too many initializers for 'int [0]' c++

In C++11, in-class member initializers are allowed, but basically act the same as initializing in a member initialization list. Therefore, the size of the array must be explicitly stated.

Stroustrup has a short explanation on his website here.

The error message means that you are providing too many items for an array of length 0, which is what int [] evaluates to in that context.

In the first example, something (actual memory allocation) is actually happening. The computer easily counts the number of items given to it and uses this as the capacity. In the second use, as part of a struct, the array is simply part of the template of a struct. In the second case, a size must explicitly be given and known at compile-time. There is no automatic counting here. It's in a similar vein to function declarations versus definitions as well as variable declarations (tells type but memory is untouched) and their invocation/use (where the program acts).

Probably cause in a struct the compiler needs you to specify the size explicitly.

C initialize array within structure (pun intended)

These are two completely different contexts:

The first is a variable declaration (with an initialiser clause).

The second is a type definition.