Question

Is it possible in C++?
For example I have a pointer to a function that takes no parameters and its return type is void:

void (*f)();

and and a function object:

class A
{
public:
    void operator()() { cout << "functor\n"; }
};

Is it possible to assign to f the address of an A object? And when I call f() to call the A functor?

I tried this but it doesn’t work:

#include <iostream>
using namespace std;

class A
{
public:
    void operator()() { cout << "functorA\n"; }
};

int main()
{
    A ob;
    ob();
    void (*f)();
    f = &ob;
    f();       // Call ob();
    return 0;
}

I get C:\Users\iuliuh\QtTests\functor_test\main.cpp:15: error: C2440: '=' : cannot convert from 'A *' to 'void (__cdecl *)(void)'
There is no context in which this conversion is possible

Is there any way to achieve this?

Answer 1

You can’t do it in the way you’ve specified, because:

1. operator() must be a nonstatic function (standards requirement)
2. a pointer to a non-static function must have an implicit parameter – the pointer to the class instance
3. your call to f() does not give any indication on which instance of the object A your function is called

Using C++11 and std::function, as Stephane Rolland pointed out, may do the trick – you’ll be specifying the pointer to the object in the binding:

std::function<void(void)> f = std::bind(&A::operator(), &ob);

(See question on using std::function on member functions)