c++ c-style zero-initialization { 0 }

Question

https://en.cppreference.com/w/cpp/language/zero_initialization shows that zero-initialization happens in the following scenario:

int array[32] = {};

; but never says anything about this:

int array[32] = { 0 };

Does the latter also zero-initialize the whole array in c++, or only the first element? If so, is it also true for structs?

Answer 1

ISO/IEC N489 §9.4.1 states

• ⁽⁵⁾ For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:

  • ^(5.1) If the element has a default member initializer (11.4), the element is initialized from that initializer.

  • ^(5.2) Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list (9.4.4).

  • ^(5.3) Otherwise, the program is ill-formed.

§9.4.4 states

• ^(3.11) Otherwise, if the initializer list has no elements, the object is value-initialized.

Value-initialization of a scalar leads to its zero-initialization. Thus, the second one explicitly initializes with 0 only array[0] and the rest of the elements will be zero-initialized.

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Thus, the following code

int array[4] = {13};

initializes array with values

{13, 0, 0, 0}