How to call a function by its name. An approach us

Question

Based on post How to call a function by its name (std::string) in C++?, tried to make a version using CLASS, but my approach does not work.

class A {
    public:
        int add(int i, int j) { return i+j; }
        int sub(int i, int j) { return i-j; }
};

typedef int (*FnPtr)(int, int);

int main(int argc, char* argv[]) {

// initialization:
std::map<std::string, FnPtr> myMap;
A a;
myMap["add"] = a.add;
myMap["sub"] = a.sub;

Returns this erro:

main.cpp:31:22: error: cannot convert ‘A::add’ from type ‘int (A::)(int, int)’ to type ‘std::map<std::basic_string<char>, int (*)(int, int)>::mapped_type {aka int (*)(int, int)}’
main.cpp:32:22: error: cannot convert ‘A::sub’ from type ‘int (A::)(int, int)’ to type ‘std::map<std::basic_string<char>, int (*)(int, int)>::mapped_type {aka int (*)(int, int)}’

Does anyone know what is the error?

Answer 1

At least as you've shown things, your class A provides nothing but problems. If you turn it into a namespace, things will be a lot easier.

namespace A {
    int add(int i, int j) { return i+j; }
    int sub(int i, int j) { return i-j; }
};

typedef int (*FnPtr)(int, int);

int main(int argc, char* argv[]) {
    std::map<std::string, FnPtr> myMap;
    myMap["add"] = A::add;
    myMap["sub"] = A::sub;
    // ...

This way, add and sub aren't member functions, so you don't get the type mismatch. At least as shown, the instance of A provided no functionality beyond calling add and sub, so a namespace accomplishes just as much good while eliminating the problems.

Answer 2

You could also solve the problem, as mentioned by Kal in the comments, using std::function and std::bind(this is C++11 so):

#include <iostream>
#include <string>
#include <map>
#include <functional>

class A {
    public:
        int add(int i, int j) { return i+j; }
        int sub(int i, int j) { return i-j; }
};


int main(int argc, char* argv[])
{
  std::map<std::string, std::function<int(int,int)>> myMap;
  A a;
  myMap["add"] = std::bind(&A::add,&a,std::placeholders::_1,std::placeholders::_2);
  myMap["sub"] = std::bind(&A::sub,&a,std::placeholders::_1,std::placeholders::_2);

  std::cout<<myMap["sub"](7,2)<<std::endl;
}