Re: How to declare and use a function pointer?

From:

"Stuart Golodetz" <sgolodetz@dNiOaSl.PpAiMpPeLxE.AcSoEm>

Newsgroups:

comp.lang.c++

Date:

Fri, 14 Jul 2006 22:59:43 +0100

Message-ID:

<CZ-dnQZlu6VMjiXZnZ2dnUVZ8t2dnZ2d@pipex.net>

"Sam Waller" <sam.waller@avmet.com> wrote in message
news:u00gb2tkp3c95h6eu1ov48ejbnbvpspp9j@4ax.com...

/*********************

**********************/

I'm trying to declare a function pointer in class A and set it in class B,
but I get
syntax errors. How can I get this to compile and run?

thanks,
Sam

class A {
  public:
     A(void) {
     }
     virtual ~A() {
     }
     void (*func)(int i, bool b); // trying to declare
a function pointer
};

/*********************

**********************/

class B {

  public:
     B(void) {
        a = new A();

        a->func = &func(int i, bool b); // <<<<<<<<<< syntax error
<<<<<<<<<<<<<<

        // Microsoft visual studio error messages about the above line:
        // error C2144: syntax error : missing ')' before type 'int'
        // error C2660: 'func' : function does not take 0 parameters
        // error C2059: syntax error : ')'

     }
     virtual ~B() {
        delete a;
     }
     A *a;
     void func(int i, bool b) {
        return;
     }

};

/***************************************************************************
  I need to have this so that different 'B'-type classes can
  change the behavior of A::func
***************************************************************************/

int main(int argc, char *argv[]) {
  B *b = new B();

  b->a->func(99, true); // should call B::func()

  delete b;

  return 0;
}

Well if you absolutely must do this for whatever capricious reason(!), then:

#include <iostream>

template <typename T>
class A
{
public:
    void (T::*func)(int i, bool b);
};

class B
{
public:
    B()
    {
        a = new A<B>;
        a->func = &B::func;
    }

    ~B()
    {
        delete a;
    }

    A<B> *a;

    void func(int i, bool b)
    {
        std::cout << "func(" << i << ", " << b << ")\n";
    }
};

int main()
{
    B b;
    (b.*(b.a->func))(99, true);
    return 0;
}

But it's really, really dubious. What are you actually trying to achieve
here? If you really want to have different func behaviour in various
classes, what's wrong with something like

class A
{
public:
    virtual void func(int i, bool b) = 0;
};

class B : public A
{
public:
    void func(int i, bool b)
    {
        // Whatever
    }
};

?

I'm not really clear what you're after here.

Hope this helps a bit! :)
Stu