can i use mem_fun like this?

Question

I want to set the function pointer at runtime. But i'm stuck here. When i use global function or static class member function, everything is ok. but, when the function is ordinary class member functions. i always got compiler errors. Here is the code:

class A   
{
    int val;
public:
    A() { val = 0; }
    A(int j) { val = j; }

    int aFun(int k) {val -= k; return val; }
};

typedef int (* func)(int );
class B
{
    func m_addr;
public:
    B(func param)
        : m_addr(param)
    {

    }
    void execute()
    {
        cout << m_addr(9) << endl;
    }
};

I'm trying to use them like this:

/* error C2355: 'this' : can only be referenced inside non-static member functions error C2064: term does not evaluate to a function taking 1 arguments class does not define an 'operator()' or a user defined conversion operator to a pointer-to-function or reference-to-function that takes appropriate number of arguments */

A a;
B b(A::aFun); 
b.execute();

after googled a lot, i found that std::mem_fun may be helpful. but i don't know how to use it. anyone can help me?

PS: i'm using Visual C++ 2010

Answer 1

Class member functions have an extra parameter passed to it the by the compiler called this , so the compiler sees aFun as begin declared and written as

int A::aFun(A* this, int k)
{
    this->val -= k;
    return this->val;
}

Static/global functions don't have this parameter and so compilation succeeds.

In order to use A::aFun you'll need an instance of class A somewhere.

Answer 2

modified as following:

static int val;

class A   
{

public:
    A() { val = 0; }
    A(int j) { val = j; }

  static  int aFun(int k) {val -= k; return val; }
};

Answer 3

I didn't get exactly what are you trying to do but I saw an error, maybe helps you.

That's a pointer to a function, isn't?

 typedef int (* func)(int );

But you don't have a function, you have a method of a class. (a member function) So the typedef shoud be probability:

typedef int (A::*func)(int );

I hope helps you a bit, you can google for pointers to member functions and you will find some examples.

Good luck!