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Added boost::initialized<T> as was agreed at http://lists.boost.org/Archives/boost/2010/04/164916.php -- see #3472

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[SVN r61883]
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Niels Dekker
2010-05-09 20:51:24 +00:00
parent e3c982287a
commit 22743ee125
6 changed files with 349 additions and 23 deletions
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value_init.htm
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@@ -33,6 +33,7 @@
<ul>
<li><a href="#val_init"><code>template class value_initialized&lt;T&gt;</code></a></li>
<li><a href="#initialized"><code>template class initialized&lt;T&gt;</code></a></li>
<li><a href="#initialized_value"><code>initialized_value</code></a></li>
</ul>
@@ -123,6 +124,12 @@ constructed by the following declaration:
</pre>
</p>
<p>
The template <a href="#initialized"><code>initialized</code></a>
offers both value-initialization and direct-initialization.
It is especially useful as a data member type, allowing the very same object
to be either direct-initialized or value-initialized.
</p>
<p>
The <code>const</code> object <a href="#initialized_value"><code>initialized_value</code></a>
allows value-initializing a variable as follows:
<pre>
@@ -340,6 +347,52 @@ the wrapped object is always performed with the <code>get()</code> idiom:</p>
<pre>value_initialized&lt;int&gt; x ;<br>get(x) = 1 ; // OK<br><br>value_initialized&lt;int const&gt; cx ;<br>get(x) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized&lt;int&gt; const x_c ;<br>get(x_c) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized&lt;int const&gt; const cx_c ;<br>get(cx_c) = 1 ; // ERROR: Cannot modify a const object<br></pre>
<h2><a name="initialized"><code>template class initialized&lt;T&gt;</code></a></h2>
<pre>namespace boost {<br><br>template&lt;class T&gt;<br>class initialized<br>{
<br> public :
<br> initialized() : x() {}
<br> explicit initialized(T const &amp; arg) : x(arg) {}
<br> operator T const &amp;() const;
<br> operator T&amp;();
<br> T const &amp;data() const;
<br> T&amp; data();
<br> void swap( value_initialized&lt;T&gt;&amp; );
<br>
<br> private :
<br> <i>unspecified</i> x ;
<br>} ;
<br>
<br>template&lt;class T&gt;
<br>T const&amp; get ( initialized&lt;T&gt; const&amp; x );
<br>
<br>template&lt;class T&gt;
<br>T&amp; get ( initialized&lt;T&gt;&amp; x );
<br>
<br>} // namespace boost
<br></pre>
The template class <code>boost::initialized&lt;T&gt;</code> supports both value-initialization
and direct-initialization, so its interface is a superset of the interface
of <code>value_initialized&lt;T&gt;</code>: Its default-constructor
value-initializes the wrapped object just like the default-constructor of
<code>value_initialized&lt;T&gt;</code>, but <code>boost::initialized&lt;T&gt;</code>
also offers an extra <code>explicit</code>
constructor, which direct-initializes the wrapped object by the specified value.
<p>
<code>initialized&lt;T&gt;</code> is especially useful when the wrapped
object must be either value-initialized or direct-initialized, depending on
runtime conditions. For example, <code>initialized&lt;T&gt;</code> could
hold the value of a data member that may be value-initialized by some
constructors, and direct-initialized by others.
On the other hand, if it is known beforehand that the
object must <i>always</i> be value-initialized, <code>value_initialized&lt;T&gt;</code>
may be preferable. And if the object must always be
direct-initialized, none of the two wrappers really needs to be used.
</p>
<h2><a name="initialized_value"><code>initialized_value</code></a></h2>
<pre>
@@ -399,6 +452,9 @@ Special thanks to Bj&ouml;rn Karlsson who carefully edited and completed this do
<p>value_initialized was reimplemented by Fernando Cacciola and Niels Dekker
for Boost release version 1.35 (2008), offering a workaround to various compiler issues.
</p>
<p><code>boost::initialized</code> was very much inspired by feedback from Edward Diener and
Jeffrey Hellrung.
</p>
<p>initialized_value was written by Niels Dekker, and added to Boost release version 1.36 (2008).
</p>
<p>Developed by <a href="mailto:fernando_cacciola@hotmail.com">Fernando Cacciola</a>,
@@ -407,9 +463,9 @@ for Boost release version 1.35 (2008), offering a workaround to various compiler
</p>
<hr>
<p>Revised 03 October 2009</p>
<p>Revised 1 May 2010</p>
<p>&copy; Copyright Fernando Cacciola, 2002, 2009.</p>
<p>&copy; Copyright Fernando Cacciola, 2002 - 2010.</p>
<p>Distributed under the Boost Software License, Version 1.0. See
<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a></p>