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<title>Converters</title>
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<link rel="home" href="../index.html" title="Chapter 1. Boost.Convert 2.0">
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_convert.converters"></a><a class="link" href="converters.html" title="Converters">Converters</a>
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</h2></div></div></div>
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<p>
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The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code>
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API plays its role by providing a <span class="emphasis"><em>uniform interface</em></span> and
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ensuring <span class="emphasis"><em>consistent behavior</em></span>. However, it is the respective
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converter which does the hard work of actual type conversion/transformation.
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</p>
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<p>
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<span class="emphasis"><em>Boost.Convert</em></span> design reflects the fact that no one converter
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is to satisfy all imaginable conversion/transformation-related user requirements.
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Consequently, <span class="emphasis"><em>extendibility</em></span> and <span class="emphasis"><em>converter pluggability</em></span>
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are important properties of <span class="emphasis"><em>Boost.Convert</em></span>. The library
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provides several converters for common type conversions with varying degrees
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of formatting support and performance. However, it is an expectation that more
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generic-purpose and custom-specific converters are to be written and deployed
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with <span class="emphasis"><em>Boost.Convert</em></span>.
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</p>
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<p>
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For a converter to be plugged in to the <span class="emphasis"><em>Boost.Convert</em></span>
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framework it needs to be a <span class="emphasis"><em>callable</em></span> with one of the signatures:
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">TypeOut</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">TypeIn</span><span class="special">></span>
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<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">TypeIn</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value_in</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">TypeOut</span><span class="special">>&</span> <span class="identifier">result_out</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">TypeOut</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">TypeIn</span><span class="special">></span>
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<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">TypeIn</span> <span class="identifier">value_in</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">TypeOut</span><span class="special">>&</span> <span class="identifier">result_out</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
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</pre>
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<p>
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if that is a general-purpose converter capable of handling many types (like
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string-to-type and type-to-string conversions). Alternatively, a purpose-built
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custom converter might only care to provide
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">TypeIn</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">TypeOut</span><span class="special">>&)</span> <span class="keyword">const</span><span class="special">;</span>
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</pre>
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<p>
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if its sole purpose is to handle one specific conversion/transformation of
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<span class="emphasis"><em>TypeIn</em></span> to <span class="emphasis"><em>TypeOut</em></span>. For example, a
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converter from the operating-system-specific MBCS string format to the UCS-2
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or UCS-4 (depending on <code class="computeroutput"><span class="keyword">wchar_t</span></code>
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size) might be one such example:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">wstring</span><span class="special">>&)</span> <span class="keyword">const</span><span class="special">;</span>
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</pre>
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<p>
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Alternatively again, an ad-hoc in-place <span class="emphasis"><em>callable</em></span> might
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be provided as a converter. For example,
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">;</span>
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</pre>
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<p>
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">v03</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">str</span><span class="special">,</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="identifier">_2</span><span class="special">,</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">string</span><span class="special">>,</span> <span class="identifier">_1</span><span class="special">))).</span><span class="identifier">value_or</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span>
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</pre>
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<p>
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</p>
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<p>
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or an old-fashioned function:
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">plain_old_func</span><span class="special">(</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value_in</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>&</span> <span class="identifier">value_out</span><span class="special">)</span>
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</pre>
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<p>
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">v01</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">str</span><span class="special">,</span> <span class="identifier">plain_old_func</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span>
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</pre>
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<p>
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</p>
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<p>
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With regard to converters the <span class="emphasis"><em>Boost.Convert</em></span> framework
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has been designed with the following requirement in mind:
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</p>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Converters shall be independent from and must not rely on the <span class="emphasis"><em>Boost.Convert</em></span>
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infrastructure.
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</p></td></tr>
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</table></div>
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<h4>
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<a name="boost_convert.converters.h0"></a>
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<span class="phrase"><a name="boost_convert.converters.implicit__emphasis_typein__emphasis__promotions_and_conversions"></a></span><a class="link" href="converters.html#boost_convert.converters.implicit__emphasis_typein__emphasis__promotions_and_conversions">Implicit
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<span class="emphasis"><em>TypeIn</em></span> Promotions and Conversions</a>
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</h4>
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<p>
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It is worth remembering that <span class="emphasis"><em>TypeIn</em></span> in the signature should
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be interpreted in the context of the potential implicit type promotions and
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conversions allowed <span class="emphasis"><em>by the language</em></span>. For example, depending
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on the context the <code class="computeroutput"><span class="identifier">take_double</span></code>
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and <code class="computeroutput"><span class="identifier">take_int</span></code> converters below
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might not do what is expected of them due to implicit <span class="emphasis"><em>int-to-double</em></span>
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promotion and value-destroying <span class="emphasis"><em>double-to-int</em></span> conversion
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applied <span class="emphasis"><em>by the compiler</em></span>:
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">take_double</span> <span class="special">{</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">>&)</span> <span class="keyword">const</span> <span class="special">{}};</span>
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<span class="keyword">struct</span> <span class="identifier">take_int</span> <span class="special">{</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">>&)</span> <span class="keyword">const</span> <span class="special">{}};</span>
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</pre>
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<p>
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="identifier">convert</span><span class="special"><</span><span class="identifier">string</span><span class="special">>(</span><span class="number">11</span><span class="special">,</span> <span class="identifier">take_double</span><span class="special">());</span> <span class="comment">// Compiler applies int-to-double promotion to call the converter.</span>
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<span class="identifier">convert</span><span class="special"><</span><span class="identifier">string</span><span class="special">>(</span><span class="number">11.23</span><span class="special">,</span> <span class="identifier">take_int</span><span class="special">());</span> <span class="comment">// Compiler applies double-to-int implicit truncation.</span>
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</pre>
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<p>
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</p>
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<p>
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code>
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API does not modify <span class="emphasis"><em>TypeIn</em></span> or interpret it in any way.
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The passed-in value and its type are delivered to the underlying converter
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as-is. Consequently, if potential implicit type promotions and conversions
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are not desirable, then it is the converter's responsibility to address that
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issue. For example, one way to disable implicit conversions might be:
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">double_only</span>
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<span class="special">{</span>
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<span class="comment">// Declared for all types.</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">TypeIn</span><span class="special">></span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">TypeIn</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">>&)</span> <span class="keyword">const</span><span class="special">;</span>
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<span class="special">};</span>
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<span class="comment">// Defined only for certain types.</span>
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<span class="keyword">template</span><span class="special"><></span> <span class="keyword">void</span> <span class="identifier">double_only</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()<</span><span class="keyword">double</span><span class="special">>(</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">>&)</span> <span class="keyword">const</span> <span class="special">{}</span>
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</pre>
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<p>
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</p>
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<p>
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</p>
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<pre class="programlisting"> <span class="identifier">convert</span><span class="special"><</span><span class="identifier">string</span><span class="special">>(</span><span class="number">11.23</span><span class="special">,</span> <span class="identifier">double_only</span><span class="special">());</span> <span class="comment">// Fine.</span>
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<span class="comment">// convert<string>(11, double_only()); // Fails: undefined reference to double_only::operator()<int></span>
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</pre>
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<p>
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2009-2016 Vladimir Batov<p>
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Distributed under the Boost Software License, Version 1.0. See 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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</p>
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</div></td>
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</tr></table>
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