Why do `std::vector` and `std::array` use different syntax for initializer lists?

Understanding the Different Initializer_list Behavior for std::vector and std::array

When using initializer lists to initialize containers, one may have noticed a difference in syntax between std::vector and std::array. While std::vector initializes using single curly braces {}, std::array requires double curly braces {{}}. To understand why this distinction exists, we delve into the fundamental properties of these containers.

std::array: An Aggregate

Unlike std::vector, which has user-defined constructors, std::array is considered an aggregate type. This means it lacks user-declared constructors, including those that accept initializer lists. The initialization of std::array, therefore, relies on aggregate initialization, a C feature inherited from C.

Aggregate Initialization in C

In C , aggregate initialization can be performed in two ways:

1. Old Style: Utilizing the =: syntax, e.g., std::array y = {{1, 2, 3, 4}};
2. Direct List Initialization: Directly passing an initializer list, e.g., std::array y{1, 2, 3, 4};

With the old style, extra braces could be elided when initializing within a declaration. However, this option is not available for direct list initialization.

A Forthcoming Change

CWG defect #1270 seeks to address this restriction, allowing brace elision in other forms of list initialization. If approved, the following syntax would become valid:

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

This change would eliminate the current disparity in syntax between std::vector and std::array, providing consistency in aggregate initialization practices.

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