[Bug c++/18975] Copying objects with mutable non-static data members

[nathan at gcc dot gnu dot org]

------- Additional Comments From nathan at gcc dot gnu dot org  2004-12-15 
14:44 -------
I am dubious about my own analysis of this. Wolfgang sent me more detailed
comparison between g++ and comeau

Hi Nathan,

I try to explain what I meant with my last comment. I have three variations
of my code example and I tried to compile them with GNU 3.3 and Comeau
online.

===========================================================================

The following code is accepted by GNU as well as by Comeau:

struct PTR
{
    PTR ();
    PTR (PTR&);
    PTR& operator= (PTR&);

private:
    PTR (const PTR&);
    PTR& operator= (const PTR&);

    void* ptr;
};


struct XYZ
{
    XYZ (PTR& p) : ptr(p) {}

    XYZ (const XYZ& src) : ptr(src.ptr) {}
    XYZ& operator= (const XYZ& src) { ptr = src.ptr; return *this; }

    mutable PTR ptr;
};


XYZ f1 ();


XYZ f2 (void) { return f1(); }
void f3 (XYZ& dst, const XYZ& src) { dst = src; }

===========================================================================

The following code is rejected by GNU (GNU flags line 28) and accepted by
Comeau:

struct PTR
{
    PTR ();
    PTR (PTR&);
    PTR& operator= (PTR&);

private:
    PTR (const PTR&);
    PTR& operator= (const PTR&);

    void* ptr;
};


struct XYZ
{
    XYZ (PTR& p) : ptr(p) {}

    XYZ& operator= (const XYZ& src) { ptr = src.ptr; return *this; }

    mutable PTR ptr;
};


XYZ f1 ();


XYZ f2 (void) { return f1(); }                     // <--- this is line 28
void f3 (XYZ& dst, const XYZ& src) { dst = src; }

===========================================================================

The following code is rejected by both GNU (GNU flags line 26 and 27) and
Comeau (Comeau flags line 27):

struct PTR
{
    PTR ();
    PTR (PTR&);
    PTR& operator= (PTR&);

private:
    PTR (const PTR&);
    PTR& operator= (const PTR&);

    void* ptr;
};


struct XYZ
{
    XYZ (PTR& p) : ptr(p) {}

    mutable PTR ptr;
};


XYZ f1 ();


XYZ f2 (void) { return f1(); }                        // <--- this is line
26
void f3 (XYZ& dst, const XYZ& src) { dst = src; }     // <--- this is line
27

===========================================================================

Two remarks:
- In my first code fragment I have defined an explicit copy constructor and
  an explicit copy assigment operator for class XYZ, in my third code
fragment
  there is neither an explicit copy constructor nor an explicit copy
assigment
  operator for class XYZ.
  Why is the first code legal and the third code illegal? The functionality
  seems to be identical.
- The EDG frontend seems to treat the mutable attribute differently in the
  generation of the implicit copy constructor than in the generation of the
  implicit copy assignment operator.

With kind regards,
W. Roehrl


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18975