How to rewrite global functions for functions in a structure?

Question

I want function f2 in structure node to call function f1 in node instead of global function f1.

#include <iostream>
#include <functional>

int f1()
{
    return 1;
}

int f2()
{
    return f1();
}

struct node
{
    int f1()
    {
        return 2;
    }
    std::function<int()> f2 = ::f2;
};

int main()
{
    node a;
    std::cout << a.f2() << "\n";
    return 0;
}

I want function f2 in structure node to call function f1 in node instead of global function f1.

Can't be done. What's the actual problem you are trying to solve? Maybe there is a better way to solve it. — john, Nov 22 '22 at 08:09
@Someprogrammerdude I think what the OP is saying, is that they want the `::f2` function to call `node::f1` merely because it has been assigned to `node::f2`. — john, Nov 22 '22 at 08:10
The problem is that `node::f2` is a kind of alias for the global `::f2` function, so while `node::f2` is indeed a member of `node`, the function itself still isn't. So it will call the global `::f1` function. Why don't you define it as a proper member function of `node` instead? — Some programmer dude, Nov 22 '22 at 08:10
Extend / implement an override for f2 to accept a callback function. Then pass ::f1 when calling. — nick, Nov 22 '22 at 08:30
`node::f2` is not a function. It is a glorified function *pointer*. It points to `::f2`. `::f2` cannot call `node::f1`. If you want `node::f2` to have a mind of its own, you need to make it a function. — n. m. could be an AI, Nov 22 '22 at 09:04

score 1 · Answer 1 · answered Nov 22 '22 at 08:24

You can achieve this by adding a (second) constructor to node which accepts a callable to assign to node::f2. Something like this, passing in a lambda to that constructor to capture the object itself:

#include <iostream>
#include <functional>

int f1()
{
    return 1;
}

int f2()
{
    return f1();
}

struct node
{
    node () = default;
    node (std::function<int()> assign_to_f2) { f2 = assign_to_f2; }

    int f1 ()
    {
        return 2;
    }

    std::function<int()> f2 = ::f2;
};

int main()
{
    node a;
    std::cout << a.f2() << "\n";
    node b ([&b] () { return b.f1 (); });
    std::cout << b.f2() << "\n";
    return 0;
}

Output:

1
2

Live Demo

How to rewrite global functions for functions in a structure?

1 Answers1