[Boost-bugs] [Boost C++ Libraries] #2548: Let's fix the logical constness of value_initialized!

Subject: [Boost-bugs] [Boost C++ Libraries] #2548: Let's fix the logical constness of value_initialized!
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2008-11-28 19:26:10

#2548: Let's fix the logical constness of value_initialized!
--------------------------+-------------------------------------------------
 Reporter:  niels_dekker  |       Owner:  fcacciola
     Type:  Bugs          |      Status:  new      
Milestone:  Boost 1.38.0  |   Component:  utility  
  Version:  Boost 1.37.0  |    Severity:  Problem  
 Keywords:                |  
--------------------------+-------------------------------------------------
 The data() member function and the conversion operator of
 boost::value_initialized<T> break the logical constness: both functions
 are declared "const", while they return a non-const reference to the T
 object that is owned by value_initialized<T>. I think both of them should
 be replaced by a pair of overloaded functions for const and non-const
 access.

 I don't think that there ever was a good reason for the data() function to
 break the logical constness. But clearly, there ''was'' a reason for the
 conversion operator to do so, at the time value_initialized was first
 released, in 2002.  At that time, some commonly used compilers would
 complain about ambiguity when the conversion operator would be overloaded
 for const and non-const, in some specific use cases. As described at
 http://www.boost.org/doc/libs/1_37_0/libs/utility/value_init.htm#val_init


 In January 2004, "deep const semantics" still wasn't feasible, apparently
 due to compiler errors in VC++ and Borland:
 ''[boost] value_initialized, deep const, and template instantiation''
 http://lists.boost.org/Archives/boost/2004/01/59987.php

 http://lists.boost.org/Archives/boost/2004/01/60020.php


 Nowadays I think that things have changed for the better.  I just replaced
 the single conversion operator by overloads for const and non-const in my
 local copy, and tested for VC++ 2003 (7.1), VC++ 2008 (9.0), and !CodeGear
 2009 (6.10).  And all of them passed the tests!

 Therefore I'd like to propose fixing the issue, according to the attached
 patch. Fernando, shall we give it a try?

 I'm aware that the proposed fix potentially breaks some user code, but I
 think that those cases are easily fixed, and I think that most Boost users
 are likely to appreciate logical constness.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/2548>
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