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From: gt5163b@prism.gatech.edu (Brian McNamara!)
Newsgroups: comp.std.c++
Subject: Re: Suggestion: making bound member functions usable
Date: Tue, 27 Mar 2001 17:49:22 CST
Organization: Georgia Institute of Technology
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Dennis Yelle <dennis51@jps.net> once said:
>"Brian McNamara!" wrote:
>> Indeed.  I wish I had a way to make this compile even:
>> 
>>    (&A::b)(a,3,_)
>> 
>> by somehow invoking an implicit conversion so that it means
>> 
>>    X(&A::b).operator()(a,3,_)
>> 
>> where X is a type admits an implicit conversion from the
>> pointer-to-member-function.  But I'm pretty sure that's impossible (and
>> reasonably so).
>
>Sorry, I have no idea what you are saying here.
>What is A?  What is b?
>And especially, what is X?

A is a class, b is a method in that class, and X is a class which has a
non-explicit constructor from &A::b.  In other words,

   struct A { void b( int, int ) {} };
   struct X { 
      typedef void (A::*F)(int,int); 
      F f; 
      X( F g ) : f(g) {}    // implicit conversion
      void operator()( A* a, int x, int y ) const {
         a->*f(x,y);
      }
   };
   int main() {
      A a;
      // (&A::b)(&a,3,4);  // would be nice
      (X(&A::b))(&a,3,4);  // legal
      (X(&A::b)).operator()(&a,3,4);  // more explicit version
   }

Except that in the general case, we'd write 'X' as a template, so it
can turn any class's (not just A's) member functions (not just
two-argument int,int->void ones) into calls that take the receiver
object as an extra parameter.

-- 
 Brian M. McNamara   lorgon@acm.org  :  I am a parsing fool!
   ** Reduce - Reuse - Recycle **    :  (Where's my medication? ;) )

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