Re: c++/9377: g++ 64bit calls wrong function -> Multi-inheritance: pointerto member function of the 2nd base calss points to wrong place

[axiong at ca dot ibm dot com]

The following reply was made to PR c++/9377; it has been noted by GNATS.

From: axiong@ca.ibm.com
To: bangerth@dealii.org, "Andrew Xiong" <axiong@ca.ibm.com>,
   gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org,
   gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c++/9377: g++ 64bit calls wrong function -> Multi-inheritance: pointer
 to member function of the 2nd base calss points to wrong place
Date: Tue, 21 Jan 2003 15:49:59 -0500

 Answers to the following questions:
 > My questions are (maybe you can send an answer to these):
 > - why do you need the cast from &B2::bar to D::*? I
 >   think that's the crucial step -- if your program is
 >   supposed to work, then this cast would not be necessary,
 >   but rather be an implicit default conversion.
 > - what's the purpose of the select() method you have there?
 >   The comment implies that without it, there's a problem.
 >   But what exactly is it?
 
 (1) The "select() method" was put there for another 64-bit
     C++ compiler, not for g++ 64-bit compiler. It does not
     influence 64-bit g++ compiling (I forgot to delete it when
     I was submitting the gnat report). it can be deleted or
     kept there.
 
 (2) The C++ code was reduced from a C++ program written to
     test the pointer to member function and the virtual function
     table. The cast may be useful - it will direct the pointer
     to where the programmer wants, and it may be helpful for
     function returning when multi-inheritance occurs,
     for example, a C++ programmer may write:
 /*************************************************************/
 // put the classes B1, B2, and D here
 
 typedef char * (D::*PMF)() ;
 
 PMF select(int flag) {
         if (flag==0) return &B1::foo ; //the cast occurs here
         else return &B2::bar ;
 }
 
 main() {
         int codition = 1;
 
         D *d1ptr = new D ;     //another kind of polymorphism?
         printf( "%s\n", (d1ptr->*select(condition)() ) ;
 
         d1ptr = (D*) new B2 ;  //with the cast, we can also do this
         printf( "%s\n", (d1ptr->*select(1))() ) ;
 
         return 0 ;
 }
 /*************************************************************/