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Subject: [Boost-commit] svn:boost r51356 - trunk/libs/utility
From: nielsdekker_at_[hidden]
Date: 2009-02-20 15:35:35

Author: niels_dekker
Date: 2009-02-20 15:35:34 EST (Fri, 20 Feb 2009)
New Revision: 51356
URL: http://svn.boost.org/trac/boost/changeset/51356

Log:
Updated value_initialized documentation and test following changeset [51355].
Text files modified:
   trunk/libs/utility/value_init.htm | 68 +++++++++++++++++++++++++++------------
   trunk/libs/utility/value_init_test.cpp | 2
   2 files changed, 47 insertions(+), 23 deletions(-)

Modified: trunk/libs/utility/value_init.htm
==============================================================================
--- trunk/libs/utility/value_init.htm (original)
+++ trunk/libs/utility/value_init.htm 2009-02-20 15:35:34 EST (Fri, 20 Feb 2009)
@@ -253,7 +253,33 @@

<h2><a name="val_init"><code>template class value_initialized<T></code></a></h2>

-<pre>namespace boost { template<class T> class value_initialized { public : value_initialized() : x() {} operator T&() const { return x ; } T& data() const { return x ; } void swap( value_initialized<T>& ); private : unspecified x ; } ; template<class T> T const& get ( value_initialized<T> const& x ) { return x.data() ; } template<class T> T& get ( value_initialized<T>& x ) { return x.data() ; } } // namespace boost </pre>
+<pre>namespace boost { template<class T> class value_initialized {
+ public :
+ value_initialized() : x() {}
+ operator T const &() const { return x ; }
+ operator T&() { return x ; }
+ T const &data() const { return x ; }
+ T& data() { return x ; }
+ void swap( value_initialized<T>& );
+ 
+ private :
+ unspecified x ;
+ } ;
+ 
+ template<class T>
+ T const& get ( value_initialized<T> const& x )
+ {
+ return x.data() ;
+ }
+ 
+ template<class T>
+ T& get ( value_initialized<T>& x )
+ {
+ return x.data() ;
+ }
+ 
+ } // namespace boost
+ </pre>

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>: 

-<pre>void watch(int); value_initialized<int> x; watch(x) ; // operator T& used. watch(x.data()); watch( get(x) ) // function get() used</pre>
+<pre>void watch(int); value_initialized<int> x;
+ watch(x) ; // operator T& used. watch(x.data()); watch( get(x) ) // function get() used</pre>

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>.

-<pre>value_initialized<int> x ; static_cast<int&>(x) = 1 ; // OK get(x) = 1 ; // OK value_initialized<int const> y ; static_cast<int&>(y) = 1 ; // ERROR: cannot cast to int& static_cast<int const&>(y) = 1 ; // ERROR: cannot modify a const value get(y) = 1 ; // ERROR: cannot modify a const value</pre>
+<pre>value_initialized<int> x ; static_cast<int&>(x) = 1 ; // OK get(x) = 1 ; // OK
+ value_initialized<int const> y ; static_cast<int&>(y) = 1 ; // ERROR: cannot cast to int& static_cast<int const&>(y) = 1 ; // ERROR: cannot modify a const value get(y) = 1 ; // ERROR: cannot modify a const value</pre>

<h3>Warning:</h3>

-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:
+The <code>value_initialized</code> implementation of Boost version 1.38.0 and older
+allowed non-const access to the wrapped object, from a constant wrapper,
+both by its conversion operator and its <code>data()</code> member function. For example:

-<pre>void foo(int); value_initialized<int> const x ; foo(x); </pre>
+<pre>value_initialized<int> const x_c ; int& xr = x_c ; // OK, conversion to int& available even though x_c is itself const.
+ xr = 2 ; </pre>

-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:
-
-<pre>value_initialized<int> const x_c ; int& xr = x_c ; // OK, conversion to int& available even though x_c is itself const. xr = 2 ; </pre>
-
-The reason for this obscure behavior is that some commonly used compilers
- just don't accept the following valid code:
+The reason for this obscure behavior was that some compilers
+ didn't accept the following valid code:

<pre>struct X { operator int&() ; operator int const&() const ; }; X x ; (x == 1 ) ; // ERROR HERE!</pre>

-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. 
+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.
+ 
 

<h3>Recommended practice: The non-member get() idiom</h3>

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:

<pre>value_initialized<int> x ; get(x) = 1 ; // OK value_initialized<int const> cx ; get(x) = 1 ; // ERROR: Cannot modify a const object value_initialized<int> const x_c ; get(x_c) = 1 ; // ERROR: Cannot modify a const object value_initialized<int const> const cx_c ; get(cx_c) = 1 ; // ERROR: Cannot modify a const object </pre>
@@ -383,9 +407,9 @@
 

<hr>
-Revised 28 August 2008
+Revised 20 February 2009

-© Copyright Fernando Cacciola, 2002, 2008.
+© Copyright Fernando Cacciola, 2002, 2009.

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>

Modified: trunk/libs/utility/value_init_test.cpp
==============================================================================
--- trunk/libs/utility/value_init_test.cpp (original)
+++ trunk/libs/utility/value_init_test.cpp 2009-02-20 15:35:34 EST (Fri, 20 Feb 2009)
@@ -260,7 +260,7 @@
 boost::value_initialized<T> const x_c ;
 BOOST_CHECK ( y == x_c ) ;
 BOOST_CHECK ( y == boost::get(x_c) ) ;
- T& x_c_ref = x_c ;
+ T& x_c_ref = const_cast<T&>( boost::get(x_c) ) ;
 x_c_ref = z ;
 BOOST_CHECK ( x_c == z ) ;

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