Passing member function with all arguments to std::function

Question

How can I create a std::function from member function without need for typing std::placeholders::_1, std::placeholders::_2, etc - I would like to "placehold" all arguments, saving only the object itself.

struct Foo{
  int bar(int,float,bool) {return 0;}
};
int baz(int,float,bool) {return 0;}
int main() {
  Foo object;
  std::function<int(int,float,bool)> fun1 = baz; // OK
  std::function<int(int,float,bool)> fun2 = std::bind(&Foo::bar, object); // WRONG, needs placeholders
}

I don't want to provide arguments at this stage, I just want to store function + object somewhere. For example I would like to have std::vector with both global functions and member functions. It was easy to do with FastDelegate (fastdelegate::MakeDelegate(object, &Class::function)).

I don't want to use lambda as it would require me to retype arguments as well. I just want old FastDelegate behaviour.

Answer 1

You can use function template which will deduce all member function parameter types, like this:

template<typename Obj, typename Result, typename ...Args>
auto make_delegate(const Obj &x, Result (Obj::*fun)(Args...)) -> // ...

And will return special delegate object, which will contain your object (or pointer to it) and just forward all passed arguments to member function of underlying object:

template<typename Obj, typename Result, typename ...Args>
struct Delegate
{
    Obj x;
    Result (Obj::*f)(Args...);

    template<typename ...Ts>
    Result operator()(Ts&&... args)
    {
        return (x.*f)(forward<Ts>(args)...);
    }
};

You will get following usage syntax:

function<int(int,float,bool)> fun = make_delegate(object, &Foo::bar);

Here is full example:

#include <functional>
#include <iostream>
#include <utility>

using namespace std;

struct Foo
{
    int bar(int x, float y, bool z)
    {
        cout << "bar: " << x << " " << y << " " << z << endl;
        return 0;
    }
};

int baz(int x, float y, bool z)
{
    cout << "baz: " << x << " " << y << " " << z << endl;
    return 0;
}

template<typename Obj, typename Result, typename ...Args>
struct Delegate
{
    Obj x;
    Result (Obj::*f)(Args...);

    template<typename ...Ts>
    Result operator()(Ts&&... args)
    {
        return (x.*f)(forward<Ts>(args)...);
    }
};

template<typename Obj, typename Result, typename ...Args>
auto make_delegate(const Obj &x, Result (Obj::*fun)(Args...))
    -> Delegate<Obj, Result, Args...>
{
    Delegate<Obj, Result, Args...> result{x, fun};
    return result;
}

int main()
{
    Foo object;
    function<int(int,float,bool)> fun[] =
    {
        baz,
        make_delegate(object, &Foo::bar) // <---- usage
    };
    for(auto &x : fun)
        x(1, 1.0, 1);
}

Output is:

baz: 1 1 1
bar: 1 1 1

Live Demo on Coliru