When Are Outer Braces Required in C Initializer Lists for `std::array` and Similar Aggregates?

When Outer Braces Cannot Be Omitted in an Initializer List

In C , outer braces are required for initializer lists when dealing with certain types of aggregate structures, specifically those that are Plain Old Datatypes (PODs) or lack user-defined constructors. Unlike most containers in the standard library, std::array falls into this category.

std::array Initialization with Braces

Consider the following example:

std::array<A, 2> a1 = 
{
    {0, 0.1},
    {2, 3.4}
};

where A is a struct with two data members. The extra set of braces enclosing {0, 0.1}, {2, 3.4} is essential because it initializes the internal array of std::array. Without them, the compiler will report "too many initializers" error.

Direct Array Initialization

This behavior is analogous to direct array initialization:

Aarray a1 =
{
    {0, 0.1},
    {2, 3.4}
};

Here, the internal array data of the Aarray struct is being initialized directly. Without the outer braces, the compiler would encounter the same error as with std::array.

Comparison with Scalar Types

When initializing arrays of scalar types like double, outer braces are optional because scalar types are not aggregates. For instance:

std::array<double, 2> a2 = {0.1, 2.3};

In this case, extra braces are not required since the data member of the array is already directly initialized by the initializer list {0.1, 2.3}.

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