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  28 <h2><span class="refentrytitle">Macro BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION</span></h2>
  29 <p>BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION &#8212; Program preconditions that can be disabled at compile-time for constructors. </p>
  30 </div>
  31 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  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
  34 </span>BOOST_CONTRACT_CONSTRUCTOR_PRECONDITION(...)</pre></div>
  35 <div class="refsect1">
  36 <a name="idm45751726005776"></a><h2>Description</h2>
  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 class of the class declaring the constructor for which preconditions are programmed:</p>
  38 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span>
  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             <span class="keyword">public</span> <span class="identifier">b</span>
  41     <span class="special">:</span> <span class="identifier">BASES</span>
  42 <span class="special">{</span>
  43     <span class="special">...</span>
  44     <span class="preprocessor">#undef</span> <span class="identifier">BASES</span>
  45
  46 <span class="keyword">public</span><span class="special">:</span>
  47     <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>
  48         <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>
  49             <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>
  50         <span class="special">}</span><span class="special">)</span><span class="special">,</span>
  51         <span class="identifier">b</span><span class="special">(</span><span class="number">1</span><span class="special">.</span><span class="number">0</span> <span class="special">/</span> <span class="keyword">float</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span><span class="special">)</span>
  52     <span class="special">{</span>
  53         <span class="special">...</span>
  54     <span class="special">}</span>
  55
  56     <span class="special">...</span>
  57 <span class="special">}</span><span class="special">;</span>
  58 </pre>
  59 <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_idm45123870517808.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>
  60 <pre class="programlisting"><span class="comment">// Guarded only by NO_PRECONDITIONS (and not also by NO_CONSTRUCTORS)</span>
  61 <span class="comment">// because for constructor's preconditions (not for postconditions, etc.).</span>
  62 <span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_PRECONDITIONS</span>
  63     <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>
  64 <span class="preprocessor">#else</span>
  65     <span class="comment">// No-op call (likely optimized away, minimal to no overhead).</span>
  66     <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>
  67 <span class="preprocessor">#endif</span>
  68 </pre>
  69 <p>Where:</p>
  70 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  71 <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>
  72 <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>
  73 </ul></div>
  74 <p>
  75 </p>
  76 <p><span class="bold"><strong>See Also:</strong></span></p>
  77 <p> <a class="link" href="boost_contract/extras.html#boost_contract.extras.disable_contract_compilation__macro_interface_" title="Disable Contract Compilation (Macro Interface)">
  78         Disable Contract Compilation</a>,  <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.constructors" title="Constructors"> Constructors</a> </p>
  79 <p>
  80 </p>
  81 <p>
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  87 <td align="right"><div class="copyright-footer">Copyright &#169; 2008-2018 Lorenzo Caminiti<p>
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  89         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>)
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