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<title>The Default Constructible Type Requirement</title>
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<link rel="home" href="../index.html" title="Chapter 1. The Boost Convert Library 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="the_boost_convert_library.convert_default_constructible"></a><a class="link" href="convert_default_constructible.html" title="The Default Constructible Type Requirement">The
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<span class="emphasis"><em>Default Constructible</em></span> Type Requirement</a>
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</h2></div></div></div>
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<p>
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In order for a type to be integrated into the <span class="emphasis"><em>boost::lexical_cast</em></span>
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framework that type needs to be <span class="emphasis"><em>Default Constructible</em></span>.
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Deeply in the bowels of the implementation a temporary-storage instance of
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the Target type is created and then populated with the conversion result. The
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<span class="emphasis"><em>boost::lexical_cast</em></span> implementation chooses the default
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constructor to create such an instance. For more details see <a href="http://www.boost.org/doc/libs/1_55_0/doc/html/boost_lexical_cast.html" target="_top"><span class="emphasis"><em>lexical_cast</em></span>
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documentation</a>.
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</p>
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<p>
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<span class="emphasis"><em>boost::convert</em></span> also creates a temporary-storage instance
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of the Target type that the plugged-in converter then populates with the conversion
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result. By default that temporary-storage is created by
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</p>
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
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<span class="special">{</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypeOut</span><span class="special">></span>
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<span class="identifier">TypeOut</span>
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<span class="identifier">convert</span><span class="special"><</span><span class="identifier">TypeOut</span><span class="special">>::</span><span class="identifier">create_storage</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="keyword">return</span> <span class="identifier">TypeOut</span><span class="special">();</span>
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<span class="special">}</span>
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<span class="special">}</span>
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</pre>
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<p>
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and, therefore, the <span class="emphasis"><em>Default Constructible</em></span> requirement
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is also the default requirement of <span class="emphasis"><em>boost::convert</em></span>.
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</p>
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<p>
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A well-designed type (in my opinion, anyway) should only have meaningful and
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unambiguous constructors... and the default constructor is not necessarily
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one of them. Consider the following <span class="emphasis"><em>direction</em></span> type as
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one such example. The type has only two meaningful states and is not <span class="emphasis"><em>Default
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Constructible</em></span>:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">direction</span>
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<span class="special">{</span>
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<span class="keyword">enum</span> <span class="identifier">value_type</span> <span class="special">{</span> <span class="identifier">up</span><span class="special">,</span> <span class="identifier">dn</span> <span class="special">};</span>
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<span class="identifier">direction</span><span class="special">(</span><span class="identifier">value_type</span> <span class="identifier">value</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">value_</span><span class="special">(</span><span class="identifier">value</span><span class="special">)</span> <span class="special">{}</span>
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<span class="keyword">private</span><span class="special">:</span> <span class="identifier">value_type</span> <span class="identifier">value_</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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For such a type the call below will not compile (due to the <span class="emphasis"><em>Default
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Constructible</em></span> requirement):
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</p>
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<pre class="programlisting"><span class="identifier">direction</span> <span class="identifier">dir1</span> <span class="special">=</span> <span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">direction</span><span class="special">>(</span><span class="identifier">str</span><span class="special">);</span> <span class="comment">// Does not compile</span>
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<span class="identifier">direction</span> <span class="identifier">dir2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="identifier">direction</span><span class="special">>::</span><span class="identifier">from</span><span class="special">(</span><span class="identifier">str</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// Does not compile</span>
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</pre>
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<p>
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However, <span class="emphasis"><em>boost::convert</em></span> will be able to handle such a
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type with little help from the user. What <span class="emphasis"><em>boost::convert</em></span>
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needs is the instructions <span class="emphasis"><em>how</em></span> to create that mentioned
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temporary-storage:
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</p>
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
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<span class="special">{</span>
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<span class="keyword">template</span><span class="special"><></span> <span class="keyword">inline</span> <span class="identifier">direction</span> <span class="identifier">convert</span><span class="special"><</span><span class="identifier">direction</span><span class="special">>::</span><span class="identifier">create_storage</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="keyword">return</span> <span class="identifier">direction</span><span class="special">(</span><span class="identifier">direction</span><span class="special">::</span><span class="identifier">up</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="special">}</span>
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</pre>
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<p>
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Now the conversion code compiles:
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</p>
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<pre class="programlisting"><span class="identifier">direction</span> <span class="identifier">dir2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="identifier">direction</span><span class="special">>::</span><span class="identifier">from</span><span class="special">(</span><span class="identifier">str</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// Compiles</span>
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</pre>
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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 © 2009-2014 Vladimir Batov<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or 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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