How do I create a packaged_task with a member function?

Question

On the program below, I'm trying to create a packaged_task with a member function:

#include <future>
using namespace std;

struct S
{
    int calc(int& a)
    {
        return a*a;
    }
};

int main()
{
    S s;
    auto bnd = std::bind(&S::calc, s);
    std::packaged_task<int(int&)> task( bnd);
    return 0;
}

Unfortunately the attempt results in an error.

How can this be done?

Answer 1

by adding up a placeholder like:

auto bnd = std::bind(&S::calc, s, std::placeholders::_1)

Answer 2

std::bind is quirky.

Replace your use of std::bind with:

template<class T, class Sig>
struct bound_member;

template<class T, class R, class...Args>
struct bound_member<T, R(Args...)> {
  T* t;
  R(T::*m)(Args...);
  R operator()(Args...args)const {
    return (t->*m)(std::forward<Args>(args)...);
};

template<class T, class R, class...Args>
bound_member<T,R(Args...)> bind_member( T* t, R(T::*m)(Args...) ) {
  return {t,m};
}
template<class T, class R, class...Args>
bound_member<T,R(Args...)> bind_member( T& t, R(T::*m)(Args...) ) {
  return {&t,m};
}
template<class T, class R, class...Args>
bound_member<T,R(Args...)> bind_member( T&& t, R(T::*m)(Args...) )
=delete; // avoid lifetime issues?

and now auto bnd = bind_member(s, S::calc); should make your code work.

There are few situations where a lambda isn't a better idea than std::bind, especially by C++14. In C++11, there are some corner cases, but even then I usually prefer to write my own binders without the quirks of std::bind.