Why Are Extra Braces Sometimes Necessary in C Initializer Lists?

When Extra Braces Are Required in an Initializer List

In C , initializer lists provide a convenient way to initialize an aggregate object or an array. However, when working with certain data structures like std::array and aggregate structures, the syntax requires extra braces, leaving many programmers wondering why.

The Need for Extra Braces in std::array

std::array is a fixed-size container that is declared as an aggregate data type, meaning it doesn't have a user-defined constructor. As an aggregate, std::array directly initializes its members, including the internal array.

To initialize the internal array directly, extra braces are required. Consider the following example:

std::array<int, 2> a1 = {{0, 1}, {2, 3}};

Here, the braces around {0, 1} and {2, 3} are essential because they indicate that the initialization is for the internal array within a1. Without these braces, the code would produce a compilation error due to too many initializers.

Lack of Extra Braces for Basic Types

In contrast to std::array, basic types like double do not require extra braces in initializer lists. This is because they are not aggregates. For example:

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

In this case, a2 is an array of double values. Since double is not an aggregate, the initialization doesn't involve any internal array, and the extra braces are not necessary.

Other Examples

The requirement for extra braces extends beyond std::array. Other types involving aggregates also require them. For instance:

struct B { int foo[2]; };
B meow1 = {1, 2};   // OK (fully-braced)
B bark1 = {{1, 2}}; // OK (extra braces for inner aggregate)

struct C { int a, b; };
C meow2 = {1, 2};    // OK (completely elided braces)
C bark2 = {{1, 2}}; // OK (extra braces for inner aggregate)

struct D { int foo[2]; };
D meow3 = {{1, 2}, {3, 4}};    // error (neither braced-elided nor fully-braced)
D bark3 = {{{1, 2}, {3, 4}}}; // OK (extra braces for inner aggregate)

Conclusion

To summarize, extra braces are required in initializer lists when initializing aggregates like std::array and certain structures because they denote the initialization of the internal array or the aggregate's members. Understanding this distinction is crucial to avoid compilation errors and ensure the correct initialization of such data structures.

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