c++ - How to initialize `std::function` with a member-function?

Question

I am trying to learn std::function and here's my code:

#include <iostream>
#include <functional>

struct Foo {
    void print_add(int i){ 
      std::cout << i << '
'; 
    }
};

typedef std::function<void(int)> fp;

void test(fp my_func)
{
  my_func(5);
}

int main(){
    Foo foo;
    test(foo.print_add);
    return 0;
}

Compiler Error:

 error: cannot convert 'Foo::print_add' from type 'void (Foo::)(int)' to type 'fp {aka std::function<void(int)>}'
     test(foo.print_add);

How can I make this work, i.e how can I pass a member function as a parameter?

Answer 1

print_add is a non-static member function of foo, which means it must be invoked on an instance of Foo; hence it has an implicit first argument, the this pointer.

Use a lambda that captures the foo instance and invokes print_add on it.

Foo foo;
test([&foo](int i){ foo.print_add(i); });

Another option is to use std::bind to bind the foo instance:

test(std::bind(&Foo::print_add, &foo, std::placeholders::_1));

Live demo