C++ Function Pointer Problems

Hello,

I'm having problems understanding/using function pointers. I'm trying to use a member function of an instantiated class i.e. non-static as a call back function and need to pass a function pointer to the caller.

I've posted an example below which will result in the same compile error in my code :

Code:

---

class MyClass {

public:
        MyClass(){};
        virtual ~MyClass(){};
        int add(int oneNumber, int twoNumber){return (oneNumber + twoNumber);};
};

int main(){
        MyClass* t = new MyClass();
        int (*fp)(int,int);        // Create a function pointer - will point to a member function of an INSTANCE of class MyClass
        fp = t->add;                // Results in a compiler error
        delete(t);
}

---

The compiler gives :

Quote:

---

test.cpp:12: error: argument of type ‘int (MyClass::)(int, int)’ does not match ‘int (*)(int, int)

---

Just how do I pass a (*) pointer ? (and what is a (*) pointer anyway ???) Unfortunately I cannot change the calling function as it is code not written by myself and it is has been written in C. How do I pass a pointer to a function of a class instance ?

I'm using Linux by the way and gcc/g++ compiler

Thanks

>> I'm having problems understanding/using function pointers. I'm trying to use a member function of an instantiated class i.e. non-static as a call back function and need to pass a function pointer to the caller.

You can't. The member function requires a pointer to *this, so logically it's type is closer to:

int ( * )( MyClass*, int, int )

>> Unfortunately I cannot change the calling function as it is code not written by myself and it is has been written in C.

The only way to do it then would be a function that uses a global variable, eg:

Code:

---

int add_callback( int a, int b )
{
        // myClassObject is defined elsewhere in the program
        return myClassObject.add( a, b );
}

---

Another option, and this is a bit of a hack:

Code:

---

int add_callback_hack( int a, int b )
{
        static MyClass* ptr = 0;
        if( a == numeric_limits< int >::max( ) )
                ptr = ( MyClass* )b;
        return ptr->add( a, b );
}

int main(){
        MyClass mc;
        add_callback_hack(numeric_limits<int>::max(), int(&mc));
        int (*fp)(int,int) = add_callback_hack;
}

---

EDIT: I honestly wouldn't recommend the hack, though, as it's error-prone and could potentially lead to some nasty bugs!

EDIT#2: Ok, so I've come up with a much better hack, and it's totally safe. :p

Code:

---

int add_callback_hack( int a, int b, MyClass* instance )
{
        static MyClass*
                ptr = 0;
        if( instance )
                ptr = instance;
        return ptr->add( a, b );
}

inline void add_callback_init( MyClass* instance )
{
        add_callback_hack( 0, 0, instance );
}

int add_callback( int a, int b )
{
        return add_callback_hack( a, b, 0 );
}

int main(){
        MyClass mc;
        add_callback_init(&mc);
        int (*fp)(int,int) = add_callback;
}

---

Just make add() static.

As Sebastiani points out, in the general case this cannot be done, because most methods need a pointer the their associated object to work with. Static methods do not, however.