Could anybody explain how to use type (fn_t) from the following example:

struct S {
int x;
void fn() { x++; }
};

typedef void (S::*fn_t)();

void main()
{
S a;
fn_t p;
p=NULL; // works
p=a.fn; // doesn't work. error expecting '('
}

sergey wrote:
> Could anybody explain how to use type (fn_t) from the following example:
> 
> struct S {
> int x;
> void fn() { x++; }
> };
> 
> typedef void (S::*fn_t)();
> 
> void main()
> {
> S a;
> fn_t p;
> p=NULL; // works  p=a.fn; // doesn't work. error expecting '(' 

Try "&a.fn" instead. After all, you're attempting to set a pointer to point at the address of something. Otherwise, the parser is expecting a method invocation, thus, it expects a '(' to follow a.fn.

sergey wrote:
> Could anybody explain how to use type (fn_t) from the following example:
> 
> struct S {
> int x;
> void fn() { x++; }
> };
> 
> typedef void (S::*fn_t)();

Generally speaking, though, if you have to resort to pointers to member functions in C++, there is something hideously wrong with your design. Pointers to functions are much more useful if you're programming in C, but really wrong if you're programming in C++. In C++, you want to take advantage of functor structures (e.g., the one's STL likes that require you to implement an operator() method.)

In article <ct66rn$2aso$1@digitaldaemon.com>, Scott Michel says...
>
>sergey wrote:
>> Could anybody explain how to use type (fn_t) from the following example:
>> 
>> struct S {
>> int x;
>> void fn() { x++; }
>> };
>> 
>> typedef void (S::*fn_t)();
>> 
>> void main()
>> {
>> S a;
>> fn_t p;
>> p=NULL; // works
>> p=a.fn; // doesn't work. error expecting '('
>
>Try "&a.fn" instead. After all, you're attempting to set a pointer to point at the address of something. Otherwise, the parser is expecting a method invocation, thus, it expects a '(' to follow a.fn.

Even this isn't right. You can only write:
p = &S::fn;
You have to include the name of the class.
Note that "a" doesn't appear at all. You probably want it to call a.fn(), but
that requires a delegate, which you can't do legally in C++ (although Borland
C++, D, and C# all provide delegates).

Sergey, I didn't intend to plug my own article again, but I think you should read it...

http://www.codeproject.com/cpp/FastDelegate.asp

It was the most popular C++ article published on the CodeProject website in 2004, so you wouldn't be wasting your time. It gives a tutorial on member function pointers, and provides a library that uses some tricks to allow you to write code like:

#include "FastDelegate.h"

struct S {
int x;
void fn() { x++; }
};

typedef FastDelegate< void () > fn_t; // no parameters, returns void

void main()
{
S a;
fn_t p;
p.clear();
p.bind(&a, &S::fn); // or: p = MakeDelegate(&a, &S::fn);
p(); // calls a.fn()
}

As Scott says, you should seriously consider if you can use functors instead. But functors won't let you do run-time polymorphism, so delegates can often be very handy.

-Don.