When to Use Braces in C Aggregate Initialization with Arrays?

Ambiguous Initializer Syntax for Aggregates Containing Arrays

In C , when initializing aggregates containing arrays, omitting curly braces can lead to confusion and errors. This is evident in the following examples:

// Error: Too many initializers
std::array<A, 2> a1 = {
    {0, 0.1},
    {2, 3.4}
};

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

Braces Required for std::array of Structures

The first example throws an error because std::array is an aggregate and lacks a user-defined constructor. Initialization of its internal array requires explicit braces, as seen in the corrected version:

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

Braces Not Required for std::array of PODs

In contrast, std::array in the second example doesn't require braces because double is a Plain Old Datatype (POD), and the array initialization is effectively handled by the default initializer.

Consistency for Aggregates

The principle of requiring braces for aggregate members applies to other types of aggregates as well:

// Valid
B meow1 = {1, 2};
B bark1 = {{1, 2}};

C meow2 = {1, 2};
C bark2 = {{1, 2}};

Ambiguity in D

However, the following example leads to an error:

// Error: Too many initializers
D meow3 = {{1, 2}, {3, 4}};
D bark3 = {{{1, 2}, {3, 4}}};

In D, the initializer for foo is itself an array. The braces in meow3 are ambiguous because they could refer to either the initialization of foo or its internal array. To resolve the ambiguity, explicit braces are required, as in bark3.

Mechanism for Initializing Aggregates

When braces are omitted in aggregate initialization, several rules apply:

1. If the initializer for a member aggregate begins with a brace, it is treated as a fully-braced initialization, and all members within the braces are initialized.
2. If the initializer does not begin with a brace, only enough initializers are taken from the list to initialize the members of the aggregate; any remaining initializers are used for the next member of the parent aggregate.

Additional Examples

• Completely-braced initialization: { {1, 3, 5}, {2, 4, 6}, {3, 5, 7} }
• Braces elided for inner aggregate: { 1, 3, 5, 2, 4, 6, 3, 5, 7 }
• Ambiguous syntax: { {1, 3, 5}, {2, 4, 6}, 3, 5, 7 } (neither fully-braced nor braces-elided)

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