Question

#include <iostream>

using namespace std;

int main(void){
    int size = -2;
    int* p = new int[size];
    cout<<p<<endl;
    return 0;
}

Above code compiles without any problem on visual studio 2010.

But

#include <iostream>

using namespace std;

int main(void){
    const int size = -2;
    int* p = new int[size];
    cout<<p<<endl;
    return 0;
}

But this code(added const keyword) gives error on compilation(size of array cannot be negative).

Why these different results ?

Answer 1

By making it a constant expression, you’ve given the compiler the ability to diagnose the problem at compile time. Since you’ve made it const, the compiler can easily figure out that the value will necessarily be negative when you pass it to new.

With a non-constant expression, you have the same problem, but it takes more intelligence on the part of the compiler to detect it. Specifically, the compiler has to detect that the value doesn’t change between the time you initialize it and the time you pass it to new. With optimization turned on, chances are pretty good at least some compilers still could/would detect the problem, because for optimization purposes they detect data flow so they’d “realize” that in this case the value remains constant, even though you haven’t specified it as const.