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4<title>Macro BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</title>
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28<h2><span class="refentrytitle">Macro BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</span></h2>
29<p>BOOST_CONTRACT_OLD_PTR_IF_COPYABLE — Program old values that can be completely disabled at compile-time for old value types that are not 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_IF_COPYABLE(...)</pre></div>
35<div class="refsect1">
36<a name="idm45988675613968"></a><h2>Description</h2>
37<p>This is used to program old value copies for types that might or might not be copyable:</p>
38<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="comment">// Type `T` might or not be copyable.</span>
39<span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
40<span class="keyword">public</span><span class="special">:</span>
41    <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>
42        <span class="identifier">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</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>
43        <span class="identifier">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</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>
44                <span class="identifier">old_expr_b</span><span class="special">)</span><span class="special">;</span>
45        <span class="identifier">BOOST_CONTRACT_PUBLIC_FUNCTION</span><span class="special">(</span><span class="keyword">this</span><span class="special">)</span>
46            <span class="special">...</span>
47            <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>
48                <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>
49                <span class="special">...</span>
50            <span class="special">}</span><span class="special">)</span>
51            <span class="special">...</span> <span class="comment">// In postconditions or exception guarantees:</span>
52                <span class="keyword">if</span><span class="special">(</span><span class="identifier">old_var_a</span><span class="special">)</span> <span class="special">...</span> <span class="comment">// Always null for non-copyable types.</span>
53                <span class="keyword">if</span><span class="special">(</span><span class="identifier">old_var_b</span><span class="special">)</span> <span class="special">...</span> <span class="comment">// Always null for non-copyable types.</span>
54            <span class="special">...</span>
55        <span class="special">;</span>
56
57        <span class="special">...</span> <span class="comment">// Function body.</span>
58    <span class="special">}</span>
59
60    <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>
61        <span class="identifier">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</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>
62        <span class="identifier">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE</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>
63                <span class="identifier">old_expr_b</span><span class="special">)</span><span class="special">;</span>
64        <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>
65            <span class="special">...</span>
66            <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>
67                <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>
68                <span class="special">...</span>
69            <span class="special">}</span><span class="special">)</span>
70            <span class="special">...</span> <span class="comment">// In postconditions or exception guarantees:</span>
71                <span class="keyword">if</span><span class="special">(</span><span class="identifier">old_var_a</span><span class="special">)</span> <span class="special">...</span> <span class="comment">// Always null for non-copyable types.</span>
72                <span class="keyword">if</span><span class="special">(</span><span class="identifier">old_var_b</span><span class="special">)</span> <span class="special">...</span> <span class="comment">// Always null for non-copyable types.</span>
73            <span class="special">...</span>
74        <span class="special">;</span>
75
76        <span class="special">...</span> <span class="comment">// Function body.</span>
77    <span class="special">}</span>
78
79    <span class="special">...</span>
80<span class="special">}</span><span class="special">;</span>
81</pre>
82<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>
83<p>1. <code class="computeroutput">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE(old_type)(old_var)</code> expands to code equivalent to the following (this leaves the old value pointer null):</p>
84<pre class="programlisting"><span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_OLDS</span>
85    <span class="comment">// This declaration does not need to use `v`.</span>
86    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr_if_copyable</span><span class="special">&lt;</span><span class="identifier">old_type</span><span class="special">&gt;</span> <span class="identifier">old_var</span>
87<span class="preprocessor">#endif</span>
88</pre>
89<p>2. <code class="computeroutput">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE(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>
90<pre class="programlisting"><span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_OLDS</span>
91    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr_if_copyable</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>
92            <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">old_expr</span><span class="special">)</span>
93<span class="preprocessor">#endif</span>
94</pre>
95<p>3. <code class="computeroutput">BOOST_CONTRACT_OLD_PTR_IF_COPYABLE(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>
96<pre class="programlisting"><span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_CONTRACT_NO_OLDS</span>
97    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr_if_copyable</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>
98            <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>
99<span class="preprocessor">#endif</span>
100</pre>
101<p>Where:</p>
102<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
103<li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>old_type</strong></span></code> is the type of the pointed old value. If this type is not copyable (i.e., <code class="computeroutput">boost::contract::is_old_value_copyable&lt;old_type&gt;::value</code> is <code class="computeroutput">false</code>), this pointer will always be null, but this library will not generate a compile-time error when this pointer is dereferenced (see <code class="computeroutput"><a class="link" href="BOOST_CONTRACT_OLD_PTR.html" title="Macro BOOST_CONTRACT_OLD_PTR">BOOST_CONTRACT_OLD_PTR</a></code>). (This is a variadic macro parameter so it can contain commas not protected by round parenthesis.) </p></li>
104<li class="listitem"><p><code class="computeroutput"><span class="bold"><strong>v</strong></span></code> is the extra trailing 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>
105<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>
106<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_IF_COPYABLE(old_type)(v, old_var, (old_expr))</code> will always work.)</p></li>
107</ul></div>
108<p>
109</p>
110<p><span class="bold"><strong>See Also:</strong></span></p>
111<p> <a class="link" href="boost_contract/extras.html#boost_contract.extras.disable_contract_compilation__macro_interface_" title="Disable Contract Compilation (Macro Interface)">
112        Disable Contract Compilation</a>,  <a class="link" href="boost_contract/extras.html#boost_contract.extras.old_value_requirements__templates_" title="Old Value Requirements (Templates)">
113        Old Value Requirements</a> </p>
114<p>
115</p>
116<p>
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