完美的转发容器元素 [英] Perfect forwarding container element

问题描述

类似于此问题，但不是完美的转发成员对象，我想知道如何完善STL容器的转发元素，即类似于

Similar to this question, but instead of perfect forwarding member of an object, I would like to know how to perfect forwarding elements of an STL container, i.e. similar to

struct X {};
void f(X&);
void f(X&&);

template <typename Vector>
void g(Vector&& v, size_t i) {
  if (is_lvalue_reference<Vector>::value) {
    f(v[i]);
  } else {
    f(move(v[i]));
  }
}

推荐答案

namespace detail {
    template<class T, class U>
    using forwarded_type = std::conditional_t<std::is_lvalue_reference<T>::value,
                                              std::remove_reference_t<U>&, 
                                              std::remove_reference_t<U>&&>;
}
template<class T, class U>
detail::forwarded_type<T,U> forward_like(U&& u) {
    return std::forward<detail::forwarded_type<T,U>>(std::forward<U>(u));
}

template <typename Vector>
void g(Vector&& v, size_t i) {
  f(forward_like<Vector>(v[i]));
}

演示。在实现中使用 std :: forward 自动防止您将rvalue进行危险的左值化。

Demo. Using std::forward in the implementation automatically prevents you from doing a dangerous forward of rvalue as lvalue.

您的实际用例

我想从中创建 vector< T> vector ， vector< U2> ，....，其中
每个元素 T 由 U1，U2，... 构造。
vector< Ui> 的每个数组可以是& 或& ;& ，我希望 Ui 能够完美地转发
。

I'd like to create vector<T> from vector<U1>, vector<U2>, ...., where each element T is constructed from U1, U2, .... Each array of vector<Ui> could be either & or &&, and I'd like the Ui to be perfectly forwarded.

这就像

template<class T, class...Vectors>
std::vector<T> make_vector(Vectors&&...vectors){
    auto n = std::min({vectors.size()...});
    std::vector<T> ret; 
    ret.reserve(n);
    for(decltype(n) i = 0; i < n; ++i)
        ret.emplace_back(forward_like<Vectors>(vectors[i])...);
    return ret;
}

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