <p>
    53. 

  53.         If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>,
    54. 

  54.         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
    55. 

  55.         also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
    56. 

  56.         can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
    57. 

  57.         and be passed by value:
    58. 

  58.       </p>
    59. 

  59. <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
    60. 

  60. <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;();</span>
    61. 

  61. </pre>
    62. 

  62. <p>
    63. 

  63.         If <code class="computeroutput"><span class="identifier">T</span></code> is <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
    64. 

  64.         also <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>
    65. 

  65.         and can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>:
    66. 

  66.       </p>
    67. 

  67. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
    68. 

  68. <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">v</span><span class="special">;</span>
    69. 

  69. <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">;</span>
    70. 

  70. </pre>
    71. 

  71. <p>
    72. 

  72.         If <code class="computeroutput"><span class="identifier">T</span></code> is not <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>, it is still possible to
    73. 

  73.         reset the value of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
    74. 

  74.         using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>:
    75. 

  75.       </p>
    76. 

  76. <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
    77. 

  77. <span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">make_another_T</span><span class="special">());</span>
    78. 

  78. </pre>
    79. 

  79. <p>
    80. 

  80.         If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>
    81. 

  81.         (both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
    82. 

  82.         <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
    83. 

  83.         also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally
    84. 

  84.         can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
    85. 

  85.       </p>
    86. 

  86. <p>
    87. 

  87.         Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
    88. 

  88.         is also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally
    89. 

  89.         can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
    90. 

  90.       </p>
    91. 

  91. <p>
    92. 

  92.         <code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span>
    93. 

  93.         required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>.
    94. 

  94.       </p>
    95. 

  95. </div>
    96. 

  96. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    97. 

  97. <td align="left"></td>
    98. 

  98. <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
    99. 

  99.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    100. 

  100.         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
    101. 

  101.       </p>
    102. 

  102. </div></td>
    103. 

  103. </tr></table>
    104. 

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