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   4 <title>Function template call_if</title>
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  26 <a name="boost.contract.call_if"></a><div class="titlepage"></div>
  27 <div class="refnamediv">
  28 <h2><span class="refentrytitle">Function template call_if</span></h2>
  29 <p>boost::contract::call_if &#8212; Select compilation and execution of functor template calls using a nullary boolean meta-function (not needed on C++17 compilers, use <code class="computeroutput">if constexpr</code> instead). </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.call_if_hpp" title="Header &lt;boost/contract/call_if.hpp&gt;">boost/contract/call_if.hpp</a>&gt;
  33
  34 </span>
  35 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Pred<span class="special">,</span> <span class="keyword">typename</span> Then<span class="special">&gt;</span>
  36   <a class="link" href="call_if_statement.html" title="Struct template call_if_statement">call_if_statement</a><span class="special">&lt;</span> <span class="identifier">Pred</span><span class="special">::</span><span class="identifier">value</span><span class="special">,</span> <span class="identifier">Then</span> <span class="special">&gt;</span> <span class="identifier">call_if</span><span class="special">(</span><span class="identifier">Then</span> f<span class="special">)</span><span class="special">;</span></pre></div>
  37 <div class="refsect1">
  38 <a name="idm45707997647376"></a><h2>Description</h2>
  39 <p>This is equivalent to <code class="computeroutput">boost::contract::call_if_c&lt;Pred::value&gt;(f)</code>. Create a call-if object with the specified then-branch functor template:</p>
  40 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">call_if</span><span class="special">&lt;</span><span class="identifier">Pred1</span><span class="special">&gt;</span><span class="special">(</span>
  41     <span class="identifier">then_functor_template1</span>
  42 <span class="special">)</span><span class="special">.</span><span class="keyword">template</span> <span class="identifier">else_if</span><span class="special">&lt;</span><span class="identifier">Pred2</span><span class="special">&gt;</span><span class="special">(</span>              <span class="comment">// Optional.</span>
  43     <span class="identifier">then_functor_template2</span>
  44 <span class="special">)</span>                                       <span class="comment">// Optionally, other `else_if` or</span>
  45 <span class="special">...</span>                                     <span class="comment">// `else_if_c`.</span>
  46 <span class="special">.</span><span class="identifier">else_</span><span class="special">(</span>                                 <span class="comment">// Optional for `void` functors,</span>
  47     <span class="identifier">else_functor_template</span>               <span class="comment">// but required for non `void`.</span>
  48 <span class="special">)</span>
  49 </pre>
  50 <p>Optional functor templates for else-if-branches and the else-branch can be specified as needed (the else-branch functor template is required if <code class="computeroutput">f</code> returns non-void).</p>
  51 <p><span class="bold"><strong>See Also:</strong></span></p>
  52 <p> <a class="link" href="../../boost_contract/extras.html#boost_contract.extras.assertion_requirements__templates_" title="Assertion Requirements (Templates)">
  53         Assertion Requirements</a></p>
  54 <p>
  55 </p>
  56 <p>
  57
  58
  59
  60 </p>
  61 <div class="variablelist"><table border="0" class="variablelist compact">
  62 <colgroup>
  63 <col align="left" valign="top">
  64 <col>
  65 </colgroup>
  66 <tbody>
  67 <tr>
  68 <td><p><span class="term">Parameters:</span></p></td>
  69 <td><div class="variablelist"><table border="0" class="variablelist compact">
  70 <colgroup>
  71 <col align="left" valign="top">
  72 <col>
  73 </colgroup>
  74 <tbody><tr>
  75 <td><p><span class="term"><code class="computeroutput">f</code></span></p></td>
  76 <td><p>Then-branch nullary functor template. The functor template call <code class="computeroutput">f()</code> is compiled and executed if and only if <code class="computeroutput">Pred::value</code> is <code class="computeroutput">true</code>. The return type of other functor template calls specified for this call-if statement (else-branch, else-if-branches, etc.) must be the same as (or implicitly convertible to) the return type of then-branch functor template call <code class="computeroutput">f()</code>.</p></td>
  77 </tr></tbody>
  78 </table></div></td>
  79 </tr>
  80 <tr>
  81 <td><p><span class="term">Template Parameters:</span></p></td>
  82 <td><div class="variablelist"><table border="0" class="variablelist compact">
  83 <colgroup>
  84 <col align="left" valign="top">
  85 <col>
  86 </colgroup>
  87 <tbody><tr>
  88 <td><p><span class="term"><code class="computeroutput">Pred</code></span></p></td>
  89 <td><p>Nullary boolean meta-function selecting which functor template call to compile and execute.</p></td>
  90 </tr></tbody>
  91 </table></div></td>
  92 </tr>
  93 <tr>
  94 <td><p><span class="term">Returns:</span></p></td>
  95 <td><p>A call-if statement so else and else-if statements can be specified if needed. Eventually, this will be the return value of the one functor template call being compiled and executed (which could also be <code class="computeroutput">void</code>). </p></td>
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 105         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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