🚀 go-pugleaf

Hi,
Igor Boukanov (Igor.Boukanov@fi.uib.no) wrote:

[example how to potentially simulate 'mutable' removed]

: And remove_constness_from is more useful than mutable because in this 
: case to make any class member looks like mutable one don't even need to 
: modify class definition! And of cause lines like 
: "remove_constness_from(this)->i = i_;"
: will show explicitly what somebody does...

This is why 'const_cast' was invented but it does not replace
'mutable'.  See dcl.type.cv section 4: "Except that any class member
declared mutable can be modified,  any  attempt  to  modify  a  const
object  during its lifetime results in undefined behavior."

Thus, the following would result in undefined behavior:

  class f
  {
    int count_;
  public:
    void count() const
    { 
      const_cast<f*>(this)->count_++;
    }
  };

  f const const_f;

  int main() { const_f.count(); return 0; }

If 'count_' is declare to be 'mutable', everything is fine. Now you
could try to argue, that it would be necessary to remove decl.type.cv
section 4 or rephrase it to make the above code well behaved. But this
would prevent the ability to put 'const' objects into write-protected
memory (like ROM) or to determine their value at compile-time.

Thus, 'mutable' is necessary and it has indeed some useful
applications.  In particular, it allows to implement classes which
maintain logical constness (instead of bitwise constness) in a
convenient way. The fact that the bits of some 'const' object will be
changed is already obvious from the (concious) declaration of the
'mutable' member.

: >From this point of view mutable keyword can be removed from C++. And it 
: will not break any real code because as I now there is no compiler that 
: can compile such code!

g++ implements 'mutable' and removing 'mutable' from the standard would
break my code.  Whether you consider this code to be "real" code is a
different question...

: And of cause this will make C++ standard shorter...

... and less flexible.
--
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 
                newsreader.  If that fails, use mailto:std-c++@ncar.ucar.edu
  comp.std.c++ FAQ: http://reality.sgi.com/austern/std-c++/faq.html
  Moderation policy: http://reality.sgi.com/austern/std-c++/policy.html
  Comments? mailto:std-c++-request@ncar.ucar.edu 
]