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  26 <a name="BOOST_CONTRACT_OLD_PTR"></a><div class="titlepage"></div>
  27 <div class="refnamediv">
  28 <h2><span class="refentrytitle">Macro BOOST_CONTRACT_OLD_PTR</span></h2>
  29 <p>BOOST_CONTRACT_OLD_PTR &#8212; Program old values that can be completely disabled at compile-time for old value types that are required to be copyable. </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_OLD_PTR(...)</pre></div>
  35 <div class="refsect1">
  36 <a name="idm45707992959344"></a><h2>Description</h2>
  37 <p>This is used to program old value copies for copyable types:</p>
  38 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
  39 <span class="keyword">public</span><span class="special">:</span>
  40     <span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="special">...</span><span class="special">)</span> <span class="special">{</span>
  41         <span class="identifier">BOOST_CONTRACT_OLD_PTR</span><span class="special">(</span><span class="identifier">old_type_a</span><span class="special">)</span><span class="special">(</span><span class="identifier">old_var_a</span><span class="special">)</span><span class="special">;</span> <span class="comment">// Null...</span>
  42         <span class="identifier">BOOST_CONTRACT_OLD_PTR</span><span class="special">(</span><span class="identifier">old_type_b</span><span class="special">)</span><span class="special">(</span><span class="identifier">old_var_b</span><span class="special">,</span> <span class="identifier">old_expr_b</span><span class="special">)</span><span class="special">;</span> <span class="comment">// Set.</span>
  43         <span class="identifier">BOOST_CONTRACT_PUBLIC_FUNCTION</span><span class="special">(</span><span class="keyword">this</span><span class="special">)</span>
  44             <span class="special">...</span>
  45             <span class="identifier">BOOST_CONTRACT_OLD</span><span class="special">(</span><span class="special">[</span><span class="special">&amp;</span><span class="special">]</span> <span class="special">{</span>
  46                 <span class="identifier">old_var_a</span> <span class="special">=</span> <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">old_expr_a</span><span class="special">)</span><span class="special">;</span> <span class="comment">// ...set.</span>
  47                 <span class="special">...</span>
  48             <span class="special">}</span><span class="special">)</span>
  49             <span class="special">...</span>
  50         <span class="special">;</span>
  51
  52         <span class="special">...</span> <span class="comment">// Function body.</span>
  53     <span class="special">}</span>
  54
  55     <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">g</span><span class="special">(</span><span class="special">...</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span>
  56         <span class="identifier">BOOST_CONTRACT_OLD_PTR</span><span class="special">(</span><span class="identifier">old_type_a</span><span class="special">)</span><span class="special">(</span><span class="identifier">old_var_a</span><span class="special">)</span><span class="special">;</span> <span class="comment">// No `v`</span>
  57         <span class="identifier">BOOST_CONTRACT_OLD_PTR</span><span class="special">(</span><span class="identifier">old_type_b</span><span class="special">)</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">old_var_b</span><span class="special">,</span> <span class="identifier">old_expr_b</span><span class="special">)</span><span class="special">;</span> <span class="comment">// `v`</span>
  58         <span class="identifier">BOOST_CONTRACT_PUBLIC_FUNCTION</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)</span>
  59             <span class="special">...</span>
  60             <span class="identifier">BOOST_CONTRACT_OLD</span><span class="special">(</span><span class="special">[</span><span class="special">&amp;</span><span class="special">]</span> <span class="special">{</span>
  61                 <span class="identifier">old_var_a</span> <span class="special">=</span> <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">old_expr_a</span><span class="special">)</span><span class="special">;</span> <span class="comment">// `v`</span>
  62                 <span class="special">...</span>
  63             <span class="special">}</span><span class="special">)</span>
  64             <span class="special">...</span>
  65         <span class="special">;</span>
  66
  67         <span class="special">...</span> <span class="comment">// Function body.</span>
  68     <span class="special">}</span>
  69
  70     <span class="special">...</span>
  71 <span class="special">}</span><span class="special">;</span>
  72 </pre>
  73 <p>This is an overloaded variadic macro and it can be used in the following different ways (note that no code is generated when  <code class="computeroutput"><a class="link" href="BOOST_CONTRACT_NO_OLDS.html" title="Macro BOOST_CONTRACT_NO_OLDS">BOOST_CONTRACT_NO_OLDS</a></code> is defined).</p>
  74 <p>1. <code class="computeroutput">BOOST_CONTRACT_OLD_PTR(old_type)(old_var)</code> expands to code equivalent to the following (this leaves the old value pointer null):</p>
  75 <pre class="programlisting"><span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_OLDS</span>
  76     <span class="comment">// This declaration does not need to use `v`.</span>
  77     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="identifier">old_type</span><span class="special">&gt;</span> <span class="identifier">old_var</span>
  78 <span class="preprocessor">#endif</span>
  79 </pre>
  80 <p>2. <code class="computeroutput">BOOST_CONTRACT_OLD_PTR(old_type)(old_var, old_expr)</code> expands to code equivalent to the following (this initializes the pointer to the old value copy, but not to be used for virtual public functions and public function overrides):</p>
  81 <pre class="programlisting"><span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_OLDS</span>
  82     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="identifier">old_type</span><span class="special">&gt;</span> <span class="identifier">old_var</span> <span class="special">=</span>
  83             <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">old_expr</span><span class="special">)</span>
  84 <span class="preprocessor">#endif</span>
  85 </pre>
  86 <p>3. <code class="computeroutput">BOOST_CONTRACT_OLD_PTR(old_type)(v, old_var, old_expr)</code> expands to code equivalent to the following (this initializes the pointer to the old value copy for virtual public functions and public function overrides):</p>
  87 <pre class="programlisting"><span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_OLDS</span>
  88     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="identifier">old_type</span><span class="special">&gt;</span> <span class="identifier">old_var</span> <span class="special">=</span>
  89             <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">old_expr</span><span class="special">)</span>
  90 <span class="preprocessor">#endif</span>
  91 </pre>
  92 <p>Where:</p>
  93 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  94 <li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>old_type</strong></span></code> is the type of the pointed old value. This type must be copyable (i.e., <code class="computeroutput">boost::contract::is_old_value_copyable&lt;old_type&gt;::value</code> is <code class="computeroutput">true</code>), otherwise this pointer will always be null and this library will generate a compile-time error when the pointer is dereferenced (see <code class="computeroutput"><a class="link" href="BOOST_CO_idm45394998674080.html" title="Macro BOOST_CONTRACT_OLD_PTR_IF_COPYABLE">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</a></code>). (This is a variadic macro parameter so it can contain commas not protected by round parenthesis.) (Rationale: Template parameters like this one are specified to this library macro interface using their own set of parenthesis <code class="computeroutput">SOME_MACRO(template_params)(other_params)</code>.) </p></li>
  95 <li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>v</strong></span></code> is the extra training parameter of type  <code class="computeroutput"><a class="link" href="boost/contract/virtual_.html" title="Class virtual_">boost::contract::virtual_</a></code><code class="computeroutput">*</code> and default value <code class="computeroutput">0</code> from the enclosing virtual public function or public function override declaring the contract. (This is not a variadic macro parameter but it should never contain commas because it is an identifier.) </p></li>
  96 <li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>old_var</strong></span></code> is the name of the old value pointer variable. (This is not a variadic macro parameter but it should never contain commas because it is an identifier.) </p></li>
  97 <li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>old_expr</strong></span></code> is the expression to be evaluated and copied in the old value pointer. (This is not a variadic macro parameter so any comma it might contain must be protected by round parenthesis and <code class="computeroutput">BOOST_CONTRACT_OLD_PTR(old_type)(v, old_var, (old_expr))</code> will always work.)</p></li>
  98 </ul></div>
  99 <p>
 100 </p>
 101 <p><span class="bold"><strong>See Also:</strong></span></p>
 102 <p> <a class="link" href="boost_contract/extras.html#boost_contract.extras.disable_contract_compilation__macro_interface_" title="Disable Contract Compilation (Macro Interface)">
 103         Disable Contract Compilation</a>,  <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.old_values" title="Old Values"> Old Values</a> </p>
 104 <p>
 105 </p>
 106 <p>
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 114         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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