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If a class has non-default constructors, the order in which class members appear in the brace initializer is the order in which the corresponding parameters appear in the constructor, not the order in which the members are declared (as with class_a in the previous example).
std::array contains a built-in array, which can be initialized via an initializer list, which is what the inner set is. The outer set is for aggregate initialization.
Designated initialization is an extension of aggregate initialization and empowers you to directly initialize the members of a class type using their names. Designated initialization is a special case of aggregate initialization.
Brace elision applies, but not in C++11. In C++14, they will apply because of http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1270 . If you are lucky, Clang will backport that to their C++11 mode (let's hope they will!).
Relevant: http://en.cppreference.com/w/cpp/language/aggregate_initialization
In short,
struct S {
int x;
struct Foo {
int i;
int j;
int a[3];
} b;
};
S s1 = { 1, { 2, 3, {4, 5, 6} } };
S s2 = { 1, 2, 3, 4, 5, 6}; // same, but with brace elision
S s3{1, {2, 3, {4, 5, 6} } }; // same, using direct-list-initialization syntax
S s4{1, 2, 3, 4, 5, 6}; // error in C++11: brace-elision only allowed with equals sign
// okay in C++14
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