得到std :: tuple的一部分 [英] get part of std::tuple
本文介绍了得到std :: tuple的一部分的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我有一个未知大小的元组(它是方法的模板参数)
I have a tuple of unknown size (it's template parametr of method)
是它的一部分(我需要丢弃它的第一个元素)
Is it way to get part of it (I need throw away first element of it)
例如,我有 tuple< int,int,int>(7,12,42)
。我想要 tuple
For example, I have tuple<int,int,int>(7,12,42)
. I want tuple<int,int>(12,42)
here
推荐答案
p>在编译时整数列表的帮助下:
With help of a compile-time integer list:
#include <cstdlib>
template <size_t... n>
struct ct_integers_list {
template <size_t m>
struct push_back
{
typedef ct_integers_list<n..., m> type;
};
};
template <size_t max>
struct ct_iota_1
{
typedef typename ct_iota_1<max-1>::type::template push_back<max>::type type;
};
template <>
struct ct_iota_1<0>
{
typedef ct_integers_list<> type;
};
我们可以通过参数包扩展来构造尾部:
We could construct the tail simply by parameter-pack expansion:
#include <tuple>
template <size_t... indices, typename Tuple>
auto tuple_subset(const Tuple& tpl, ct_integers_list<indices...>)
-> decltype(std::make_tuple(std::get<indices>(tpl)...))
{
return std::make_tuple(std::get<indices>(tpl)...);
// this means:
// make_tuple(get<indices[0]>(tpl), get<indices[1]>(tpl), ...)
}
template <typename Head, typename... Tail>
std::tuple<Tail...> tuple_tail(const std::tuple<Head, Tail...>& tpl)
{
return tuple_subset(tpl, typename ct_iota_1<sizeof...(Tail)>::type());
// this means:
// tuple_subset<1, 2, 3, ..., sizeof...(Tail)-1>(tpl, ..)
}
用法:
#include <cstdio>
int main()
{
auto a = std::make_tuple(1, "hello", 7.9);
auto b = tuple_tail(a);
const char* s = nullptr;
double d = 0.0;
std::tie(s, d) = b;
printf("%s %g\n", s, d);
// prints: hello 7.9
return 0;
}
(在ideone:http://ideone.com/Tzv7v ;该代码适用于g ++ 4.5到4.7和clang ++ 3.0)
(On ideone: http://ideone.com/Tzv7v; the code works in g++ 4.5 to 4.7 and clang++ 3.0)
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