Can size of an array be determined at runtime?

Question

The code compiles and runs, but is it legal? AFAIK, size of static array must be known at compile time.

#include <iostream>
void foo(const unsigned size){
    double arr[size];
    for(auto& i:arr){
        i=2;
        std::cout << i;
    }
}

int main(){
    unsigned size;
    std::cin >> size;
    foo(size);
}

What exactly do you mean by "static" array??? The array in your example is local (automatic), not static. — AnT stands with Russia, Oct 01 '14 at 06:52
GingerPlusPlus, I think you should edit your question, `static` does not have the meaning that you think. — BlueTrin, Oct 01 '14 at 07:21
@AndreyT I have no idea how can I improve the title, please edit my question if you think you can improve it. — GingerPlusPlus, Oct 01 '14 at 07:26

score 3 · Accepted Answer · answered Oct 01 '14 at 06:51

3

Strictly speaking, this is not legal. Variable length arrays (VLA) are not supported by standard C++. VLA are supported by most compilers an an extension but using those may give you non portable behavior.

Also, there is no static array in your source code. To do so you would need to use the static keyword on the declaration.

answered Oct 01 '14 at 06:51

Alok Save

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IIRC Runtime-sized arrays are included in C++14. – user657267 Oct 01 '14 at 06:59
@user657267, I believe they were cut towards the end, n3797 §8.3.4/1 has the size as a constant expression – Niall Oct 01 '14 at 07:14
So, avoid constructions like that? – GingerPlusPlus Oct 01 '14 at 07:14
@Niall wasn't that `std::dynarray`? Or were they both axed? – user657267 Oct 01 '14 at 07:21
@user657267, both. See the note at the top of this page as well; http://en.cppreference.com/w/cpp/container/dynarray – Niall Oct 01 '14 at 07:22
@Niall right you are, here's the [TS](http://www.open-std.org/Jtc1/sc22/wg21/docs/papers/2013/n3820.html). – user657267 Oct 01 '14 at 07:28

BlueTrin · Answer 2 · 2014-10-01T10:58:38.497

1

As user657267 mentioned, Runtime-size arrays have been discussed for C++14 in N3639 and another similar feature, std::dynarray has been discussed, however they have been rejected for C++14.

Currently, this is not considered as part of the standard although some compilers implemented this as non-portable extensions, gcc for example is one of these compilers (possibly because this is legal in C99).

Most of the time, you can just use a vector to the same effect.

edited Oct 01 '14 at 10:58

answered Oct 01 '14 at 07:19

BlueTrin

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I was wrong, VLAs and `std::dynarray` have been moved to a separate TS and are not part of C++14. – user657267 Oct 01 '14 at 07:31
Yeah I saw that after, but was the decision final ? – BlueTrin Oct 01 '14 at 08:53
Yeah I linked the [TS](http://www.open-std.org/Jtc1/sc22/wg21/docs/papers/2013/n3820.html) in the other comments. – user657267 Oct 01 '14 at 08:57

score 0 · Answer 3 · answered Oct 01 '14 at 07:05

In your example there is no array with the static storage duration. There is a local array within a function with automatic storage duration. The size of the array shall be a constant expression.

If the code you showed is compiled then it means it is a feature of the compiler that supports Variable Length Arrays in C++ the same way as in C.

Can size of an array be determined at runtime?

3 Answers3