How to assign a struct function pointer to a struct function

Question

I have a struct Game with a function pointer called onBegin

struct Game {
  // ...
  void (*onBegin)(Game&);
  // ...
};

What I am attempting to do is allow the user to create their own onBegin function, in which they could say

void CustomFunc(Game& g) {
  // Do something
}

Game g = Game();
g.onBegin = *CustomFunc;

What I am attempting to do is make a function and then set the pointer onBegin to point at that default function.

struct Game {
public:
  void (*onBegin)(Game&);
private:
  void defualtOnBegin(Game&);
};

// In the constructor
Game::Game() {
  // ...
  this->onBegin = this->defaultOnBegin; // This is what is giving me the error
}

I receive the error: a pointer to a bound function may only be used to call the function and do not know what is wrong here.

it's a member function pointer, differs from free functions. — apple apple
– apple apple, Commented May 12, 2022 at 5:23
so if i wanted to achieve what im getting at, i couldnt make the default function apart of the game struct? — QuBit Gaming
– QuBit Gaming, Commented May 12, 2022 at 5:50
Or make it static. It gets the instance by its parameter anyway. — Sebastian
– Sebastian, Commented May 12, 2022 at 5:55

Phlar · Accepted Answer · 2022-05-12 09:07:32Z

1

What I am attempting to do is allow the user to create their own onBegin function...

You could achieve that in different ways, however as you want to go for a function-pointer approach, you might want to utilize std::function like:

#include <iostream>
#include <functional>

struct Game {
    public:
        Game(std::function<void(Game&)> customOnBeginFnc = nullptr) {
            if(customOnBeginFnc) {
                customOnBeginFnc(*this);
            } else {
                defaultOnBegin(*this);
            }
        }

    private:
        void defaultOnBegin(Game&) {
            std::cout << "Default 'onBegin'\n";
        }
};

void customOnBegin(Game&) {
    std::cout << "Custom 'onBegin'\n";
}

int main() {

    {
        std::cout << "Starting a 'default' game...\n";
        Game g;
    }

    {
        std::cout << "Starting a 'customized' game...\n";
        Game g(customOnBegin);
    }
}

Run it here.

The advantage of that approach would be that you are not limited to free functions but you could also bind a member function to it via a lambda or std::bind.

edited May 12, 2022 at 9:07

answered May 12, 2022 at 9:01

Phlar

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1 Comment

apple apple Over a year ago

although std::function is a valid approach, the code you present doesn't benefit from it.

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