Using variable to initialize an array in C++

Question

As C++ primer said, we can't use variable as the dimension of builtin array, so the following code did not work

int length = 3;
int array[length] = {0, 1, 2};

the error is

error: variable-sized object may not be initialized

But why following code works?

int length = 3;
int array[length];

Add `-pedantic-errors` to your compiler flags. Also throw in `-Wall -Wextra -Wfatal-errors` for good measure. — Quentin, Sep 20 '17 at 14:49
@hyde - To my knowledge, gcc was the only compiler with VLA support in C++. — StoryTeller - Unslander Monica, Sep 20 '17 at 15:10
@StoryTeller https://clang.llvm.org/docs/LanguageExtensions.html#introduction — hyde, Sep 20 '17 at 15:28
@StoryTeller Just expanding on your *"To my knowledge, gcc was the only compiler..."*, for benefit of all readers. — hyde, Sep 20 '17 at 15:49

score 3 · Answer 1 · answered Sep 20 '17 at 14:51

3

This is an extension by your compiler called a Variable Length Array (VLA) and is not in the C++ standard which means that this code can break at any moment you switch compilers or the compiler vendor decides to no longer support this feature. If you want a variable length array that does not depend on this extension but instead on the standard you should use a std::vector.

answered Sep 20 '17 at 14:51

Hatted Rooster

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Thanks. But now in my env it works, why the elements in the array is not 0? – danche Sep 20 '17 at 14:53
@danche Because `array` is still uninitialized, it's not zero-initialized. – Hatted Rooster Sep 20 '17 at 14:54
"or the compiler vendor decides to no longer support this feature", well, for extensions which are rather well established and documented, and have existed for longer than C++ has been standardized, I think that's just FUD. Features can't be retroactively removed from current versions. I'd be more worried about using latest C++ standard features and encountering a compiler bug, which will silently alter code behavior when it gets fixed in a minor version bump, or when compiling with different compiler... – hyde Sep 20 '17 at 15:46

Using variable to initialize an array in C++

1 Answers1