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Member function pointers

Started by neoztar Mar 11, 2003 at 2:33 PM 3 replies 400+ views

Original Post

Mar 11, 2003 at 2:33 PM


class a {};
class c {};
class b: public c, public a {
  int func(int, int);
};

typedef long (a::*funcptr)(long, long);

void f(a *obj, funcptr ptr) {
  a->*ptr(1, 2);
}

void main() {
  b myobj;
  f(&myobj, &b::func); // Error
}

It gives me the error: cannot convert parameter 2 from ''long (__thiscall b::*)(long,long)'' to long (__thiscall a::*)(long,long)''
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civguy

Mar 11, 2003 at 2:57 PM

Isn''t it pretty obvious why it doesn''t work? What if someone did this:

  
int main() {
  b myobj;
  a myotherobj;
  f(&myotherobj, &b::func);
}

Now what would happen inside f? A function that''s not part of ''a'' gets executed through an object of type ''a''. Uh, oh.

cgoat

Mar 11, 2003 at 3:43 PM

What you might want is:

  
class a
{
public:
   virtual long func(long, long) = 0;
};

class c
{
};

class b: public c, public a
{
public:
   long func(long n1, long n2);
};

typedef long (a::*funcptr)(long, long);

void f(a *obj, funcptr ptr)
{
   (obj->*ptr)(1, 2);
}

void main()
{
   b myobj;

   f(&myobj, &a::func); // Error

}

That will end up calling B''s function, or the defined function of anything derived from A.

neoztar

Author

neoztar

Mar 11, 2003 at 4:06 PM

Thanks! I see it now. Guess I''ll have to do it another way...

// Website // Google // GameDev // NeHe // MSDN // OpenGL Extensions //

thehurricane

Mar 11, 2003 at 4:24 PM

This might help:

http://www.parashift.com/c++-faq-lite/pointers-to-members.html

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