我想声明一个具有 N 个参数的 lambda 函数,其中 N 是模板参数。类似...
template <int N>
class A {
std::function<void (double, ..., double)> func;
// exactly n inputs
};
我想不出用元函数范例来做到这一点的方法。
您可以使用嵌套的 typedef
n_ary_function
编写模板 type
。该类型可以如下使用:
template <int N>
class A {
typename n_ary_function<N, double>::type func;
};
以下代码片段包含
n_ary_function
的定义:
template <std::size_t N, typename Type, typename ...Types>
struct n_ary_function
{
using type = typename n_ary_function<N - 1, Type, Type, Types...>::type;
};
template <typename Type, typename ...Types>
struct n_ary_function<0, Type, Types...>
{
using type = std::function<void(Types...)>;
};
元
template
采用模板、计数和类型,并使用类型的N
副本调用模板:
template<template<class...>class target, unsigned N, class T, class... Ts>
struct repeat_type_N: repeat_type_N<target, N-1, T, T, Ts...> {};
template<template<class...>class target, class T, class... Ts>
struct repeat_type_N<target, 0, T, Ts...> {
typedef target<Ts...> type;
};
template<template<class...>class target, unsigned N, class T>
using repeat_type_N_times = typename repeat_type_N<target, N, T>::type;
现在,我们使用它:
template<typename... Ts> using operation=void(Ts...);
template<unsigned N, class T> using N_ary_op = repeat_type_N_times< operation, N, T >;
template<unsigned N> using N_double_func = N_ary_op<N,double>;
我们测试一下:
void three_doubles(double, double, double) {}
int main() {
N_double_func<3>* ptr = three_doubles;
std::function< N_double_func<3> > f = three_doubles;
}
并获胜。
您到底使用
double, double, double
做什么完全取决于您在上述系统中的情况。例如,您可以使用 lambda 来初始化 std::function
。
您可以将
double, double, double
打包到 template<class...>struct type_list{};
中,这样您就可以将其作为一个参数传递给另一个 template
,然后专门对其进行解包。
A
repeat_type
对于大 N
具有较少的递归:
// package for types. The typedef saves characters later, and is a common pattern in my packages:
template<class...>struct types{typedef types type;};
// Takes a target and a `types`, and applies it. Note that the base has no implementation
// which leads to errors if you pass a non-`types<>` as the second argument:
template<template<class...>class target, class types> struct apply_types;
template<template<class...>class target, class... Ts>
struct apply_types<target, types<Ts...>>{
typedef target<Ts...> type;
};
// alias boilerplate:
template<template<class...>class target, class types>
using apply_types_t=typename apply_types<target,types>::type;
// divide and conquer, recursively:
template<unsigned N, class T, class Types=types<>> struct make_types:make_types<
(N+1)/2, T, typename make_types<N/2, T, Types>::type
> {};
// terminate recursion at 0 and 1:
template<class T, class... Types> struct make_types<1, T, types<Types...>>:types<T,Types...> {};
template<class T, class Types> struct make_types<0, T, Types>:Types{};
// alias boilerplate:
template<unsigned N, class T>
using make_types_t=typename make_types<N,T>::type;
// all of the above reduces `repeat_type_N_t` to a one-liner:
template<template<class...>class target, unsigned N, class T>
using repeat_type_N_times = apply_types_t<target, make_types_t<N,T>>;
对于大型
N
,上述可以显着减少编译时间,并处理template
堆栈溢出。
您不能直接执行此操作。
你可以做这样的事情
template <unsigned N> class UniformTuple;
template <>
class UniformTuple <0>
{
};
template <unsigned N>
class UniformTuple : public UniformTuple <N-1>
{
public:
template <typename... Args>
UniformTuple (double arg, Args... args)
: UniformTuple <N-1> (args...)
, m_value (arg)
{
}
private:
double m_value;
};
template <int N>
class A
{
std :: function <void (const UniformTuple <N> &)> func;
};
为了完整起见,这是一个不使用递归的解决方案:
template <class Ret, class Arg, class Idx>
struct n_ary_function_;
template <class Ret, class Arg, std::size_t... Idx>
struct n_ary_function_<Ret, Arg, std::index_sequence<Idx...>> {
template <class T, std::size_t>
using id = T;
using type = std::function<Ret(id<Arg, Idx>...)>;
};
template <class Ret, class Arg, std::size_t N>
using n_ary_function = typename n_ary_function_<
Ret, Arg, std::make_index_sequence<N>
>::type;
NoSid 的解决方案非常有创意(本质上是一一“追加”类型)。我编写的另一个解决方案可能需要更少的脑力劳动,如下所示使用
std::index_sequence
(这是创建未知大小的参数包的一种非常自然的方法):
#include <utility>
#include <functional>
#include <type_traits>
template<typename T, long U>
struct reduce
{
using type = T;
};
template<typename U, typename IndexSequence>
struct FunctionHolderImpl;
template<typename U, long ... Indices>
struct FunctionHolderImpl<U, std::index_sequence<Indices...>>
{
using value = std::function<void(typename reduce<U, Indices>::type...)>;
};
template<long N>
struct FunctionHolder
{
using func = FunctionHolderImpl<double, std::make_index_sequence<N>>::value;
};