How do i write a pointer-to-member-function with std::function?

Question

I know how to declare int fn(double) inside of std::function (std::function). I know how to write a pointer-to-member-function (

Answer 1

A member function is not a function. It is not itself anything you can call. All you can do is call a member function of an instance object. Only the pair of pointer-to-member-function and object constitutes a callable entity.

To bind an instance to a PTMF and obtain something callable, use bind:

#include 

struct Foo
{
    double bar(bool, char);
};

Foo x;
using namespace std::placeholders;
std::function f = std::bind(&Foo::bar, x, _1, _2);
f(true, 'a'); //...

As with lambdas, bind expressions have an unknowable type, and the conversion to std::function (as well as the actual dispatch) is potentially expensive. If possible, it is preferable to use auto for the type of the bind expression.