Why is the C++ initializer_list behavior for std::vector and std::array different?

Question

Code:

std::vector<int> x{1,2,3,4}; std::array<int, 4> y{{1,2,3,4}}; 

Why do I need double curly braces for std::array?

Answer 1

std::array<T, N> is an aggregate: it doesn't have any user-declared constructors, not even one taking a std::initializer_list. Initialization using braces is performed using aggregate initialization, a feature of C++ that was inherited from C.

The "old style" of aggregate initialization uses the =:

std::array<int, 4> y = { { 1, 2, 3, 4 } }; 

With this old style of aggregate initialization, extra braces may be elided, so this is equivalent to:

std::array<int, 4> y = { 1, 2, 3, 4 }; 

However, these extra braces may only be elided "in a declaration of the form T x = { a };" (C++11 §8.5.1/11), that is, when the old style = is used . This rule allowing brace elision does not apply for direct list initialization. A footnote here reads: "Braces cannot be elided in other uses of list-initialization."

There is a defect report concerning this restriction: CWG defect #1270. If the proposed resolution is adopted, brace elision will be allowed for other forms of list initialization, and the following will be well-formed:

std::array<int, 4> y{ 1, 2, 3, 4 }; 

(Hat tip to Ville Voutilainen for finding the defect report.)