Confused about const

[Pelle Månsson]

On 03/20/2010 01:58 AM, Paul D. Anderson wrote:
> I created a struct, call it "S", and some functions that operate on S. But I'm confused about when const is useful.
>
> Being an old Java programmer, I use 'const' the same as I used 'final' in Java. So most of my functions look like this:
>
> S for(const S a, const S b) {
>      S x = a;
>      S y = b;
>      // do some stuff
>      return a;
> }
>
> I declare the parameters const and then copy them to work on them.
>
> I get error messages about not implicitly casting const S to S. So I can make an explicit cast:
>
>      S x = cast(S) a;
>      S y = cast(S) b;
>
> and the error messages go away.
>
> But I think I must have gone off the rails somewhere -- all I want is for the caller to be sure the function doesn't alter a and b. But I end up with lots of casts to/from const. What am I missing??
>
> Paul

I think you misunderstand const. If you flag an argument with const it 
means you're unable to ever change anything reachable from that 
argument, so if you have a const reference, you can not alter anything 
reachable from that reference.

If you have this:

struct S {
     int[] xs;
}
S foo(const S s) {
     return cast(S)s;
}
void main() {
     S s;
     s.xs = [1,4,5];
     S fromconst = foo(s);
     fromconst.xs[0] = 4; //<-- there you alter something you
                          // should not be able to alter.
}

The compiler protects you from this, unless you cast your constness away.

If you want to copy, you'll need to implement the proper deep copying 
yourself. :)