Why is new int[n] valid when int array[n] is not?

Question

For the following code:

foo(int n){
    int array[n];
}

I understand that this is invalid syntax and that it is invalid because the c++ sta

Answer 1

Because it has different semantics:

If n is a compile-time constant (unlike in your example):

int array[n]; //valid iff n is compile-time constant, space known at compile-time

But consider when n is a runtime value:

int array[n]; //Cannot have a static array with a runtime value in C++
int * array = new int[n]; //This works because it happens at run-time, 
                          // not at compile-time! Different semantics, similar syntax.

In C99 you can have a runtime n for an array and space will be made in the stack at runtime. There are some proposals for similar extensions in C++, but none of them is into the standard yet.