Question

AFAIK this code is not a valid c++ code by standard:

int a = 5;
int b[a];

but it seems many compilers can compile that code (mostly with warning) and it just behaves as expected. Am I wrong is is it compilers being nice to me?

Answer 1

It is called variable length array (VLA) which is not allowed by Standard C++, any version of C++, though some GCC supports this as an extension.

If you’re using GCC, then

• Compile it with -pedantic option, you will see warning.
• Compile it with -pedantic -Werror option, you will see warning turned into error.

VLA is allowed only by C99, though not by other versions of C.