Function Variable

• Sometimes, you need to store a function in a variable that you can call later.
• For example, DeepTensor (a minimal pytorch like autograd framework)'s Value object requires backward_ variable, that stores the function to be called when value::backward() is called.

---

Older way (before C++11)

• Before C++11, we used to use function-pointers.

#include <iostream>

int sum(int a, int b){
    return a+b;
}

int main(){
    int(*fn)(int, int) = nullptr;
    if(!fn){
        std::cout<<"fn is null.\n";
    }

    fn = sum;
    std::cout<<"calling fn: "<<fn(2,3)<<"\n";

}

---

After C++11

• From C++11 you can use std::function to store functions.

std::function<return_type(parameter_type1, parameter_type2, ...)>

#include <iostream>

int sum(int a, int b){
    return a+b;
}

int main(){
    std::function<int(int, int)> fn = nullptr;
    if(!fn){
        std::cout<<"fn is null.\n";
    }

    fn = sum;
    std::cout<<"calling fn: "<<fn(2,3)<<"\n";

}