获取可变参数模板的所有最后参数 [英] Obtain all-but-last parameter of variadic template

问题描述

我有一个这样声明的ctor:

I have a ctor declared like this:

template<typename... Coords>
MyClass<T>(vector<T> values, Coords... coords) { /* code */ }

我希望它看起来像这样:

I want it to look like this:

template<typename... Coords>
MyClass<T>(Coords... coords, vector<T> values) { /* code */ }

，但标准要求可变参数为最后一个参数.如果我写类似

but the standard requires the variadic argument to be the last. If I write something like

template<typename... Args>
MyClass<T>(Args... coordsThenValues) { /* code */ }

我如何将 coordsThenValues 拆分为最后一个参数包 Coords ... coords 和最后一个参数 vector< T>值?

how would I split coordsThenValues into an all-but-last parameter pack Coords... coords and the last parameter vector<T> values?

推荐答案

您喜欢元组吗?

你喜欢像元组一样转发吗?

How do you like forward as tuple?

struct foo {
  template<class...Ts>
  foo(Ts&&...ts):
    foo(
      magic<0>{}, // sent it to the right ctor
      std::index_sequence< sizeof...(ts)-1 >{}, // the last shall be first
      std::make_index_sequence<sizeof...(ts)-1>{}, // the first shall be last
      std::forward_as_tuple(std::forward<Ts>(ts)...) // bundled args
    )
  {}
private:
  template<size_t>
  struct magic {};
  template<size_t...I0s, size_t...I1s, class...Ts>
  foo(
    magic<0>, // tag
    std::index_sequence<I0s...>, // first args
    std::index_sequence<I1s...>, // last args
    std::tuple<Ts&&...> args // all args
  ):
    foo(
      magic<1>{}, // dispatch to another tagged ctor
      std::get<I0s>(std::move(args))..., // get first args
      std::get<I1s>(std::move(args))... // and last args
    )
  {}
  // this ctor gets the args in an easier to understand order:
  template<class...Coords>
  foo(magic<1>, std::vector<T> values, Coords...coords) {
  }
};

在这里，公共ctor将参数打包成一个引用元组(可能是l，可能是r).它还获得了两组索引.

here the public ctor packs up the arguments into a tuple of references (possibly l, possibly r). It also gets two sets of indexes.

然后将其发送到 magic< 0> ctor，后者将参数变乱，以便最后一个在前(假设索引正确).

It then sends it to magic<0> ctor, which shuffles the arguments around so that the last one is first (assuming the indexes are right).

magic< 1> ctor首先获取矢量，然后获取坐标.

The magic<1> ctor gets the vector first, then the coords.

基本上，我将参数打乱了，所以最后一个变成了第一个.

Basically I shuffled the arguments around so the last one became first.

魔术的存在是为了让我不必过多考虑过载解析，并且使我要转发给哪个ctor明确.如果没有它，它可能会起作用，但是当我对ctor转发进行疯狂的操作时，我喜欢添加标签.

magic just exists to make me not have to think much about overload resolution, and make which ctor I'm forwarding to explicit. It would probably work without it, but I like tagging when I'm doing something crazy with ctor forwarding.

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