Question

I don’t really understand why I can’t have a variable size array on the stack, so something like

foo(int n) {
   int a[n];
}

As I understand the stack(-segment) of part of the data-segment and thus it is not of “constant size”.

Answer 1

Variable Length Arrays(VLA) are not allowed in C++ as per the C++ standard.
Many compilers including gcc support them as a compiler extension, but it is important to note that any code that uses such an extension is non portable.

C++ provides std::vector for implementing a similar functionality as VLA.

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There was a proposal to introduce Variable Length Arrays in C++11, but eventually was dropped, because it would need large changes to the type system in C++. The benefit of being able to create small arrays on stack without wasting space or calling constructors for not used elements was considered not significant enough for large changes in C++ type system.