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  28. <h2><span class="refentrytitle">Macro BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION</span></h2>
    29. 

  29. <p>BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION &#8212; Program preconditions that can be disabled at compile-time for constructors. </p>
    30. 

  30. </div>
    31. 

  31. <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
    32. 

  32. <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="reference.html#header.boost.contract_macro_hpp" title="Header &lt;boost/contract_macro.hpp&gt;">boost/contract_macro.hpp</a>&gt;
    33. 

  33. 

  34. </span>BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION(...)</pre></div>
    35. 

  35. <div class="refsect1">
    36. 

  36. <a name="idm45707992490480"></a><h2>Description</h2>
    37. 

  37. <p>This is used together with <code class="computeroutput"><a class="link" href="BOOST_CONTRACT_CONSTRUCTOR.html" title="Macro BOOST_CONTRACT_CONSTRUCTOR">BOOST_CONTRACT_CONSTRUCTOR</a></code> to specify contracts for constructors. Constructors that do not have preconditions do not use this macro. When at least one of the class constructors uses this macro,  <code class="computeroutput"><a class="link" href="boost/contract/constructor_precondition.html" title="Class template constructor_precondition">boost::contract::constructor_precondition</a></code> must be the first and private base of the class declaring the constructor for which preconditions are programmed:</p>
    38. 

  38. <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span>
    39. 

  39.     <span class="preprocessor">#define</span> <span class="identifier">BASES</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">u</span><span class="special">&gt;</span><span class="special">,</span> \
    40. 

  40.             <span class="keyword">public</span> <span class="identifier">b</span>
    41. 

  41.     <span class="special">:</span> <span class="identifier">BASES</span>
    42. 

  42. <span class="special">{</span>
    43. 

  43.     <span class="keyword">friend</span> <span class="keyword">class</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">access</span><span class="special">;</span>
    44. 

  44. 

  45.     <span class="keyword">typedef</span> <span class="identifier">BOOST_CONTRACT_BASE_TYPES</span><span class="special">(</span><span class="identifier">BASES</span><span class="special">)</span> <span class="identifier">base_types</span><span class="special">;</span>
    46. 

  46.     <span class="preprocessor">#undef</span> <span class="identifier">BASES</span>
    47. 

  47. 

  48.     <span class="special">...</span>
    49. 

  49. 

  50. <span class="keyword">public</span><span class="special">:</span>
    51. 

  51.     <span class="keyword">explicit</span> <span class="identifier">u</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">:</span>
    52. 

  52.         <span class="identifier">BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION</span><span class="special">(</span><span class="identifier">u</span><span class="special">)</span><span class="special">(</span><span class="special">[</span><span class="special">&amp;</span><span class="special">]</span> <span class="special">{</span>
    53. 

  53.             <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special">!=</span> <span class="number">0</span><span class="special">)</span><span class="special">;</span>
    54. 

  54.         <span class="special">}</span><span class="special">)</span><span class="special">,</span>
    55. 

  55.         <span class="identifier">b</span><span class="special">(</span><span class="number">1</span> <span class="special">/</span> <span class="identifier">x</span><span class="special">)</span>
    56. 

  56.     <span class="special">{</span>
    57. 

  57.         <span class="special">...</span>
    58. 

  58.     <span class="special">}</span>
    59. 

  59. 

  60.     <span class="special">...</span>
    61. 

  61. <span class="special">}</span><span class="special">;</span>
    62. 

  62. </pre>
    63. 

  63. <p><code class="computeroutput">BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION(class_type)(f)</code> expands to code equivalent to the following (note that when  <code class="computeroutput"><a class="link" href="BOOST_CO_idm45394999662416.html" title="Macro BOOST_CONTRACT_NO_PRECONDITIONS">BOOST_CONTRACT_NO_PRECONDITIONS</a></code> is defined, this macro trivially expands to a default constructor call that is internally implemented to do nothing so this should have minimal to no overhead):</p>
    64. 

  64. <pre class="programlisting"><span class="comment">// Guarded only by NO_PRECONDITIONS (and not also by NO_CONSTRUCTORS)</span>
    65. 

  65. <span class="comment">// because for constructor's preconditions (not for postconditions, etc.).</span>
    66. 

  66. <span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_PRECONDITIONS</span>
    67. 

  67.     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">class_type</span><span class="special">&gt;</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
    68. 

  68. <span class="preprocessor">#else</span> <span class="comment">// No-op call (likely optimized away, minimal to no overhead).</span>
    69. 

  69.     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">constructor_precondition</span><span class="special">&lt;</span><span class="identifier">class_type</span><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span>
    70. 

  70. <span class="preprocessor">#endif</span>
    71. 

  71. </pre>
    72. 

  72. <p>Where:</p>
    73. 

  73. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    74. 

  74. <li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>class_type</strong></span></code> is the type of the class containing the constructor for which preconditions are being programmed. (This is a variadic macro parameter so it can contain commas not protected by round parenthesis.) </p></li>
    75. 

  75. <li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>f</strong></span></code> is the nullary functor called by this library to check constructor preconditions <code class="computeroutput">f()</code>. Assertions within this functor call are usually programmed using  <code class="computeroutput"><a class="link" href="BOOST_CONTRACT_ASSERT.html" title="Macro BOOST_CONTRACT_ASSERT">BOOST_CONTRACT_ASSERT</a></code>, but any exception thrown by a call to this functor indicates a contract failure (and will result in this library calling  <code class="computeroutput"><a class="link" href="boost/contract/precondition_failure.html" title="Function precondition_failure">boost::contract::precondition_failure</a></code>). This functor should capture variables by (constant) value, or better by (constant) reference to avoid extra copies. (This is a variadic macro parameter so it can contain commas not protected by round parenthesis.)</p></li>
    76. 

  76. </ul></div>
    77. 

  77. <p>
    78. 

  78. </p>
    79. 

  79. <p><span class="bold"><strong>See Also:</strong></span></p>
    80. 

  80. <p> <a class="link" href="boost_contract/extras.html#boost_contract.extras.disable_contract_compilation__macro_interface_" title="Disable Contract Compilation (Macro Interface)">
    81. 

  81.         Disable Contract Compilation</a>,  <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.constructors" title="Constructors"> Constructors</a> </p>
    82. 

  82. <p>
    83. 

  83. </p>
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  84. <p>
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  85. </p>
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  86. </div>
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    89. 

  89. <td align="left"></td>
    90. 

  90. <td align="right"><div class="copyright-footer">Copyright &#169; 2008-2019 Lorenzo Caminiti<p>
    91. 

  91.         Distributed under the Boost Software License, Version 1.0 (see accompanying
    92. 

  92.         file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
    93. 

  93.       </p>
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