Comment by wahern

C never had {} until C23. In C {0} was the only way to explicitly zero-initialize a structure in a generic manner. It works because in C initializer lists are applied to members as-if nested structures are flattened out lexically.

However, a long time ago C++ went in a completely different direction with initializer lists, and gcc and clang started emitting warnings (in C mode) about otherwise perfectly valid C code, thus the adoption of C++'s {} for C23. {0} is still technically valid C23, though, as well as valid C89, C90, C99, and C11. In fact, reading both C23 and C89 I'm struck by how little the language has changed:

C89 3.5.7p16:

> If the aggregate contains members that are aggregates or unions, or if the first member of a union is an aggregate or union, the rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the members of the subaggregate or the first member of the contained union. Otherwise, only enough initializers from the list are taken to account for the members of the first subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next member of the aggregate of which the current subaggregate or contained union is a part.

C23 6.7.10p21:

> If the aggregate or union contains elements or members that are aggregates or unions, these rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only enough initializers from the list are taken to account for the elements or members of the subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next element or member of the aggregate of which the current subaggregate or contained union is a part.