c++ changing array size - is it black magic or an illusion? 🤔

Question

Traditional rules state array size cannot change. however, with this array function it appears the array is changing in size. So question for the experts: is it actually changing size? Or is it an illusion (the array actually has 22 idx and only reports how many are being used)? I honestly don't have a clue!

The array size is initially set at 7. Yet it is eventually increased to 22 and back to 5.

However, in the other example (c++ array size), the array is set to have 5 idx. Even as they are empty, the size is always reported as 5.

But in the case of the function, the array size *is* actually being reported as changing...

And while size_arr is being passed to the function, the cout of the function never actually references that variable, it should only report the actual size:
cout << sizeof(arr)/sizeof(int) << endl;

as it does in the accompanying example.

So, is it black magic or an illusion?

https://code.sololearn.com/cB7UChgXGIGJ/?ref=app

https://code.sololearn.com/cCBZXjZzTx17/?ref=app

Answer 1

KrOW
You're probably right, I'm probably "gaming the system" so to speak.

Even though the array name doesn't change each time the function is called it's a new array, but with the same name.

I'm a newbie but not aware that there's any changes regarding making array lengths variable. I guess that's why there are vectors.

Wouldn't a compiler allowing that be kind of bad? I don't know much about compilers but would think that if the code was run on a different compiler or OS there could be errors.

Answer 2

The array, tecnically, is not changing.. Every time that you call the randArr function a new stack frame is created and a new array is allocated with specified size.
Anyway, i dont be update of c++ standards, but variable length array would be an extension of compiler not part of c++ standard.
In theory :

int a= 7;
int c[a];

would be compilable in c++ specs from what i remember

Answer 3

it seem that gcc and clang support it but can be disabled ... I dont know if and why is supported by default by compilers (maybe some flag used by SL compiler?)