Partial Binding of Function Arguments

Question

Is there a way to partially bind the first/last n arguments of a callable object (e.g. function) without explicitly specifying the rest of the arguments?

std::bind() seems to require that all the arguments are be bound, those that are to be left should be bound to std::placeholders::_1,_2,_3 etc.

Is it possible to write a bind_first()/bind_last() for partial binding starting from the first/last argument and that automagically inserts the placeholders for any remaining unbound arguments in their original order in their the original position?

Answer 1

Neither Boost nor the standard library bind fill in the blanks automatically. You could write such a gadget yourself if you have a rainy evening to fill; here's an example for trailing arguments of a plain function only:

#include <tuple>
#include <type_traits>
#include <utility>

template <typename F, typename ...Args> struct trailing_binder;

template <typename R, typename ...Frgs, typename ...Args>
struct trailing_binder<R(Frgs...), Args...>
{
    template <typename ...Brgs>
    trailing_binder(R (*f)(Frgs...), Brgs &&... brgs)
    : the_function(f)
    , the_args(std::forward<Brgs>(brgs)...)
    { }

    template <unsigned int ...I> struct intlist {};

    template <typename ...Brgs>
    typename std::enable_if<sizeof...(Brgs) + sizeof...(Args) == sizeof...(Frgs), R>::type
    operator()(Brgs &&... brgs)
    {
        return unwrap(std::integral_constant<bool, 0 == sizeof...(Args)>(),
                      intlist<>(),
                      std::forward<Brgs>(brgs)...);
    }

private:
    template <unsigned int ...I, typename ...Brgs>
    R unwrap(std::false_type, intlist<I...>, Brgs &&... brgs)
    {
        return unwrap(std::integral_constant<bool, sizeof...(I) + 1 == sizeof...(Args)>(),
                      intlist<I..., sizeof...(I)>(),
                      std::forward<Brgs>(brgs)...);
    }

    template <unsigned int ...I, typename ...Brgs>
    R unwrap(std::true_type, intlist<I...>, Brgs &&... brgs)
    {
        return the_function(std::get<I>(the_args)..., std::forward<Brgs>(brgs)...);
    }

    R (*the_function)(Frgs...);
    std::tuple<Args...> the_args;
};

template <typename R, typename ...Args, typename ...Frgs>
trailing_binder<R(Frgs...), Args...> trailing_bind(R (*f)(Frgs...), Args &&... args)
{
    return trailing_binder<R(Frgs...), typename std::decay<Args>::type...>(f, std::forward<Args>(args)...);
}

Usage:

int f(int a, int b, int c, int d) { return a + b + c + d; }

int main()
{
    auto b = trailing_bind(f, 1);
    return b(3, 8, 13);
}