#include<iostream>
using namespace std;
const int vals[] = {0, 1, 2, 3, 4};
int newArray[ vals[2] ]; //"error: array bound is not an integer constant"
int main(){
return vals[2];
}
//returns 2 if erroneous line is removed
Why doesn't this work?
Unfortunately you can't do that in standard C++ because vals[2] is not a constant expression! In the coming standard you would have constexpr(implemented in g++ 4.6) to request compile-time evaluation easily:
#include<iostream>
using namespace std;
constexpr int vals[] = {0, 1, 2, 3, 4};
int newArray[ vals[2] ]; // vals[2] is a constant expression now!
int main(){
return vals[2];
}
It's possible that the value of a const expression is not even known at compile time. For example, you can initialize a constant with something returned from a function, like
const int size = rand(); // random size
So it is not that constant as you might think
The C++ compiler can only allocate an array with a size known at compile time. If you want to allocated a variable size piece of memory, use the new operator.
