initializer_list和move语义 [英] initializer_list and move semantics

问题描述

我允许将元素移出 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> 需要特殊的编译器注意，并且没有像C ++标准库的正常容器那样的值语义，我宁愿安全而不是问。

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，这将无法正常工作;你仍然会得到副本。我对此感到非常惊讶，因为我认为 initializer_list 存在以保持临时数组，直到 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.

开始和结束 initializer_list return const T * ，因此 move 在你的代码中是 T const&&& - 一个不可变的右值引用。这样的表达式不能有意义地移动。它将绑定到 T const& 类型的函数参数，因为rvalues绑定到const lvalue引用，你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.

这可能是因为编译器可以选择使 initializer_list 一个静态初始化的常量，但似乎它会更清洁在编译器的判断下使其类型 initializer_list 或 const initializer_list ，因此用户不知道是否期望一个开始和结束的可变结果。

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 支持的移动类型。这只是一个初稿，它没有在任何地方实现，但你可以看到它的更多的分析的问题。

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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