std :: forward_as_tuple将参数传递给2个构造函数 [英] std::forward_as_tuple to pass arguments to 2 constructors

问题描述

我想传递多个参数以在函数内构造两个对象，方法与 std :: pair< T1，T2>（std :: piecewise_construct，...）有效。

I would like to pass multiple arguments in order to construct two objects inside a function, the same way std::pair<T1, T2>(std::piecewise_construct, ...) works.

所以我写了

template <typename Args0..., typename Args1...>
void f(std::tuple<Arg0> args0, std::tuple<Args1> args1) {
   Object0 alpha(...);
   Object1 beta(...);
   ...
}

所以我可以打电话

f(std::forward_as_tuple(..., ..., ...), std::forward_as_tuple(..., ...))

但是我不知道如何构造 Object0 和 Object1 。我已经检查了标准库的 std :: pair 的源代码，它们似乎使用复杂的内部函数来获取args0和args1的索引。您是否知道如何执行此操作？

But I don't know how to construct Object0 and Object1. I have checked the source code of my standard library for std::pair and they seems to use complicated internal functions to get the indices of args0 and args1. Do you have any idea on how to do that?

推荐答案

C ++ 17将具有 make_from_tuple 进行此操作，但是您可以使用C ++ 11编写。这是从 cppreference （对于C ++ 11）中盗取的C ++ 14版本您可以使用std :: index_sequence 的实现>实施C ++ 14 make_integer_sequence ）。

C++17 will have make_from_tuple for doing this, but you can write this in C++11. Here's a C++14 version stolen from cppreference (for C++11 you can use the implementation of std::index_sequence from Implementation C++14 make_integer_sequence).

namespace detail {
template <class T, class Tuple, std::size_t... I>
constexpr T make_from_tuple_impl( Tuple&& t, std::index_sequence<I...> )
{
  return T(std::get<I>(std::forward<Tuple>(t))...);
}
} // namespace detail

template <class T, class Tuple>
constexpr T make_from_tuple( Tuple&& t )
{
    return detail::make_from_tuple_impl<T>(std::forward<Tuple>(t),
        std::make_index_sequence<std::tuple_size<std::decay_t<Tuple>>::value>{});
}

使用此实用程序，实现 f 轻而易举：

With this utility, the implementation of f is a breeze:

template <typename... Args0, typename... Args1>
void f(std::tuple<Args0...> args0, std::tuple<Args1...> args1) {
   auto alpha = make_from_tuple<Object0>(args0);
   auto beta = make_from_tuple<Object1>(args1);
}

为了使其更通用，我建议您仅推导那些类型元组并对其进行完美转发：

To make it more generic, I'd recommend just deducing the types of those tuples and perfect-forwarding them:

template <typename T0, typename T1>
void f(T0&& args0, T1&& args1) {
   auto alpha = make_from_tuple<Object0>(std::forward<T0>(args0));
   auto beta = make_from_tuple<Object1>(std::forward<T1>(args1));
}

实时C ++ 11演示

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