受这个问题的启发，我从头开始编写了自己的预绑定。虽然它最终看起来和其他所有人都非常相似，但我保证它是原创的 :) - 称之为趋同进化。
不过味道稍有不同。一方面，它转发给它的构造函数，但您可能更喜欢使用std::decay（它在某些方面更有意义，但我不喜欢到处写std::ref）。另外，我添加了对嵌套预绑定的支持，因此 prebind(foo, prebind(GetRandomNumber))() 与 prebind(foo)(GetRandomNumber()) 相同。
#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)...);
}
也可以轻松更改为 postbind。
用法如下：
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;
}