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

In the expression

new int[n]

int[n] is not the type. C++ treats "new with arrays" and "new with non-arrays" differently. The N3337 standard draft has this to say about new:

When the allocated object is an array (that is, the noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type), the new-expression yields a pointer to the initial element (if any) of the array.

The noptr-new-declarator refers to this special case (evaluate n and create the array of this size), see:

noptr-new-declarator:

    [ expression ] attribute-specifier-seq_opt

    noptr-new-declarator [ constant-expression ] attribute-specifier-seq_opt

However you can't use this in the "usual" declarations like

int array[n];

or in the typedef

typedef int variable_array[n];

This is different with C99 VLAs, where both are allowed.

Should I be using vectors instead?

Yes, you should. You should use vectors all the time, unless you have a very strong reason to do otherwise (there was one time during the last 7 years when I used new - when I was implementing vector for a school assignment).