C ++非静态函数作为变量

Question

   class manip{
public:
    int t;
    manip(int tom){
        t = tom;
    }
    int sub(int val){
        return t - val;
    }
    int add(int val){
        return t + val;
    }
    int perform(int(manip::*key)(int), int val){
        return (this->*key)(val);
    }
};

int main() {
    manip g(5);
    cout << g.perform(&manip::add, 9) << ":" << g.perform(&manip::sub, 9);

(This is just a simplified version of a problem im trying to solve in a larger piece of code)

the problem lies here

    int(b::*func)(int) = b::add;

    int c = func(2);

this gives me a syntax error on the second line (because i have no reference to the "this" data). How do i change it so that the function being called isnt b::add but rather inst.add.

Edit : Posted a working version of the code. Thanks Speed8ump

Answer 1

in your example 'func' is a member function pointer. It must be used on an instance of the data type it is a member of:

int c = (inst.*func)(2);

Answer 2

You need to give a parameter pointing to the struct (or aA).

int main(){
    b inst(5)
    int(b::*func)(int, struct b*) = b::add;
    int c = func(2, &inst);
}

int add(int a, struct b* ptr){
    return a + ptr->aA;
}

Answer 3

If your compiler is new enough you can avoid function pointers and use std::function instead

 int(b::*func)(int) = b::add;

becomes

#include <functional>
#include <iostream>
 class manip{
public:
    int t;
    manip(int tom){
        t = tom;
    }
    int sub(int val){
        return t - val;
    }
    int add(int val){
        return t + val;
    }
    int perform(std::function<int(manip*,int)>f, int val){
        return f(this, val);
    }
};

int main() {
    manip g(5);
    std::cout << g.perform(&manip::add, 9) << ":" << g.perform(&manip::sub, 9);
}