Why does C++ compiler allows to create an array of unknown size on stack at compile time?

Question

I try to understand, what happens when creating an array of unknown size on stack at compile time. Let consider this code:

int main()
{
    int x;
    cin >> x;

    int tab[x];
}

I found a lot of information about this saying that you can not create an array of unknown size on the stack, but I didn't find any information why does the C++ compiler allows it, or maybe some of them do? What happens when creating such array? Is it even created on stack or already on heap?

Does the GCC compiler have some option to turn on, thanks to which such a constructions would be considered as errors or at least warnings?

1) VLAs are non-standard C++ (as in - standard C++ doesn't allow them). Though, some compiler extensions make them possible. 2) "_why does the C++ compiler allows it_" There's no such thing as **the** C++ compiler. There are many implementations of it. — Algirdas Preidžius, Apr 16 '18 at 17:13
The ancient practice was to use dynamic memory when declaring arrays at run-time. Now, in C++, you should use `std::vector` instead. — Thomas Matthews, Apr 16 '18 at 17:16
btw its not called "table" but array and `x` is not "unknown" it is just unknown at compile time, but once the user entered a number you can create an array of that size (just not in this way if you want to write standard c++) — 463035818_is_not_an_ai, Apr 16 '18 at 17:17
I dont understand why this question gets downvoted. If I didnt know the answer I also wouldnt know where to find more information on this. Also the duplicate is strictly speaking not a duplicate. — 463035818_is_not_an_ai, Apr 16 '18 at 17:19
This is, strictly speaking, undefined behaviour. *Anything* is possible/valid. — Jesper Juhl, Apr 16 '18 at 17:23
Thank you I already found an option to prohibit VLAs (-Werror=vla) — Tom, Apr 16 '18 at 17:25

score 4 · Accepted Answer · answered Apr 16 '18 at 17:20

4

C++ does not permit Variable Length Arrays (VLAs).

However, the most recent C standard does, so it can sometimes be found as an extension, such as it is with the GCC.

When compiling, make sure so explicitly select a language (chose C++17 or later if you can) and ask for pedantic (strictly standards-conforming) behavior.

answered Apr 16 '18 at 17:20

Dúthomhas

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Thank you. Right now I can only use C++1z, but it still allows the VLAs, so I have to use this compiler option: (-Werror=vla) – Tom Apr 16 '18 at 17:29
1

It turns out that also the flag `-pedantic-errors` can be used instead of `-Werror=vla` – Tom Apr 16 '18 at 17:46

Why does C++ compiler allows to create an array of unknown size on stack at compile time?

1 Answers1