为什么不能简单初始化(带大括号)2D std::array? [英] Why can't simple initialize (with braces) 2D std::array?

问题描述

<块引用>

可能的重复:
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):

 std::array,2>一个 {{1,2,3}，{4,5,6}};

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

为什么我不能使用这种语法?

我找到了一些变通方法，一个非常有趣的额外括号，但只是想知道为什么第一种最简单的方法无效?

解决方法:

//额外的大括号std::array,2>一个 {{{1,2,3}，{4,5,6}}};//显式转换std::array,2>一个 {std::array{1,2,3},std::array{4,5,6}};

[更新]

好的，感谢 KerrekSB 和评论，我明白了.所以看起来我的例子中大括号太少了，就像这个C例子一样:

struct B {整数数组[3];};结构体{B数组[2];};B b = {{1,2,3}};A a = {{{{1,2,3}},{{4,5,6}}}};

解决方案

std::array 是一个集合，包含一个 C 数组.要初始化它，您需要为类本身使用外大括号，为 C 数组使用内大括号:

std::arraya1 = { { 1, 2, 3 } };

将此逻辑应用于二维数组给出:

std::array, 2>a2 { { { {1, 2, 3} }, { { 4, 5, 6} } } };//^ ^ ^ ^ ^ ^//||||||//|+-|-+------------|-+//+-|-+-|------------+---- C++类大括号//||//+---+--- 成员 C 数组大括号

Possible Duplicate:
c++ why initializer_list behavior for std::vector and std::array are different

I defined simple 2D array (3X2):

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

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

Why can't I use this syntax?

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

Workarounds:

  // 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}
  };

[UPDATE]

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}}
}};

解决方案

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 } };

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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