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<a name="boost_convert.default_converter"></a><a class="link" href="default_converter.html" title="Default Converter">Default Converter</a>
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<p>
The explicit converter as in
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">boost</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="string">"123"</span><span class="special">,</span> <span class="identifier">converter</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
</pre>
<p>
provides considerable flexibility, configurability and efficiency. However,
in certain contexts that might be not that important or even counter-productive
if, for example, an application relies on certain consistent behavior associated
with one particular converter type and configuration. To accommodate such a
scenario <span class="emphasis"><em>Boost.Convert</em></span> introduces the concept of the
<span class="emphasis"><em>default converter</em></span> implemented as <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">by_default</span></code>.
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
There is no default converter set by default.
</p></td></tr>
</table></div>
<p>
Consequently, without additional configuration steps the following call will
fail to compile:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">boost</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="string">"123"</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// No converter provided</span>
</pre>
<p>
However, after <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">by_default</span></code>
is defined simply as:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">by_default</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">cstream</span> <span class="special">{};</span>
</pre>
<p>
</p>
<p>
or potentially configured with additional formatting:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">by_default</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">cstream</span>
<span class="special">{</span>
<span class="identifier">by_default</span><span class="special">()</span> <span class="special">{</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">uppercase</span><span class="special">)(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
the code compiles and deploys <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">cstream</span></code> when <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code> is called without an explicitly supplied
converter:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// No explicit converter provided. boost::cnv::by_default is used.</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">boost</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="string">"F"</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">s</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;(</span><span class="number">255</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="string">"bad"</span><span class="special">);</span>
<span class="comment">// 'i' and 's' are converted using boost::cnv::cstream</span>
<span class="comment">// with std::uppercase and std::hex formatting applied.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">i</span> <span class="special">==</span> <span class="number">15</span><span class="special">);</span> <span class="comment">// 15(10) = F(16)</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">s</span> <span class="special">==</span> <span class="string">"FF"</span><span class="special">);</span> <span class="comment">// 255(10) = FF(16)</span>
</pre>
<p>
</p>
<p>
The trade-off for the convenience is the rigid converter configuration (which
in certain contexts might be the desired behavior) and a potential performance
impact. When a converter is not provided explicitly, the default converter
is created, potentially configured, deployed and destroyed for every <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code> call. Consequently, if efficiency of this
particular component is important, then the implementation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">by_default</span></code>
will need to take that into account and to make sure those operations are cheap.
</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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