Question

I tried

const int i[] = { 1, 2, 3, 4 };
float f[i[3]]; // g++ cries "error: array bound is not an integer constant"

int main()
{
   const int j[] = { 0, 1, 2, 3 };
   float g[j[3]];                   // compiler is happy :)

   return 0;
}

What is the difference between the two aggregates? How come referring to a const aggregate’s element inside main() is valid when it’s invalid at the global scope?

Answer 1

In C++ sizes in array declaration have to be Integral Constant Expressions (ICE). By definition, ICE in C++ cannot include a value taken from an array, regardless of whether it is a const array or not. So, in both cases i[3] and j[3] are not ICEs and cannot be used as sizes in array declarations. For this reason both declarations – f and g – are illegal in C++.

However, since you are using GCC, in the second case a non-standard compiler extension comes into play. When you are declaring an automatic array, GCC allows you to specify a non-constant size (i.e. run-time size) for the array (this is basically C99’s VLAs carried over to C++). This is why it is “happy”, even though the code is not legal in C++. Compile in -ansi -pedantic-error mode and you should get a diagnostic message (an error) for each declaration.