How to pass method of a class into another function in C++?

Question

Say I have a class and a function ``` class A { int a; int b; int mul() { return a+b; } }; ... void func(int af, int bf, int (*fc)()) { ... } ``` In the main function, the function is supposed to use the methods of class A ``` int main { A as; ... func(as.a, as.b, as.mul); } ``` However, I can't do that, the compiler keeps telling me that I'm passing ``` (int&, int&, 'unresolved overloaded function type') ``` into a function of candidate ``` (int, int, void(*)()). ``` Why is that, and how to pass method of a class into another function? Oh, I think I should make the problem a little bit clearer. `func(...)` is actually a function from an algorithm I'm working on. And `class A` is a model that will use the algorithm to do the simulation. So I don't think I'll specifically use the instance of A in function B, but only pass A's methods and components in and work with them. Update: Some have mentioned using static methods in class A. However, this is still a partial solution. Using static…

Accepted Answer

First of all, `mul` is a member function of `A`, so you can't invoke it as if it was a global function or a `static` member function: it needs an object on which the function is invoked:

```
A::mul(); // ERROR!
A a;
a.mul(); // OK
```

This said, you could change the definition of `func` to accept a pointer-to-member-function:

```
void func(int af, int bf, void (A::*fc)())
//                        ^^^^^^^^^^^^^^^
```

However, this change alone won't make you progress too much, because you're still not passing the concrete instance of `A` on which the member function should be invoked. In other words, this would be illegal:

```
fc(); // ERROR!
```

To over come this limitation, you could pass a reference or a pointer to an instance of `A` together to `func`, and call it as done below:

```
void func(A* pA, int af, int bf, void (A::*fc)())
{
    ...
    (pA->*func)();
}
```

However, if `func()` has the object on which the member function should be invoked passed in as an argument, it is not clear what is the purpose of `af` and `bf`.

Possibly, if your `mul()` member function does not need to work on any concrete instance of `A`, and still you want to make it a member of `A`, you could declare it as `static`:

```
class A
{
    int a;
    int b;
    static void mul();
//  ^^^^^^
};
```

This would make your original call to `func()` compile. However, in this case it is not clear whether this would make sense, since the fact that `mul()` accept no arguments suggests that it is supposed to work on the `a` and `b` members of the object it is invoked on.

How to pass method of a class into another function in C++?

Solution

Problem