Variable length arrays (VLA) in C and C++

Yes your reasoning is correct, that is how these different forms of array declarations and definitions are viewed by C and C++.

As others already stated, VLA with a veritable variable length (non-const) in global scope is difficult to make sense. What would the evaluation order be, e.g if the the length expression would refer to an object of a different compilation unit? C++ doesn't have VLA, but it has dynamic initialization of objects at file scope. And already this gives you quite a head ache, if you have to rely on evaluation order.

This leaves the small gap for C concerning length expressions that contain a const qualified object, which isn't allowed. This comes from the fact that such objects are not considered "integer constant expressions" by the C standard. This could perhaps change in future versions, but up to now the C committee didn't find it necessary to allow for such a thing: there are enum constants that play that role in C. Their only limitation is that they are limited to int in C, it would be nice to also have them size_t.

C++ doesn't support VLAs, period. The reason the second code snippet works in C++ is that the const keyword creates a compile-time constant in C++; in C, it doesn't.

C99 doesn't support VLAs outside of block scope, period, regardless of how you declare the size variable. Note that C2011 makes VLA support optional.