Is the sizeof operator evaluated differently for VLAs by gcc?

Question

g++ allows Variable Length Arrays (VLA) as an extension. The results of sizeof operator on VLAs are interesting:

int main ()
{
  char size = 20, a[10], b[size];
  cout<<"sizeof(a) = "<<sizeof(a)<<endl;  // sizeof(a) = 10, (can be used as template param)
  cout<<"sizeof(b) = "<<sizeof(b)<<endl;  // sizeof(b) = 20 !! (can't used be as template param)
}

In case of sizeof(b), is g++ not following the standard where sizeof is evaluated only at compile time? Is sizeof overloaded?

Answer 1

VLAs are an exception to the rule that the operand of sizeof is not evaluated, as specified in C99, 6.5.3.4/2:

If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

This behaviour is a g++ extension; in Standard C++ (up to and including C++14) the operand of sizeof is never evaluated (and VLAs are not permitted).