From mboxrd@z Thu Jan  1 00:00:00 1970
From: Don Mies <dmies@bluekite.com>
To: gcc-bugs@gcc.gnu.org, bug-gcc@gnu.org, gcc@gcc.gnu.org
Subject: Casting Overloaded Function Pointer Problem...Repost...
Date: Mon, 18 Sep 2000 11:06:00 -0000
Message-id: <39C6596B.5C12890@bluekite.com>
X-SW-Source: 2000-09/msg00409.html

 
Gentlemen,
This is a reposting of an earlier request.  I received no responses
that helped in resolving my problem so I will try posting the query
again...
I have run into a problem trying to use the g++ compiler on a Sun
Solaris 7 system.
I've been trying to compile a version of the Microsoft MFC library
with the g++ compiler and have run into a problem that I don't know
how to resolve.
Here is a highly condensed test file that illustrates the problem.
--------------------------------------------
     1
     2  class
CCmdTarget;
     3  class
CWnd;
     4  class
CRecordView;
     5
     6  typedef
int (CCmdTarget::*PMSG)(int);
     7
     8  class
CCmdTarget {};
     9
    10
    11  class
CWnd : public CCmdTarget
    12  {
    13   
public:
    14     
int OnMove (int x, int y);
    15  };
    16
    17
    18  class
CRecordView : public CWnd
    19  {
    20   
public:
    21     
int OnMove (int);
    22   
protected:
    23     
void OnMove (int cx, int cy);
    24   
private:
    25     
static const PMSG _messageEntries[];
    26  };
    27
    28
    29  ////////////////////////////////////////////////////////////
    30
    31  const
PMSG CRecordView::_messageEntries[] =
    32  {
    33     
(PMSG)&CRecordView::OnMove
    34  };
  ----------------------------------------
If I compile the above with the Sun CC compiler, I get no complaints
or warnings.  When I compile it with g++ (gcc version 2.95.2 19991024
(release)) I get the following error messages:
    t.C:34: no matches converting function `OnMove' to
type `int (CCmdTarget::*)(int)'
    t.C:21: candidates are: int CRecordView::OnMove(int)
    t.C:23:                
void CRecordView::OnMove(int, int)
If I comment out the overloaded function declaration at line 23, it
will compile
with no errors.
The compiler appears to be ignoring the cast operator at line 33 when
it determines
the function signature.
I received one response to my initial posting with the following information:
> Your code is essentially asking GCC to convert the pointer
> to the member function into a p-to-m-f of the base class.
> If I'm interpreting this correctly, according to the C++
> Standard, section 13.4, this is not allowed:
>
> [Note: there are no standard conversions (clause 4) of one
> pointer-to-function type into another. In particular, even
> if B is a public base of D, we have
>
>       D* f();
>       B* (*p1)() = &f; // error
>       void g(D*);
>       void (*p2)(B*) = &g; // error
>
> -end note]
>
> Therefore, I think GCC may be right in complaining about
> this code.
However, I don't believe the above interpretation is incorrect as
my example program above is doing an explicit cast and not
a standard cast as the above response contends.  In addition,
if I comment out line 23 to remove the overloading function
declaration, it will compile with no errors and the casting
operation is still in effect and unchanged.
Sections 5.1.10.6, 5.2.10.9, and 5.4.5 of the C++ Standard seem
to indicate that the above construct should be legal but the g++
compiler appears to disagree.
Is there a way to fix this without making major changes to the source
files
(I don't understand  the MFC library well enough to make changes
to it)?
Thank you,
 
Don Mies
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Thank you.
 
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