Confused about const

[Paul D. Anderson]

bearophile Wrote:

> Paul D. Anderson:
> 
> > 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.
> 
> This program compiles:
> 
> struct S { int x; }
> S foo(const S a) {
>     S other = a;
>     return other;
> }
> void main() {}
> 
> 
> So, what's that you actually write? When possible show programs that run! It's one of the things I have learnt from the Python groups.
> 
> Bye,
> bearophile

My struct has a dynamic array as a member -- that seems to be the problem. This code doesn't compile:

struct S {
 int x;
 int[] a;
}

S foo(const S b) {
    S other = b;
    return other;
}
void main() {}

FYI, I'm using DMD version 2.30.