Using variable to initialize an array in C++ [duplicate]

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
– Quentin, Commented Sep 20, 2017 at 14:49

Hatted Rooster · Accepted Answer · 2017-09-20 14:51:14Z

2

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, 2017 at 14:51

Hatted Rooster

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3 Comments

danche Over a year ago

Thanks. But now in my env it works, why the elements in the array is not 0?

Hatted Rooster Over a year ago

@danche Because array is still uninitialized, it's not zero-initialized.

hyde Over a year ago

"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...

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