Variadic templates and std::bind

Question

Given the following templated function, how can I change it to take advantage of variadic templates? That is to say, to replace std::bind placeholders with a variadic parameter instead of P1 and P2? At the moment I have one of these functions per arity, with arity zero having no P parameter, up to arity 9 having P1 to P9 parameters. I was hoping to collapse this into a single function if possible.

template<typename R, typename T, typename U, typename P1, typename P2>
void Attach(R (T::*f)(P1, P2), U p)
{
    AttachInternal(p, std::bind(f, 
                                p.get(), 
                                std::placeholders::_1, 
                                std::placeholders::_2));
}

Answer 1

You can (partially) specialize std::is_placeholder for specializations of a custom template. This way, you can introduce a placeholder generator via the usual int_sequence technique.

From [func.bind.isplace]/2

The implementation shall provide a definition that has the BaseCharacteristic of integral_constant<int, J> if T is the type of std::placeholders::_J, otherwise it shall have a BaseCharacteristic of integral_constant<int, 0>. A program may specialize this template for a user-defined type T to have a BaseCharacteristic of integral_constant<int, N> with N > 0 to indicate that T should be treated as a placeholder type.

The usual int_sequence:

#include <cstddef>

template<int...> struct int_sequence {};

template<int N, int... Is> struct make_int_sequence
    : make_int_sequence<N-1, N-1, Is...> {};
template<int... Is> struct make_int_sequence<0, Is...>
    : int_sequence<Is...> {};

The custom placeholder template and specialization of is_placeholder:

template<int> // begin with 0 here!
struct placeholder_template
{};

#include <functional>
#include <type_traits>

namespace std
{
    template<int N>
    struct is_placeholder< placeholder_template<N> >
        : integral_constant<int, N+1> // the one is important
    {};
}

I'm not sure where to introduce the 1; the places I considered are all not optimal.

Using it to write some binder:

template<class Ret, class... Args, int... Is>
void my_bind(Ret (*p)(Args...), int_sequence<Is...>)
{
    auto x = std::bind(p, placeholder_template<Is>{}...);
    x( Args(42)... );
}

template<class Ret, class... Args>
void my_bind(Ret (*p)(Args...))
{
    my_bind(p, make_int_sequence< sizeof...(Args) >{});
}

Usage example of the binder:

#include <iostream>

void foo(double, char, int) { std::cout << __PRETTY_FUNCTION__ << "\n"; }
void bar(bool, short) { std::cout << __PRETTY_FUNCTION__ << "\n"; }

int main()
{
    my_bind(foo);
    my_bind(bar);
}

Answer 2

I would like to propose a more simple solution to the problem to bind a member function to a variable number of placeholders:

template<typename R, typename T, typename U, typename... Args>
std::function<R(Args...)> Attach(R (T::*f)(Args...), U p)
{
    return [p,f](Args... args)->R { return (p->*f)(args...); };
};

A simple example of the usage looks like that

class CrazyAdd
{
public:

    CrazyAdd(double val)
    : m_crazyPart(val)
    {}

    double add(double a, double b)
    {
        return a+b+m_crazyPart;
    }

private:
    double m_crazyPart;
};

void main() {

    CrazyAdd myAdd(39);

    // Create a function object that is bound to myAdd
    auto f = Attach(&CrazyAdd::add, &myAdd);

    // Call the function with the parameters    
    std::cout << f(1,2) << std::endl;   // outputs 42
}

Personally I think that's another good example why Scott Meyer recommends lambdas instead of std::bind.