如何从对象std :: tuple获取对元组尾部的引用<Head,Tail…>
[英]How to get a reference to the tail of a tuple from an object std::tuple<Head,Tail…>
问题描述
I have defined: 
我定义了：
template<class Head,class ...Tail>
struct elem{
std::tuple<Head,Tail...> dm;
};
I have the functions head(), tail() and others for the class elem, but they create copies, they return a copy of the head of tuple or the tail of tuple, but I need a reference to the tail of this->dm. 我有elem类的函数head（），tail（）和其他函数，但是它们创建了副本，它们返回元组的头部或元组的尾部的副本，但是我需要引用this->的尾部DM。
For the head is easy, std::get<0>(this->dm) give me the reference. 因为头很容易，所以std::get<0>(this->dm)给我参考。
Is it possible for the tail? 尾巴有可能吗？ By tail, I mean all the elements after the first one. 尾巴，是指第一个元素之后的所有元素。
1 个解决方案
解决方案1
2 已采纳 2019-03-16 13:01:40
Such a thing would only be possible if a tuple<A, B, C> were stored internally as something equivalent to a cons cell: 仅当tuple<A, B, C>在内部存储为等同于cons单元格的东西时tuple<A, B, C>此类事情才有可能：
struct __tuple_A_B_C {
A car;
tuple<B, C> cdr;
A& head() { return car; }
tuple<B, C>& tail() { return cdr; }
};
But that isn't the case - all you know is that you have subobjects of types A , B , and C . 但这不是事实-您所知道的是您具有类型A ， B和C子对象。 Their layout is completely unspecified - and you definitely don't know whether or not implementations use recursion like this to implement tuple . 它们的布局是完全不确定的-并且您绝对不知道实现是否使用像这样的递归来实现tuple 。 They're allowed to, but I'm not sure any do. 他们被允许，但我不确定有什么办法。
The best you could do is, given a tuple<A, B, C> return a tuple<B&, C&> . 最好的办法是给定一个tuple<A, B, C>返回一个tuple<B&, C&> 。 In C++17, that's not so bad to implement: 在C ++ 17中，实现起来并不难：
template<class Head,class ...Tail>
struct elem{
std::tuple<Head,Tail...> dm;
auto tail() {
return std::apply([](auto&, auto&... rest){
return std::tie(rest...);
}, dm);
}
};
But if you really want this recursive cons-cell-like approach, you're probably better off actually implementing your own recursion so you get the desired behavior: 但是，如果您真的想要这种类似于cons-cell的递归方法，那么最好实际实现自己的递归，这样您就可以得到所需的行为：
template <class Head, class... Tail>
struct elem {
Head head;
elem<Tail...> tail;
};
template <class Head>
struct elem<Head> {
Head head;
};
Depends on what you're actually doing. 取决于您的实际操作。
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