How to call a function that is in an array of function pointers?

Question

I have an array that contains function pointers to functions that are inside my GameBoard class. How can I call one of those functions?

class myClass {
    int plusOne(int val) {return val + 1;}
    int minusOne(int val) {return val - 1;}
    int keepTheSame(int val) {return val;}

    int (myClass::*functions[3])(int val) {&myClass::plusOne, &myClass::minusOne, &myClass::keepTheSame};

    // Get coordinates of all positions that flip when placing disc at x,y
    vector<pair<int, int>> getFlipDiscs(pair<int, int> moveCoord, int color) {
        int a = (*functions[0])(2); // error: indirection requires pointer operand
    }
};

I've looked on the internet for member function pointers and arrays of function pointers and found quite a lot of information, but I wasn't able to combine those in a working solution.

Dietmar Kühl · Accepted Answer · 2018-08-18 17:16:58Z

4

Your function pointers are actually pointer to member functionss. To call any of these you’ll need to provide the object of the class you want to call them on. I think this should work:

(this->*(functions[0]))(2);

That said, I recommend not to pursue this direction! Sooner or later you’ll want to register other operations and usin an array of std::function<Signature> objects is more flexible.

answered Aug 18, 2018 at 17:16

Dietmar Kühl

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Comments

Palo · Accepted Answer · 2018-08-18 17:13:17Z

0

Don't you complicate it too much? How about this one:

#include <stdio.h>

int plusOne(int val) {return val + 1;}
int minusOne(int val) {return val - 1;}
int keepTheSame(int val) {return val;}

int (*functions[3])(int val) {plusOne, minusOne, keepTheSame};

int main()
{
    int a = (functions[1])(2); // no error
    printf("%d\n", a);
}

answered Aug 18, 2018 at 17:13

Palo

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3 Comments

JXD Over a year ago

It gives the error: 'Reference to non-static member function must be called'. Remember the code I gave is inside the GameBoard class, as noted in the question. Therefore these three functions are member functions.

JXD Over a year ago

I updated the question to better match the situation

Palo Over a year ago

Yes, this code only works as is, not inside the class. Maybe if the functions are simple and generic enough, one can keep them outside of a class as C++ is not strictly object-oriented. But Dietmar's magic solution seems to work.

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