Subject: [Boost-commit] svn:boost r56547 - in branches/release: boost/utility libs/utility
From: nielsdekker_at_[hidden]
Date: 2009-10-03 06:19:09
Author: niels_dekker
Date: 2009-10-03 06:19:09 EDT (Sat, 03 Oct 2009)
New Revision: 56547
URL: http://svn.boost.org/trac/boost/changeset/56547
Log:
Merged value_init from the trunk, including fix of #2548, regarding "const value_initialized".
Properties modified:
branches/release/boost/utility/value_init.hpp (contents, props changed)
branches/release/libs/utility/value_init.htm (contents, props changed)
Text files modified:
branches/release/boost/utility/value_init.hpp | 14 ++++++-
branches/release/libs/utility/value_init.htm | 70 ++++++++++++++++++++++++++-------------
2 files changed, 59 insertions(+), 25 deletions(-)
Modified: branches/release/boost/utility/value_init.hpp
==============================================================================
--- branches/release/boost/utility/value_init.hpp (original)
+++ branches/release/boost/utility/value_init.hpp 2009-10-03 06:19:09 EDT (Sat, 03 Oct 2009)
@@ -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 @@
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 @@
::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 )
{
Modified: branches/release/libs/utility/value_init.htm
==============================================================================
--- branches/release/libs/utility/value_init.htm (original)
+++ branches/release/libs/utility/value_init.htm 2009-10-03 06:19:09 EDT (Sat, 03 Oct 2009)
@@ -253,7 +253,33 @@
<h2><a name="val_init"><code>template class value_initialized<T></code></a></h2>
-<pre>namespace boost {<br><br>template<class T><br>class value_initialized<br>{<br> public :<br> value_initialized() : x() {}<br> operator T&() const { return x ; }<br> T& data() const { return x ; }<br> void swap( value_initialized<T>& );<br><br> private :<br> <i>unspecified</i> x ;<br>} ;<br><br>template<class T><br>T const& get ( value_initialized<T> const& x )<br>{<br> return x.data() ;<br>}<br><br>template<class T><br>T& get ( value_initialized<T>& x )<br>{<br> return x.data() ;<br>}<br><br>} // namespace boost<br></pre>
+<pre>namespace boost {<br><br>template<class T><br>class value_initialized<br>{
+<br> public :
+<br> value_initialized() : x() {}
+<br> operator T const &() const { return x ; }
+<br> operator T&() { return x ; }
+<br> T const &data() const { return x ; }
+<br> T& data() { return x ; }
+<br> void swap( value_initialized<T>& );
+<br>
+<br> private :
+<br> <i>unspecified</i> x ;
+<br>} ;
+<br>
+<br>template<class T>
+<br>T const& get ( value_initialized<T> const& x )
+<br>{
+<br> return x.data() ;
+<br>}
+<br>
+<br>template<class T>
+<br>T& get ( value_initialized<T>& 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&'</code> whose wrapped object (data member of type <code>T</code>)
@@ -271,7 +297,8 @@
<code>T&</code>, the member function <code>data()</code>, or the
non-member function <code>get()</code>: </p>
-<pre>void watch(int);<br>value_initialized<int> x;<br><br>watch(x) ; // operator T& used.<br>watch(x.data());<br>watch( get(x) ) // function get() used</pre>
+<pre>void watch(int);<br>value_initialized<int> x;
+<br><br>watch(x) ; // operator T& 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 @@
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<int> x ; <br>static_cast<int&>(x) = 1 ; // OK<br>get(x) = 1 ; // OK<br><br>value_initialized<int const> y ; <br>static_cast<int&>(y) = 1 ; // ERROR: cannot cast to int&<br>static_cast<int const&>(y) = 1 ; // ERROR: cannot modify a const value<br>get(y) = 1 ; // ERROR: cannot modify a const value</pre>
+<pre>value_initialized<int> x ; <br>static_cast<int&>(x) = 1 ; // OK<br>get(x) = 1 ; // OK
+<br><br>value_initialized<int const> y ; <br>static_cast<int&>(y) = 1 ; // ERROR: cannot cast to int&<br>static_cast<int const&>(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<int> const x ;<br>foo(x);<br></pre>
+<pre>value_initialized<int> const x_c ;<br>int& xr = x_c ; // OK, conversion to int& available even though x_c is itself const.
+<br>xr = 2 ; </pre>
-<p>But notice that this conversion operator is to <code>T&</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<int> const x_c ;<br>int& xr = x_c ; // OK, conversion to int& 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&() ;<br> operator int const&() 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<int> x ;<br>get(x) = 1 ; // OK<br><br>value_initialized<int const> cx ;<br>get(x) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized<int> const x_c ;<br>get(x_c) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized<int const> const cx_c ;<br>get(cx_c) = 1 ; // ERROR: Cannot modify a const object<br></pre>
@@ -383,9 +407,9 @@
</p>
<hr>
-<p>Revised 28 August 2008</p>
+<p>Revised 03 October 2009</p>
-<p>© Copyright Fernando Cacciola, 2002, 2008.</p>
+<p>© 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 @@
<br>
</body>
-</html>
\ No newline at end of file
+</html>