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Merged value_init from the trunk, including fix of #2548, regarding "const value_initialized".

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[SVN r56547]
This commit is contained in:
Niels Dekker
2009-10-03 10:19:09 +00:00
parent f8bef7ba95
commit c131cbd0b2
2 changed files with 59 additions and 25 deletions
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14
include/boost/utility/value_init.hpp
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@ -8,6 +8,7 @@
// 24 Dec 2007 (Refactored and worked around various compiler bugs) Fernando Cacciola, Niels Dekker
// 23 May 2008 (Fixed operator= const issue, added initialized_value) Niels Dekker, Fernando Cacciola
// 21 Ago 2008 (Added swap) Niels Dekker, Fernando Cacciola
// 20 Feb 2009 (Fixed logical const-ness issues) Niels Dekker, Fernando Cacciola
//
#ifndef BOOST_UTILITY_VALUE_INIT_21AGO2002_HPP
#define BOOST_UTILITY_VALUE_INIT_21AGO2002_HPP
@ -90,7 +91,12 @@ class value_initialized
wrapper_address()->wrapper::~wrapper();
}
T& data() const
T const & data() const
{
return wrapper_address()->data;
}
T& data()
{
return wrapper_address()->data;
}
@ -100,12 +106,16 @@ class value_initialized
::boost::swap( this->data(), arg.data() );
}
operator T&() const { return this->data(); }
operator T const &() const { return this->data(); }
operator T&() { return this->data(); }
} ;
template<class T>
T const& get ( value_initialized<T> const& x )
{

70
value_init.htm
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@ -253,7 +253,33 @@ its internal data, prior to constructing the object that it contains.
<h2><a name="val_init"><code>template class value_initialized&lt;T&gt;</code></a></h2>
<pre>namespace boost {<br><br>template&lt;class T&gt;<br>class value_initialized<br>{<br> public :<br> value_initialized() : x() {}<br> operator T&amp;() const { return x ; }<br> T&amp; data() const { return x ; }<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 ( value_initialized&lt;T&gt; const&amp; x )<br>{<br> return x.data() ;<br>}<br><br>template&lt;class T&gt;<br>T&amp; get ( value_initialized&lt;T&gt;&amp; x )<br>{<br> return x.data() ;<br>}<br><br>} // namespace boost<br></pre>
<pre>namespace boost {<br><br>template&lt;class T&gt;<br>class value_initialized<br>{
<br> public :
<br> value_initialized() : x() {}
<br> operator T const &amp;() const { return x ; }
<br> operator T&amp;() { return x ; }
<br> T const &amp;data() const { return x ; }
<br> T&amp; data() { return x ; }
<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 ( value_initialized&lt;T&gt; const&amp; x )
<br>{
<br> return x.data() ;
<br>}
<br>
<br>template&lt;class T&gt;
<br>T&amp; get ( value_initialized&lt;T&gt;&amp; x )
<br>{
<br> return x.data() ;
<br>}
<br>
<br>} // namespace boost
<br></pre>
<p>An object of this template class is a <code>T</code>-wrapper convertible
to <code>'T&amp;'</code> whose wrapped object (data member of type <code>T</code>)
@ -271,7 +297,8 @@ its internal data, prior to constructing the object that it contains.
<code>T&amp;</code>, the member function <code>data()</code>, or the
non-member function <code>get()</code>: </p>
<pre>void watch(int);<br>value_initialized&lt;int&gt; x;<br><br>watch(x) ; // operator T&amp; used.<br>watch(x.data());<br>watch( get(x) ) // function get() used</pre>
<pre>void watch(int);<br>value_initialized&lt;int&gt; x;
<br><br>watch(x) ; // operator T&amp; used.<br>watch(x.data());<br>watch( get(x) ) // function get() used</pre>
<p>Both <code>const</code> and non-<code>const</code> objects can be wrapped.
Mutable objects can be modified directly from within the wrapper but constant
@ -281,37 +308,34 @@ non-member function <code>get()</code>: </p>
is swappable as well, by calling its <code>swap</code> member function
as well as by calling <code>boost::swap</code>.</p>
<pre>value_initialized&lt;int&gt; x ; <br>static_cast&lt;int&amp;&gt;(x) = 1 ; // OK<br>get(x) = 1 ; // OK<br><br>value_initialized&lt;int const&gt; y ; <br>static_cast&lt;int&amp;&gt;(y) = 1 ; // ERROR: cannot cast to int&amp;<br>static_cast&lt;int const&amp;&gt;(y) = 1 ; // ERROR: cannot modify a const value<br>get(y) = 1 ; // ERROR: cannot modify a const value</pre>
<pre>value_initialized&lt;int&gt; x ; <br>static_cast&lt;int&amp;&gt;(x) = 1 ; // OK<br>get(x) = 1 ; // OK
<br><br>value_initialized&lt;int const&gt; y ; <br>static_cast&lt;int&amp;&gt;(y) = 1 ; // ERROR: cannot cast to int&amp;<br>static_cast&lt;int const&amp;&gt;(y) = 1 ; // ERROR: cannot modify a const value<br>get(y) = 1 ; // ERROR: cannot modify a const value</pre>
<h3>Warning:</h3>
<p>Both the conversion operator and the <code>data()</code> member function
are <code>const</code> in order to allow access to the wrapped object
from a constant wrapper:</p>
<p>The <code>value_initialized</code> implementation of Boost version 1.40.0 and older
allowed <i>non-const</i> access to the wrapped object, from a constant wrapper,
both by its conversion operator and its <code>data()</code> member function. For example:</p>
<pre>void foo(int);<br>value_initialized&lt;int&gt; const x ;<br>foo(x);<br></pre>
<pre>value_initialized&lt;int&gt; const x_c ;<br>int&amp; xr = x_c ; // OK, conversion to int&amp; available even though x_c is itself const.
<br>xr = 2 ; </pre>
<p>But notice that this conversion operator is to <code>T&amp;</code> although
it is itself <code>const</code>. As a consequence, if <code>T</code> is
a non-<code>const</code> type, you can modify the wrapped object even from
within a constant wrapper:</p>
<pre>value_initialized&lt;int&gt; const x_c ;<br>int&amp; xr = x_c ; // OK, conversion to int&amp; available even though x_c is itself const.<br>xr = 2 ; </pre>
<p>The reason for this obscure behavior is that some commonly used compilers
just don't accept the following valid code:</p>
<p>The reason for this obscure behavior was that some compilers
didn't accept the following valid code:</p>
<pre>struct X<br>{<br> operator int&amp;() ;<br> operator int const&amp;() const ; <br>};<br>X x ;<br>(x == 1 ) ; // ERROR HERE!</pre>
<p>These compilers complain about ambiguity between the conversion operators.
This complaint is incorrect, but the only workaround that I know of is
to provide only one of them, which leads to the obscure behavior just explained.<br>
<p>The current version of <code>value_initialized</code> no longer has this obscure behavior.
As compilers nowadays widely support overloading the conversion operator by having a <code>const</code> and a <code>non-const</code> version, we have decided to fix the issue accordingly. So the current version supports the idea of logical constness.
<br>
</p>
<h3>Recommended practice: The non-member get() idiom</h3>
<p>The obscure behavior of being able to modify a non-<code>const</code>
wrapped object from within a constant wrapper can be avoided if access to
wrapped object from within a constant wrapper (as was supported by previous
versions of <code>value_initialized</code>)
can be avoided if access to
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>
@ -383,9 +407,9 @@ for Boost release version 1.35 (2008), offering a workaround to various compiler
</p>
<hr>
<p>Revised 28 August 2008</p>
<p>Revised 03 October 2009</p>
<p>&copy; Copyright Fernando Cacciola, 2002, 2008.</p>
<p>&copy; Copyright Fernando Cacciola, 2002, 2009.</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>
@ -394,4 +418,4 @@ for Boost release version 1.35 (2008), offering a workaround to various compiler
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