I'm a beginning C++ programmer. So, I just learned that gcc has an extension that allows variably sized array without having to dynamically allocate memory. I want to know if this variably sized array is allocated in the stack or heap.
3 Answers
Conceptually it's allocated with automatic storage duration, so in terms of implementation, you can think of it as being on the stack.
Do consider using std::vector as an alternative though as that's standard and therefore portable C++.
3 Comments
Surya
It's vector dynamic? I've heard that vector is comparatively slower
Hatted Rooster
@SuryaK That's premature optimization. Don't do that.
std::vector is blazingly fast. If you think not, benchmark it, then come back. Stick to using std::vector now.
πάντα ῥεῖ
@SuryaK "I've heard that vector is comparatively slower" Not significantly if used right. Make use of the initial allocation count, or use
reserve().The variable sized array is allocated in the stack.
2 Comments
Surya
So, it means it is a bad idea to use VLAs to hold large data and better to use dynamically allocated array or std::vector. Right?
Sanjay-sopho
I'd recommend dynamically allocated array over VLAs
VLA's are not supported by the C++ standard, although some compilers such as GCC do have them as an extension.
std::vector <> VLA in the GCC implementation.
- std::vector is resizable and allocates memory on the heap.
- VLA is not resizable, is limited by the maximum stack size and doesn't allocate memory.
So there is a flexibility difference, and there can be a performance difference especially if the array creation happens regularly (such as in a tight loop).
That said, some of these differences can sometimes be mitigated by for example, moving the 'array' outside of loops etc
