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Added test and documentation for convenience class initialized_value, that was added with changeset [42815]

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[SVN r42816]
This commit is contained in:
Niels Dekker 2008-01-16 09:37:25 +00:00
parent 8ce58b1675
commit a511007d0f
2 changed files with 56 additions and 4 deletions
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52
value_init.htm
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@ -32,7 +32,8 @@
</dl>
<ul>
<li><a href="#val_init"><code>value_initialized&lt;T&gt;</code></a></li>
<li><a href="#val_init"><code>template class value_initialized&lt;T&gt;</code></a></li>
<li><a href="#initialized_value"><code>class initialized_value</code></a></li>
</ul>
<a href="#acknowledgements">Acknowledgements</a><br>
@ -52,6 +53,9 @@ union and class types.
Moreover, <code>value_initialized</code> offers a workaround to various
compiler issues regarding value-initialization.
Furthermore a convenience class, <code>initialized_value</code> is provided,
to avoid repeating the type name when retrieving the value from a
<code>value_initialized&lt;T&gt;</code> object.
<br>
</p>
@ -117,6 +121,16 @@ constructed by the following declaration:
<pre>
value_initialized&lt;T&gt; var;
</pre>
</p>
<p>
The convenience class <a href="#initialized_value"><code>initialized_value</code></a>
allows value-initializing a variable as follows:
<pre>
T var = initialized_value();
</pre>
This form of initialization is also very similar to <code>T4 var4 = T4()</code>,
but robust against the aforementioned compiler issues.
</p>
<h2><a name="details"></a>Details</h2>
@ -298,6 +312,40 @@ 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_value"><code>class initialized_value</code></a></h2>
<pre>
namespace boost {
class initialized_value
{
public :
template &lt;class T&gt; operator T() const ;
};
} // namespace boost
</pre>
The class <code>initialized_value</code> provides a convenient way to get
an initialized value: its conversion operator provides an appropriate
<em>value-initialized</em> object for any CopyConstructible type.
Suppose you need to have an initialized variable of type <code>T</code>.
You could do it as follows:
<pre>
T var = T();
</pre>
But as mentioned before, this form suffers from various compiler issues.
The template <code>value_initialized</code> offers a workaround:
<pre>
T var = get( value_initialized&lt;T&gt;() );
</pre>
Unfortunately both forms repeat the type name, which
is rather short now (<code>T</code>), but could of course be
more like <code>Namespace::Template&lt;Arg&gt;::Type</code>.
Instead, one could use <code>initialized_value</code> as follows:
<pre>
T var = initialized_value();
</pre>
<h3><a name="references">References</a></h3>
[1] Bjarne Stroustrup, Gabriel Dos Reis, and J. Stephen Adamczyk wrote
various papers, proposing to extend the support for brace-enclosed <em>initializer lists</em>
@ -326,7 +374,7 @@ for Boost release version 1.35 (2008), offering a workaround to various compiler
</p>
<hr>
<p>Revised 15 January 2008</p>
<p>Revised 16 January 2008</p>
<p>&copy; Copyright Fernando Cacciola, 2002, 2008.</p>

8
value_init_test.cpp
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@ -7,7 +7,7 @@
// Test program for "boost/utility/value_init.hpp"
//
// 21 Agu 2002 (Created) Fernando Cacciola
// 15 Jan 2008 (Added tests regarding compiler issues) Fernando Cacciola, Niels Dekker
// 16 Jan 2008 (Added tests regarding compiler issues and initialized_value) Fernando Cacciola, Niels Dekker
#include <cstring> // For memcmp.
#include <iostream>
@ -52,7 +52,7 @@ struct NonPODBase
struct NonPOD : NonPODBase
{
NonPOD () : id() {}
NonPOD ( std::string const& id_) : id(id_) {}
explicit NonPOD ( std::string const& id_) : id(id_) {}
friend std::ostream& operator << ( std::ostream& os, NonPOD const& npod )
{ return os << '(' << npod.id << ')' ; }
@ -192,6 +192,10 @@ bool test ( T const& y, T const& z )
boost::value_initialized<T> x ;
BOOST_CHECK ( y == x ) ;
BOOST_CHECK ( y == boost::get(x) ) ;
T initializedValue = boost::initialized_value() ;
BOOST_CHECK ( y == initializedValue ) ;
static_cast<T&>(x) = z ;
boost::get(x) = z ;
BOOST_CHECK ( x == z ) ;