问题
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?
回答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);
}
回答2:
Inspired by the question, I wrote my own prebind from scratch. It ended up looking pretty similar to everyone else though, but I promise it's original :) - call it convergent evolution.
It has a slightly different flavour though. For one thing, it forwards to it's constructor, but you may prefer to use std::decay
(it makes more sense in some ways, but I don't like writing std::ref
everywhere). For another I added support for nested prebinds so prebind(foo, prebind(GetRandomNumber))()
is the same as prebind(foo)(GetRandomNumber())
.
#include <tuple>
#include <type_traits>
using namespace std;
struct pb_tag {}; //use inheritance to mark prebinder structs
//result_of_t will be defined by default in c++1y
template<typename T > using result_of_t = typename result_of<T>::type;
template<typename T> using is_prebinder = is_base_of<pb_tag, typename remove_reference<T>::type >;
//ugly sequence generators for something different
template<int N, int ...S> struct seq : seq<N-1, N, S...> {};
template<int ...S> struct seq<0, S...> {typedef seq type;};
//these three functions are only for nested prebind. they map
//T t -> T t and Prebind<f, T...> -> f(T...)
template<typename T>
auto dispatchee(T&& t, false_type) -> decltype(forward<T>(t)){
return forward<T>(t);
}
template<typename T>
auto dispatchee(T&& t, true_type) -> decltype(t())
{
return t();
}
template<typename T>
auto expand(T&& t) -> decltype(dispatchee(forward<T>(t), is_prebinder<T>()))
{
return dispatchee(forward<T>(t), is_prebinder<T>());
}
template<typename T> using expand_type = decltype(expand(declval<T>()));
//the functor which holds the closure in a tuple
template<typename f, typename ...ltypes>
struct prebinder : public pb_tag
{
tuple<f, ltypes...> closure;
typedef typename seq<sizeof...(ltypes)>::type sequence;
prebinder(f F, ltypes... largs) : closure(F, largs...) {}
template<int ...S, typename ...rtypes>
result_of_t<f(expand_type<ltypes>..., rtypes...)>
apply(seq<0, S...>, rtypes&& ... rargs){
return (get<0>(closure))(expand(get<S>(closure))... , forward<rtypes>(rargs)...);
}
template<typename ...rtypes>
result_of_t<f(expand_type<ltypes>..., rtypes...)>
operator() (rtypes&& ... rargs){
return apply(sequence(), forward<rtypes>(rargs)...);
}
};
template<typename f, typename ...ltypes>
prebinder<f, ltypes...> prebind(f&& F, ltypes&&... largs)
{
return prebinder<f, ltypes...>(forward<f>(F), forward<ltypes>(largs)...);
}
It can be easily changed to postbind as well.
Usage looks like:
int g(int a){ return 1 + a; }
int h(){ return 1; }
int i(int a, int b, int c, int d){
return 1 + a + b + c + d;
}
int main()
{
//completely bound
auto a = prebind(g, 1);
cout << a() << endl;
//nested bind by reference
auto b = prebind(g, a);
cout << b() << endl;
get<1>(a.closure) = 2;
cout << b() << endl;
//bind to prebinder
auto c = prebind(b);
cout << c() << endl;
//nested bind of temp to temp
auto d = prebind(prebind(g), prebind(h));
cout << d() << endl;
//and the one you wanted orginally
auto e = prebind(i, 1, 1, 1);
cout << e(1) << endl;
return 0;
}
来源:https://stackoverflow.com/questions/21216762/partial-binding-of-function-arguments