initializer_list 和移动语义 [英] initializer_list and move semantics

问题描述

我可以将元素移出 std::initializer_list 吗?

Am I allowed to move elements out of a std::initializer_list<T>?

#include <initializer_list>
#include <utility>

template<typename T>
void foo(std::initializer_list<T> list)
{
    for (auto it = list.begin(); it != list.end(); ++it)
    {
        bar(std::move(*it));   // kosher?
    }
}

由于 std::intializer_list<T> 需要特别的编译器注意并且不像 C++ 标准库的普通容器那样具有值语义，我宁愿安全而不是抱歉和询问.>

Since std::intializer_list<T> requires special compiler attention and does not have value semantics like normal containers of the C++ standard library, I'd rather be safe than sorry and ask.

推荐答案

不，这不会按预期工作；你仍然会得到副本.我对此感到非常惊讶，因为我认为 initializer_list 的存在是为了保留一个临时数组，直到它们被 move 'd.

No, that won't work as intended; you will still get copies. I'm pretty surprised by this, as I'd thought that initializer_list existed to keep an array of temporaries until they were move'd.

begin 和 end for initializer_list 返回 const T *，所以 move 在您的代码中是 T const && — 一个不可变的右值引用.这样的表达不能有意义地移开.它将绑定到 T const & 类型的函数参数，因为右值确实绑定到 const 左值引用，并且您仍然会看到复制语义.

begin and end for initializer_list return const T *, so the result of move in your code is T const && — an immutable rvalue reference. Such an expression can't meaningfully be moved from. It will bind to an function parameter of type T const & because rvalues do bind to const lvalue references, and you will still see copy semantics.

可能的原因是编译器可以选择使 initializer_list 成为静态初始化的常量，但将其类型设为 initializer_list 似乎更清晰或 const initializer_list 由编译器自行决定，因此用户不知道是否期望 const 或 begin 和 的可变结果结束.但这只是我的直觉，可能有一个很好的理由我错了.

Probably the reason for this is so the compiler can elect to make the initializer_list a statically-initialized constant, but it seems it would be cleaner to make its type initializer_list or const initializer_list at the compiler's discretion, so the user doesn't know whether to expect a const or mutable result from begin and end. But that's just my gut feeling, probably there's a good reason I'm wrong.

更新:我写了 ISO 提案，用于initializer_list 支持仅移动类型.这只是一个初稿，还没有在任何地方实施，但您可以查看它以进一步分析问题.

Update: I've written an ISO proposal for initializer_list support of move-only types. It's only a first draft, and it's not implemented anywhere yet, but you can see it for more analysis of the problem.

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