在c ++中创建一个非常长的数组时会发生什么？

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.

Answer 2

The compiler is being nice. :)

It's actually part of the C standard, and some compilers (like GCC) extend C++ with this feature.

Answer 3

This is the C99 standards Variable Length Array (or VLA), many compilers that can compile C99, often allow some of its features to be used in non-standard conforming C++ code.

G++ is one of these compilers, see here .