why does creating a 2D array with C++ std::array require an extra pair of {} [duplicate]

Question

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}}
}};

Answer 1

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