🚀 go-pugleaf

Hi,

Philippe Verdy (100105.3120@compuserve.com) wrote:
[lengthy description of problems with bitwise instead of logical
constness elided]

: Is there any way (or proposal) to explicitly specify members
: of a class definition which are relaxed on constness certi-
: fication ?

Yes: it is possible to declare a data member of an object to be
'mutable'.  This allows to modify this member even in a 'const'
function. The main idea which leads to a reasonable use of 'mutable' is
the concept of logical versus bitwise constness: In C++ bitwise
constness is generally assumed. You cannot change the member of a
constant object. However, sometimes this is not what you want (auxilary
marks and cached values are two examples).  Instead, logical constness
is desired: The representation of the object might change but the value
represented stays the same. Note, however, that the language does not
(and can not) enforce that logical constness is maintained if the
object has mutable members. It is the responsibility of the programmer
to ensure logical constness.

: Would not it be great if we could simply define such members
: as "unconst" like in the following example :

:   class T {
:   public:
:     T()                   { ...; _mark = 0; }
:     virtual ~T();

:     virtual void show() const;
:   protected:
:     void mark() const     { _mark = 1; } // RELAXED HERE
:     void unmark() const   { _mark = 0; } // RELAXED HERE
:     bool ismarked() const { return _mark; }
:   private:
:     unconst char _mark; // HERE IS WHERE THIS APPLIES
:   }

If you replace 'unconst' by 'mutable' you have what will become
standard. Since the 'mutable' keyword is a recent addition to the
DWP, it is not yet supported by all compilers.
--
dietmar.kuehl@uni-konstanz.de
http://www.informatik.uni-konstanz.de/~kuehl
I am a realistic optimist - that's why I appear to be slightly pessimistic
---
[ comp.std.c++ is moderated.  To submit articles: try just posting with      ]
[ your news-reader.  If that fails, use mailto:std-c++@ncar.ucar.edu         ]
[ FAQ:      http://reality.sgi.com/employees/austern_mti/std-c++/faq.html    ]
[ Policy:   http://reality.sgi.com/employees/austern_mti/std-c++/policy.html ]
[ Comments? mailto:std-c++-request@ncar.ucar.edu                             ]