function pointers generate 'invalid use of non-static member function' error

c++, function-pointers

Solution

The syntax to declare a function pointer to member methods is:

int (A::*plus)(int, int) = &A::add;
int (A::*minus)(int, int) = &A::subtract;

To invoke member methods use .* or ->* operator:

 (a_plus.*plus)(7, 5);

Also have a look at http://msdn.microsoft.com/en-us/library/b0x1aatf(v=vs.80).aspx

Hope this helps.

Complete code:

     #include <iostream>

    using namespace std;

    class A
    {
    public:
    int add(int first, int second)
    {
        return first + second;
    }

    int subtract(int first, int second)
    {
        return first - second;
    }

    int operation(int first, int second, int (A::*functocall)(int, int))
    {
        return (this->*functocall)(first, second);
    }
    };

    int main()
    {
        int  a, b;
        A a_plus, a_minus;
        int (A::*plus)(int, int) = &A::add;
        int (A::*minus)(int, int) = &A::subtract;
        a = a_plus.operation(7, 5, plus);
        b = a_minus.operation(20, a, minus);
        cout << "a = " << a << " and b = " << b << endl;
        return 0;
    }

Problem

I am trying to grasp pointer function concept in a better way. So I have a very simple and working example as: ``` #include <iostream> using namespace std; int add(int first, int second) { return first + second; } int subtract(int first, int second) { return first - second; } int operation(int first, int second, int (*functocall)(int, int)) { return (*functocall)(first, second); } int main() { int a, b; int (*plus)(int, int); int (*minus)(int, int); plus = &add; minus = &subtract; a = operation(7, 5, add); b = operation(20, a, minus); cout << "a = " << a << " and b = " << b << endl; return 0; } ``` So far so good, Now I need to group the functions in a class, and select add or subtract based on the function pointer that i use. So I just make a small modification as: ``` #include <iostream> using namespace std; class A { public: int add(int first, int second) { return first + second; } int subtract(int first, int second) { return first - second; } int operation(int first, int second, int (*functocall)(int, int)) { return (*functocall)(first, second); } }; int main() { int a, b; A a_plus, a_minus; int (*plus)(int, int) = A::add; int (*minus)(int, int) = A::subtract; a = a_plus.operation(7, 5, plus); b = a_minus.operation(20, a, minus); cout << "a = " << a << " and b = " << b << endl; return 0; } ``` and the obvious error is: ``` ptrFunc.cpp: In function ‘int main()’: ptrFunc.cpp:87:29: error: invalid use of non-static member function ‘int A::add(int, int)’ ptrFunc.cpp:88:30: error: invalid use of non-static member function ‘int A::subtract(int, int)’ ``` coz I haven't specified which object to invoke(and I don't want to use static methods for now) EDIT: several comments and answers suggested that the non-static version(as I have written) is not possible.(thanks to all) So, Modifying the class in the following manner also wont work: ``` #include <iostream> using namespace std; class A { int res; public: A(int choice) { int (*plus)(int, int) = A::add; int (*minus)(int, int) = A::subtract; if(choice == 1) res = operation(7, 5, plus); if(choice == 2) res = operation(20, 2, minus); cout << "result of operation = " << res; } int add(int first, int second) { return first + second; } int subtract(int first, int second) { return first - second; } int operation(int first, int second, int (*functocall)(int, int)) { return (*functocall)(first, second); } }; int main() { int a, b; A a_plus(1); A a_minus(2); return 0; } ``` generated this error: ``` ptrFunc.cpp: In constructor ‘A::A(int)’: ptrFunc.cpp:11:30: error: cannot convert ‘A::add’ from type ‘int (A::)(int, int)’ to type ‘int (*)(int, int)’ ptrFunc.cpp:12:31: error: cannot convert ‘A::subtract’ from type ‘int (A::)(int, int)’ to type ‘int (*)(int, int)’ ``` may I know how to solve this issue please? thanks

Original source