为什么不能简单地初始化(带大括号)2D std::array?

2022-01-18 00:00:00 initialization c++ c++11 stl

可能重复:
c++ 为什么 std::vector 的 initializer_list 行为和std::array 不同

我定义了简单的二维数组(3X2):

I defined simple 2D array (3X2):

  std::array<std::array<int,3>,2> a {
    {1,2,3},
    {4,5,6}
  };

我很惊讶这个初始化不起作用，出现 gcc4.5 错误:too many initializers for 'std::array<std::array<int, 3u>, 2u>'

I was surprised this initialization does not work, with gcc4.5 error: too many initializers for 'std::array<std::array<int, 3u>, 2u>'

为什么我不能使用这种语法?

Why can't I use this syntax?

我找到了解决方法，一个非常有趣的额外大括号，但只是想知道为什么第一个最简单的方法无效?

I found workarounds, one very funny with extra braces, but just wonder why the first, easiest approach is not valid?

解决方法:

  // EXTRA BRACES
  std::array<std::array<int,3>,2> a {{
    {1,2,3},
    {4,5,6}
  }};

  // EXPLICIT CASTING
  std::array<std::array<int,3>,2> a {
    std::array<int,3>{1,2,3},
    std::array<int,3>{4,5,6}
  };

[更新]

好的，感谢 KerrekSB 和评论，我明白了.因此，我的示例中的大括号似乎太少了，就像在这个 C 示例中一样:

Ok, thanks to KerrekSB and comments I get the difference. So it seems that there is too little braces in my example, like in this C example:

struct B {
  int array[3];
};
struct A {
  B array[2];
};

B b = {{1,2,3}};
A a = {{
     {{1,2,3}},
     {{4,5,6}}
}};

推荐答案

std::array 是一个包含 C 数组的聚合.要初始化它，你需要类本身的外大括号和 C 数组的内大括号:

std::array<T, N> is an aggregate that contains a C array. To initialize it, you need outer braces for the class itself and inner braces for the C array:

std::array<int, 3> a1 = { { 1, 2, 3 } };

将此逻辑应用于二维数组会得到:

Applying this logic to a 2D array gives this:

std::array<std::array<int, 3>, 2> a2 { { { {1, 2, 3} }, { { 4, 5, 6} } } };
//                                   ^ ^ ^ ^            ^ ^
//                                   | | | |            | |
//                                   | +-|-+------------|-+
//                                   +-|-+-|------------+---- C++ class braces
//                                     |   |
//                                     +---+--- member C array braces

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