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

Answer 1

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

template
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
struct Delegate
{
    Obj x;
    Result (Obj::*f)(Args...);

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

You will get following usage syntax:

function fun = make_delegate(object, &Foo::bar);

Here is full example:

#include 
#include 
#include 

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
struct Delegate
{
    Obj x;
    Result (Obj::*f)(Args...);

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

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

int main()
{
    Foo object;
    function 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