为什么 std::vector 和 std::array 的 C++ initializer_list 行为不同?
代码:
std::vector<int> x{1,2,3,4};
std::array<int, 4> y{{1,2,3,4}};
为什么 std::array 需要双花括号?
Why do I need double curly braces for std::array?
推荐答案
std::array 是一个聚合:它没有任何用户声明的构造函数，甚至没有一个采用 std::initializer_list.使用大括号进行初始化是使用聚合初始化来执行的，这是从 C 继承的 C++ 特性.
std::array<T, N> is an aggregate: it doesn't have any user-declared constructors, not even one taking a std::initializer_list. Initialization using braces is performed using aggregate initialization, a feature of C++ that was inherited from C.
聚合初始化的旧风格"使用=:
The "old style" of aggregate initialization uses the =:
std::array<int, 4> y = { { 1, 2, 3, 4 } };
使用这种老式的聚合初始化，额外的大括号可能会被省略，所以这相当于:
With this old style of aggregate initialization, extra braces may be elided, so this is equivalent to:
std::array<int, 4> y = { 1, 2, 3, 4 };
然而，这些额外的大括号只能在T x = { a };"形式的声明中被省略(C++11 §8.5.1/11)，也就是说，当使用旧样式 = 时.这条允许大括号省略的规则不适用于直接列表初始化.这里的脚注是:在列表初始化的其他用途中不能省略大括号."
However, these extra braces may only be elided "in a declaration of the form T x = { a };" (C++11 §8.5.1/11), that is, when the old style = is used . This rule allowing brace elision does not apply for direct list initialization. A footnote here reads: "Braces cannot be elided in other uses of list-initialization."
有关于此限制的缺陷报告:CWG 缺陷 #1270.如果提议的决议获得通过，则其他形式的列表初始化将允许省略大括号，以下内容将是良构的:
There is a defect report concerning this restriction: CWG defect #1270. If the proposed resolution is adopted, brace elision will be allowed for other forms of list initialization, and the following will be well-formed:
std::array<int, 4> y{ 1, 2, 3, 4 };
(感谢 Ville Voutilainen 找到缺陷报告.)
(Hat tip to Ville Voutilainen for finding the defect report.)
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