我正在尝试使用模板参数包的累积产品初始化静态和常量数组:
template <int ...D>
class Foo
{
static const std::array<const Size, sizeof...(D)> _array;
};
template <int...D> const std::array<const int, sizeof...(D)> Foo<D...>::_array =
{ cumulative_product<D...>() };
如何编写函数cumulative_product&lt;&gt;(),以便将D ...转换为D ...的累积积? E.g。
Foo<1,2,3,4>::_array;// = {1,1*2,1*2*3,1*2*3*4} = {1,2,6,24}.
解决方案:非常感谢@bogdan提供的优秀C ++ 14解决方案,以及对C ++ 11解决方案的改进。
#include <array>
#include <iostream>
#define CPP_VERSION 11
#if CPP_VERSION >= 14
// Credit: @bogdan at http://stackoverflow.com/q/37373602/6367128
template<int... Args> constexpr std::array<int, sizeof...(Args)> cumulative_product(int seed = 1) { return{ { seed *= Args ... } }; }
#elif CPP_VERSION == 11
// Acknowledgement: Huge thank you to @bogdan for making the code more portable, concise and readable!
namespace details
{
template<int N, int i, int j, int ...Args> struct c_product_gen // N counts down to zero
{
static constexpr std::array<int, sizeof...(Args)+1> get() { return c_product_gen<N - 1, i*j, Args..., i*j>::get(); }
};
template<int i, int j, int ...Args> struct c_product_gen<0, i, j, Args...> // The end point of the template recursion
{
static constexpr std::array<int, sizeof...(Args)+1> get() { return { { j, Args... } }; }
};
}
template<int... Args> constexpr std::array<int, sizeof...(Args)> cumulative_product() { return details::c_product_gen<sizeof...(Args), 1, Args...>::get(); }
#else // CPP_VERSION < 11
template<int... Args> constexpr std::array<int, sizeof...(Args)> cumulative_product()
{
static_assert(false, "C++ version 11 or greater is required.");
return std::array<int, sizeof...(Args)>();
}
#endif
int main()
{
constexpr auto a = cumulative_product<1,2,3,4,5>();
for(auto i : a) std::cout << i << ' '; // Output: 1 2 6 24 120
std::cout << '\n';
}
答案 0 :(得分:3)
#include <array>
#include <utility>
#include <cstddef>
template <int... D, std::size_t... Is>
constexpr std::array<int, sizeof...(D)> cumulative_product(std::index_sequence<Is...>)
{
static_assert(sizeof...(D), "Missing initializers");
int a[]{ D... };
for (int i = 1; i < int(sizeof...(D)); ++i)
{
a[i] *= a[i-1];
}
return {{ a[Is]... }};
}
template <int... D>
constexpr auto cumulative_product()
{
return cumulative_product<D...>(std::make_index_sequence<sizeof...(D)>{});
}
template <int... D>
struct Foo
{
static constexpr std::array<int, sizeof...(D)> _array = cumulative_product<D...>();
};