在元组上泛化for_each以接受可变数量的参数
Generalizing for_each over a tuple to accept a variable number of arguments
目前,我有:
template <unsigned I,
unsigned N,
typename Tuple,
typename UnaryFunction>
struct for_;
template <unsigned N, typename Tuple, typename UnaryFunction>
struct for_<N, N, Tuple, UnaryFunction> {
static
void call(const Tuple&, UnaryFunction) {}
};
template <unsigned I,
unsigned N,
typename Tuple,
typename UnaryFunction>
struct for_ {
static
void call(Tuple&& x, UnaryFunction f) {
f(get<I>(x));
for_<I + 1, N, Tuple, UnaryFunction>::call(std::forward<Tuple>(x), f);
}
};
template <typename Tuple, typename UnaryFunction>
inline
void for_each(Tuple&& x, UnaryFunction f) {
for_<0,
tuple_size<
typename std::remove_const<
typename std::remove_reference<Tuple>::type
>::type
>::value,
Tuple,
UnaryFunction>::call(std::forward<Tuple>(x), f);
}
是否有可能普遍化这一点,可能通过可变模板,采取任何数量的元组参数?
编辑:我将如何使用我无法定义的内容:
if (i != e) {
std::array<Tuple, 2> x;
std::get<0>(x) = *i;
std::get<1>(x) = *i;
++i;
std::for_each (i, e, [&x](const Tuple& y) {
for_each(std::get<0>(x), y, assign_if(std::less));
for_each(std::get<1>(x), y, assign_if(std::greater));
});
}
编辑:改为使用右值引用和std::forward
我不确定这是你所期望的,但我会把它贴出来-也许有人会发现它有帮助。
namespace std {
template<int I, class Tuple, typename F> struct for_each_impl {
static void for_each(const Tuple& t, F f) {
for_each_impl<I - 1, Tuple, F>::for_each(t, f);
f(get<I>(t));
}
};
template<class Tuple, typename F> struct for_each_impl<0, Tuple, F> {
static void for_each(const Tuple& t, F f) {
f(get<0>(t));
}
};
template<class Tuple, typename F>
F for_each(const Tuple& t, F f) {
for_each_impl<tuple_size<Tuple>::value - 1, Tuple, F>::for_each(t, f);
return f;
}
}
函子:
struct call_tuple_item {
template<typename T>
void operator()(T a) {
std::cout << "call_tuple_item: " << a << std::endl;
}
};
主要功能:
std::tuple<float, const char*> t1(3.14, "helloworld");
std::for_each(t1, call_tuple_item());
您可以在这里查看我的答案以获取扩展元组的提示
如何将元组扩展为可变模板函数的参数?
请参阅下面我将使用的map(UnaryFunction, Tuple&&...)
实现,以及我一直在试图让它完全工作的代码(for_aux
, last
等)。
#include <array>
#include <iostream>
#include <tuple>
namespace detail {
struct static_ {
private:
static_() = delete;
static_(const static_&) = delete;
static_& operator=(const static_&) = delete;
};
template <unsigned... Args>
struct max;
template <unsigned Head, unsigned... Tail>
struct max<Head, Tail...>: private static_ {
static const unsigned value = Head > max<Tail...>::value
? Head
: max<Tail...>::value;
};
template <>
struct max<>: private static_ {
static const unsigned value = 0;
};
template <unsigned... Args>
struct min;
template <unsigned Head, unsigned... Tail>
struct min<Head, Tail...>: private static_ {
static const unsigned value = Head < min<Tail...>::value
? Head
: min<Tail...>::value;
};
template <>
struct min<>: private static_ {
static const unsigned value = 0;
};
template <typename... Args>
struct for_aux;
template <typename A, typename B>
struct for_aux<A, B>: private static_ {
static
void call(A&& a, B b) {
b(std::forward(a));
}
};
template <typename A, typename B, typename C>
struct for_aux<A, B, C>: private static_ {
static
void call(A&& a, B&& b, C c) {
c(std::forward(a), std::forward(b));
}
};
template <typename A, typename B, typename C, typename D>
struct for_aux<A, B, C, D>: private static_ {
static
void call(A&& a, B&& b, C&& c, D d) {
d(std::forward(a), std::forward(b), std::forward(c));
}
};
// template <typename Head, typename... Tail>
// struct for_aux: private static_ {
// static
// void call(Tail&&... x, Head f) {
// f(std::forward(x)...);
// }
// };
template <typename... Args>
struct last;
template <typename X>
struct last<X>: private static_ {
typedef X type;
};
template <typename Head, typename... Tail>
struct last<Head, Tail...>: private static_ {
typedef typename last<Tail...>::type type;
};
template <unsigned I,
unsigned N,
typename UnaryFunction,
typename... Tuples>
struct map;
template <unsigned N, typename UnaryFunction, typename... Tuples>
struct map<N, N, UnaryFunction, Tuples...>: private static_ {
static
void call(UnaryFunction, const Tuples&...) {}
};
template <unsigned I,
unsigned N,
typename UnaryFunction,
typename... Tuples>
struct map: private static_ {
static
void call(UnaryFunction f, Tuples&&... x) {
f(std::get<I>(std::forward<Tuples>(x))...);
map<I + 1,
N,
UnaryFunction,
Tuples...>::call(f, std::forward<Tuples>(x)...);
}
};
template <typename Tuple>
struct tuple_size: private static_ {
enum {
value = std::tuple_size<
typename std::remove_const<
typename std::remove_reference<Tuple>::type
>::type
>::value
};
};
}
template <typename UnaryFunction, typename... Tuples>
inline
void map(UnaryFunction f, Tuples&&... x) {
detail::map<0,
detail::max<
detail::tuple_size<Tuples>::value...
>::value,
UnaryFunction,
Tuples...
>::call(f, std::forward<Tuples>(x)...);
}
using namespace std;
struct f {
template <typename T, typename U>
void operator()(const T& i, const U& j) {
cout << i << " " << j << endl;
}
};
int main() {
const array<int, 2> x = {{2}};
const tuple<double, char> y(1.1, 'a');
map(f(), x, y);
}
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