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.