C++11：正确的 std::array 初始化？
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C++11: Correct std::array initialization?
提问by Byzantian
If I initialize a std::array as follows, the compiler gives me a warning about missing braces
如果我按如下方式初始化 std::array，编译器会给我一个关于缺少大括号的警告
std::array<int, 4> a = {1, 2, 3, 4};
This fixes the problem:
这解决了这个问题：
std::array<int, 4> a = {{1, 2, 3, 4}};
This is the warning message:
这是警告消息：
missing braces around initializer for 'std::array<int, 4u>::value_type [4] {aka int [4]}' [-Wmissing-braces]
Is this just a bug in my version of gcc, or is it done intentionally? If so, why?
这只是我的 gcc 版本中的一个错误，还是故意完成的？如果是这样，为什么？
采纳答案by Pubby
This is the bare implementation of std::array:
这是的裸实现std::array：
template<typename T, std::size_t N>
struct array {
T __array_impl[N];
};
It's an aggregate struct whose only data member is a traditional array, such that the inner {}is used to initialize the inner array.
它是一个聚合结构，其唯一的数据成员是传统数组，因此内部{}用于初始化内部数组。
Brace elision is allowed in certain cases with aggregate initialization (but usually not recommended) and so only one brace can be used in this case. See here: C++ vector of arrays
在聚合初始化的某些情况下允许大括号省略（但通常不推荐），因此在这种情况下只能使用一个大括号。请参见此处：C++ 数组向量
回答by Draco Ater
According to cppreference. Double braces are required only if =is omitted.
根据cppreference。仅当=省略时才需要双大括号。
// construction uses aggregate initialization
std::array<int, 3> a1{ {1,2,3} }; // double-braces required
std::array<int, 3> a2 = {1, 2, 3}; // except after =
std::array<std::string, 2> a3 = { {std::string("a"), "b"} };
回答by Amit G.
Double-braces required in C++11 prior to the CWG 1270 (not needed in C++11 after the revision and in C++14 and beyond):
CWG 1270 之前的 C++11 中需要双大括号（修订后的 C++11 和 C++14 及更高版本中不需要）：
// construction uses aggregate initialization
std::array<int, 3> a1{ {1, 2, 3} }; // double-braces required in C++11 prior to the CWG 1270 revision
// (not needed in C++11 after the revision and in C++14 and beyond)
std::array<int, 3> a2 = {1, 2, 3}; // never required after =
