Why does std::array require double curly braces for initializer lists while std::vector doesn't?

C Initializer_List Behavior Discrepancy for std::vector and std::array

When using initializer lists for C containers, a perplexing difference arises between std::vector and std::array. Let's explore the reasons behind this behavior.

Problem:

Consider the following code:

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

Why is it necessary to use double curly braces for std::array but not for std::vector?

Answer:

The behavior stems from the nature of std::array as an aggregate. Aggregates do not have user-declared constructors, including ones that accept initializer lists. Consequently, aggregate initialization for std::array is performed through "old style" initialization using the = syntax:

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

In this old style, extra braces may be omitted, resulting in the equivalent code:

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

However, this brace elision is only allowed when using the old style initialization with the = syntax. Direct list initialization, which does not use the = syntax, does not allow brace elision. This limitation is governed by C 11 §8.5.1/11.

Proposed Resolution:

A defect report (CWG defect #1270) has been raised to address this limitation. If the proposed resolution is adopted, brace elision will be allowed for all forms of list initialization, including the following:

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

This change would bring consistency to the behavior of std::vector and std::array when using initializer lists.

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