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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_convert.converters"></a><a class="link" href="converters.html" title="Converters">Converters</a>
</h2></div></div></div>
<p>
The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code>
API plays its role by providing a <span class="emphasis"><em>uniform interface</em></span> and
ensuring <span class="emphasis"><em>consistent behavior</em></span>. However, it is the respective
converter which does the hard work of actual type conversion/transformation.
</p>
<p>
<span class="emphasis"><em>Boost.Convert</em></span> design reflects the fact that no one converter
is to satisfy all imaginable conversion/transformation-related user requirements.
Consequently, <span class="emphasis"><em>extendibility</em></span> and <span class="emphasis"><em>converter pluggability</em></span>
are important properties of <span class="emphasis"><em>Boost.Convert</em></span>. The library
provides several converters for common type conversions with varying degrees
of formatting support and performance. However, it is an expectation that more
generic-purpose and custom-specific converters are to be written and deployed
with <span class="emphasis"><em>Boost.Convert</em></span>.
</p>
<p>
For a converter to be plugged in to the <span class="emphasis"><em>Boost.Convert</em></span>
framework it needs to be a <span class="emphasis"><em>callable</em></span> with one of the signatures:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
<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">&amp;</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">&lt;</span><span class="identifier">TypeOut</span><span class="special">&gt;&amp;</span> <span class="identifier">result_out</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
<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">&lt;</span><span class="identifier">TypeOut</span><span class="special">&gt;&amp;</span> <span class="identifier">result_out</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
</pre>
<p>
if that is a general-purpose converter capable of handling many types (like
string-to-type and type-to-string conversions). Alternatively, a purpose-built
custom converter might only care to provide
</p>
<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">&amp;,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">TypeOut</span><span class="special">&gt;&amp;)</span> <span class="keyword">const</span><span class="special">;</span>
</pre>
<p>
if its sole purpose is to handle one specific conversion/transformation of
<span class="emphasis"><em>TypeIn</em></span> to <span class="emphasis"><em>TypeOut</em></span>. For example, a
converter from the operating-system-specific MBCS string format to the UCS-2
or UCS-4 (depending on <code class="computeroutput"><span class="keyword">wchar_t</span></code>
size) might be one such example:
</p>
<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">&amp;,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">wstring</span><span class="special">&gt;&amp;)</span> <span class="keyword">const</span><span class="special">;</span>
</pre>
<p>
Alternatively again, an ad-hoc in-place <span class="emphasis"><em>callable</em></span> might
be provided as a converter. For example,
</p>
<p>
</p>
<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>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
</p>
<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">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">str</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">assign</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">_2</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">string</span><span class="special">&gt;,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">placeholders</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>
</pre>
<p>
</p>
<p>
or an old-fashioned function:
</p>
<p>
</p>
<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">&amp;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;&amp;</span> <span class="identifier">value_out</span><span class="special">)</span>
</pre>
<p>
</p>
<p>
</p>
<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">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
</pre>
<p>
</p>
<p>
With regard to converters the <span class="emphasis"><em>Boost.Convert</em></span> framework
has been designed with the following requirement in mind:
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Converters shall be independent from and must not rely on the <span class="emphasis"><em>Boost.Convert</em></span>
infrastructure.
</p></td></tr>
</table></div>
<h4>
<a name="boost_convert.converters.h0"></a>
<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
<span class="emphasis"><em>TypeIn</em></span> Promotions and Conversions</a>
</h4>
<p>
It is worth remembering that <span class="emphasis"><em>TypeIn</em></span> in the signature should
be interpreted in the context of the potential implicit type promotions and
conversions allowed <span class="emphasis"><em>by the language</em></span>. For example, depending
on the context the <code class="computeroutput"><span class="identifier">take_double</span></code>
and <code class="computeroutput"><span class="identifier">take_int</span></code> converters below
might not do what is expected of them due to implicit <span class="emphasis"><em>int-to-double</em></span>
promotion and value-destroying <span class="emphasis"><em>double-to-int</em></span> conversion
applied <span class="emphasis"><em>by the compiler</em></span>:
</p>
<p>
</p>
<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">&lt;</span><span class="identifier">string</span><span class="special">&gt;&amp;)</span> <span class="keyword">const</span> <span class="special">{}};</span>
<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">&lt;</span><span class="identifier">string</span><span class="special">&gt;&amp;)</span> <span class="keyword">const</span> <span class="special">{}};</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">convert</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;(</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>
<span class="identifier">convert</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;(</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>
</pre>
<p>
</p>
<p>
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code>
API does not modify <span class="emphasis"><em>TypeIn</em></span> or interpret it in any way.
The passed-in value and its type are delivered to the underlying converter
as-is. Consequently, if potential implicit type promotions and conversions
are not desirable, then it is the converter's responsibility to address that
issue. For example, one way to disable implicit conversions might be:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">double_only</span>
<span class="special">{</span>
<span class="comment">// Declared for all types.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">TypeIn</span><span class="special">&gt;</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">&lt;</span><span class="identifier">string</span><span class="special">&gt;&amp;)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
<span class="comment">// Defined only for certain types.</span>
<span class="keyword">template</span><span class="special">&lt;&gt;</span> <span class="keyword">void</span> <span class="identifier">double_only</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()&lt;</span><span class="keyword">double</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">string</span><span class="special">&gt;&amp;)</span> <span class="keyword">const</span> <span class="special">{}</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"> <span class="identifier">convert</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;(</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>
<span class="comment">// convert&lt;string&gt;(11, double_only()); // Fails: undefined reference to double_only::operator()&lt;int&gt;</span>
</pre>
<p>
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2009-2020 Vladimir Batov<p>
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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