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  49.   <div class="section" id="boost-hof-static-function">
    50. 

  50. <h1>BOOST_HOF_STATIC_FUNCTION<a class="headerlink" href="#boost-hof-static-function" title="Permalink to this headline">¶</a></h1>
    51. 

  51. <div class="section" id="header">
    52. 

  52. <h2>Header<a class="headerlink" href="#header" title="Permalink to this headline">¶</a></h2>
    53. 

  53. <div class="highlight-cpp"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&lt;boost/hof/function.hpp&gt;</span><span class="cp"></span>
    54. 

  54. </pre></div>
    55. 

  55. </div>
    56. 

  56. </div>
    57. 

  57. <div class="section" id="description">
    58. 

  58. <h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
    59. 

  59. <p>The <code class="docutils literal"><span class="pre">BOOST_HOF_STATIC_FUNCTION</span></code> macro allows initializing a function object from a
    60. 

  60. <code class="docutils literal"><span class="pre">constexpr</span></code> expression. It uses the best practices as outlined in
    61. 

  61. <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html">N4381</a>.
    62. 

  62. This includes using <code class="docutils literal"><span class="pre">const</span></code> to avoid global state, compile-time
    63. 

  63. initialization of the function object to avoid the <a class="reference external" href="https://isocpp.org/wiki/faq/ctors#static-init-order">static initialization
    64. 

  64. order fiasco</a>, and an
    65. 

  65. external address of the function object that is the same across translation
    66. 

  66. units to avoid possible One-Definition-Rule(ODR) violations.</p>
    67. 

  67. <p>In C++17, this achieved using the <code class="docutils literal"><span class="pre">inline</span></code> keyword. However, on older
    68. 

  68. compilers it is initialized using a reference to a static member variable.
    69. 

  69. The static member variable is default constructed, as such the user variable
    70. 

  70. is always default constructed regardless of the expression.</p>
    71. 

  71. <p>By default, all functions defined with <code class="docutils literal"><span class="pre">BOOST_HOF_STATIC_FUNCTION</span></code> use the
    72. 

  72. <a class="reference internal" href="reveal.html"><span class="doc">reveal</span></a> adaptor to improve error messages.</p>
    73. 

  73. </div>
    74. 

  74. <div class="section" id="example">
    75. 

  75. <h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
    76. 

  76. <div class="highlight-cpp"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&lt;boost/hof.hpp&gt;</span><span class="cp"></span>
    77. 

  77. <span class="cp">#include</span> <span class="cpf">&lt;cassert&gt;</span><span class="cp"></span>
    78. 

  78. 

  79. <span class="k">struct</span> <span class="n">sum_f</span>
    80. 

  80. <span class="p">{</span>
    81. 

  81.     <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="p">,</span> <span class="k">class</span> <span class="nc">U</span><span class="o">&gt;</span>
    82. 

  82.     <span class="n">T</span> <span class="k">operator</span><span class="p">()(</span><span class="n">T</span> <span class="n">x</span><span class="p">,</span> <span class="n">U</span> <span class="n">y</span><span class="p">)</span> <span class="k">const</span>
    83. 

  83.     <span class="p">{</span>
    84. 

  84.         <span class="k">return</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">;</span>
    85. 

  85.     <span class="p">}</span>
    86. 

  86. <span class="p">};</span>
    87. 

  87. 

  88. <span class="n">BOOST_HOF_STATIC_FUNCTION</span><span class="p">(</span><span class="n">sum</span><span class="p">)</span> <span class="o">=</span> <span class="n">sum_f</span><span class="p">();</span>
    89. 

  89. <span class="n">BOOST_HOF_STATIC_FUNCTION</span><span class="p">(</span><span class="n">partial_sum</span><span class="p">)</span> <span class="o">=</span> <span class="n">boost</span><span class="o">::</span><span class="n">hof</span><span class="o">::</span><span class="n">partial</span><span class="p">(</span><span class="n">sum_f</span><span class="p">());</span>
    90. 

  90. 

  91. <span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
    92. 

  92.     <span class="n">assert</span><span class="p">(</span><span class="n">sum</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="o">==</span> <span class="n">partial_sum</span><span class="p">(</span><span class="mi">1</span><span class="p">)(</span><span class="mi">2</span><span class="p">));</span>
    93. 

  93. <span class="p">}</span>
    94. 

  94. </pre></div>
    95. 

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  108.             &#169; Copyright 2016, Paul Fultz II.
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  110.           Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.5.6.
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  111.           <p>Distributed under the Boost Software License, Version 1.0.
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  112.           (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 
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