Assigning C++ function pointers to member functions of the same object

Question

How do I get the function pointer assignments (and maybe the rest) in test.calculate to work?

#include <iostream>

class test {

    int a;
    int b;

    int add (){
        return a + b;
    }

    int multiply (){
        return a*b;
    }

    public:
    int calculate (char operatr, int operand1, int operand2){
        int (*opPtr)() = NULL;

        a = operand1;
        b = operand2;

        if (operatr == '+')
            opPtr = this.*add;
        if (operatr == '*')
            opPtr = this.*multiply;

        return opPtr();
    }
};

int main(){
    test t;
    std::cout << t.calculate ('+', 2, 3);
}

Answer 1

There are several problems with your code.

First, int (*opPtr)() = NULL; isn't a pointer to a member function, its a pointer to a free function. Declare a member function pointer like this:

int (test::*opPtr)() = NULL;

Second, you need to specify class scope when taking the address of a member function, like this:

if (operatr == '+') opPtr = &test::add;
if (operatr == '*') opPtr = &test::multiply;

Finally, to call through a member function pointer, there is special syntax:

return (this->*opPtr)();

Here is a complete working example:

#include <iostream>

class test {

    int a;
    int b;

    int add (){
        return a + b;
    }

    int multiply (){
        return a*b;
    }

    public:
    int calculate (char operatr, int operand1, int operand2){
        int (test::*opPtr)() = NULL;

        a = operand1;
        b = operand2;

        if (operatr == '+') opPtr = &test::add;
        if (operatr == '*') opPtr = &test::multiply;

        return (this->*opPtr)();
    }
};

int main(){
    test t;
    std::cout << t.calculate ('+', 2, 3);
}

Answer 2

Like this int (test::*opPtr)() = NULL;. Refer http://www.parashift.com/c++-faq-lite/pointers-to-members.html#faq-33.1

Edit: Also use if (operatr == '+') opPtr = &test::add; instead of [..] = this.add and return (this->(opPtr))(); instead of return opPtr();. In fact, use typedefs and macros like the FAQ says and probably member function paramaters instead of class members a and b.