Why Does `std::array` Require Double Curly Braces for Initialization, While `std::vector` Only Needs Single Curly Braces?

Initializer List Behavior Difference in std::vector and std::array

In C , the initializer_list behavior for std::vector and std::array differs significantly. Consider the following code snippets:

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

Why does std::array require double curly braces, while std::vector only requires single curly braces?

Explanation

std::vector has a user-defined constructor that takes a std::initializer_list as an argument. This allows for direct list initialization using single curly braces.

On the other hand, std::array is an aggregate type that does not have any user-defined constructors. Instead, it uses aggregate initialization, a feature inherited from C. Aggregate initialization allows for initialization using double curly braces.

The older syntax for aggregate initialization using braces and the = sign is also valid:

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

However, this syntax allows for brace elision in certain cases, which is not permitted for direct list initialization with std::array. A footnote in the C standard (C 11 §8.5.1/11) states that "Braces cannot be elided in other uses of list-initialization."

Defect Report and Proposed Resolution

A defect report (CWG defect #1270) has been filed to address this restriction. If the proposed resolution is adopted, brace elision will be allowed for std::array direct list initialization, making the following valid:

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

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