Why Do `std::vector` and `std::array` Require Different Initialization Syntaxes?

Why the Divergent Behavior of std::Vector and std::Array Initializer Lists?

When initializing objects of both std::vector and std::array data structures in C , a noticeable difference arises in their syntax. std::vectors can be initialized with braces, as in:

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

However, when it comes to std::arrays, double curly braces are required:

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

The reason for this discrepancy stems from the underlying nature of these data structures. std::arrays are aggregates—objects without user-defined constructors or even one explicitly taking a std::initializer_list.

Initialization with braces for std::arrays thus falls under aggregate initialization, inherited from C. In aggregate initialization, the "old style" using =: is employed:

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

This syntax allows for the elision of extra braces in "a declaration of the form T x = { a };". However, this rule does not extend to other forms of list initialization, including the one used in the second code snippet.

A CWG defect report (#1270) highlights this restriction. Should the proposed resolution be accepted, brace elision will be extended to include other list initialization forms, potentially making the following a valid syntax:

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

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