如何生成可变参数包? [英] How do I generate a variadic parameter pack?
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问题描述
给定一个不相关的输入,是否可以生成一个非类型的参数包?我的意思是,我想转换这个:
Is it possible to generate a non-type parameter pack given an unrelated input? What I mean is, I would like like to turn this:
template <typename D, size_t... Offsets> struct VecGeneric;
template <typename N, size_t C> struct VecGenericData;
template <typename N, size_t D> struct TVecN;
template <typename N> struct TVecN<N,2> : public VecGeneric<VecGenericData<N,2>,0,1> { };
template <typename N> struct TVecN<N,3> : public VecGeneric<VecGenericData<N,3>,0,1,2> { };
template <typename N> struct TVecN<N,4> : public VecGeneric<VecGenericData<N,4>,0,1,2,3> { };
template <typename N> struct TVecN<N,5> : public VecGeneric<VecGenericData<N,5>,0,1,2,3,4> { };
template <typename N> struct TVecN<N,6> : public VecGeneric<VecGenericData<N,6>,0,1,2,3,4,5> { };
template <typename N> struct TVecN<N,7> : public VecGeneric<VecGenericData<N,7>,0,1,2,3,4,5,6> { };
// ...
如下所示:
template <typename D, size_t... Offsets> struct VecGeneric;
template <typename N, size_t C> struct VecGenericData;
template <typename N, size_t D> struct TVecN : public VecGeneric<VecGenericData<N,D>,IntPack<D>...> { };
推荐答案
构建整数序列 0 ,1,2,...,N-1
给定 N
通常递归地完成;例如:
Constructing the integer sequence 0,1,2,...,N-1
given N
is typically done recursively; for example:
template<size_t... Is> struct index_sequence {};
namespace detail
{
template<size_t N, size_t... Is>
struct make_index_sequence_h
: make_index_sequence_h<N-1, N-1, Is...>
{};
template<size_t... Is>
struct make_index_sequence_h<0, Is...>
{
using type = index_sequence<Is...>;
};
}
template<size_t N>
using make_integer_sequence = typename detail::make_integer_sequence_h<N>::type;
(Àla C ++ 1y StdLib)
(À la C++1y StdLib)
要使用这些整数,您可以从 index_sequence
类型中推导出这些整数:
To use these integers, you deduce them from the index_sequence
type:
namespace detail
{
template<typename N, class IndexSequence>
struct helper;
template<typename N, size_t... Is>
struct helper<N, index_sequence<Is...>> // deduce the integers
{
using type = VecGeneric<VecGenericData<N, sizeof...(Is)>, Is...>;
};
}
上面是一个元函数,它将计算从。
使用别名模板简化用法:
The above is a metafunction that will "compute" the type you want to derive from. Simplify the usage using an alias template:
namespace detail
{
template<typename N, size_t D>
using helper_t = typename helper<N, make_index_sequence<D>>::type;
}
结果:
template <typename D, size_t... Offsets> struct VecGeneric
{
// just a quick test:
void print()
{
std::cout << __PRETTY_FUNCTION__ << "\n";
}
};
template <typename N, size_t C> struct VecGenericData {};
template <typename N, size_t D> struct TVecN : public detail::helper_t<N, D> {};
int main()
{
TVecN<int, 5>().print();
}
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