Question

GCC gives no error when you initialize a variable-sized array as long as the variable is const, but when it isn’t, it won’t compile.

What’s the reason behind this? What’s so wrong with doing:

int size = 7;
int test[size] = {3, 4, 5};

That won’t compile at all, but if I don’t initialize test[] then it does compile! That doesn’t make any sense to me, because as far as I know a stack frame needs to be made to fit this array according to its size(7 ints) no matter what(which means the integer literals I use don’t really have any meaning, if I’m not mistaken), so what difference does it make if I initialize it or not?

Just another one of my crazy C++ design questions…

Thanks!

Answer 1

• The size of the array must be a constant integral expression.
• An integral literal is a constant integral expression. (int arr[5];)

• A constant integral variable initialized with a constant expression is a constant expression. (const int j = 4; const int i = j; int a[i];)

• A constant variable initialized with a non-constant expression is not a constant expression

   int x = 4;  // x isn't constant expression because it is not const
   const int y = x; //therefore y is not either
   int arr[y]; //error)