invariant/const class storage class

[Neil Vice]

Nicolas Sicard wrote:
> Hi,
> I had a look at http://www.digitalmars.com/d/2.0/class.html#ConstClass.
> Does this mean that I should be able to write something like :
> 
>     module test;
> 
>     invariant class Thing {
>         private int _field;
> 
>         this(int field) {
>             _field = field;
>         }
> 
>         int field() {
>             return _field;
>         }
>     }
> 
> ?
> It doesn't compile:
> 
>     Error: cannot implicitly convert expression (this) of type
>     invariant(Thing) to test.Thing
> 
> The error doesn't mention any line. I don't undestand it.
> I thought it would be a way of saying that instances of a class are 
> always const or immutable.
> Sorry if I said something stupid :-)  ...
> 
> Nicolas

My understanding is that by applying a const/invariant specifier to a 
class type you are effectively indicating that its "this" reference has 
that specifier.

So in this case, in the constructor where you attempt to assign to 
_field, you are attempting to modify an invariant "this" variable, which 
is not permitted.

The compiler error message is due to it attempting to implicitly cast 
the invariant(Thing) to a non-invariant Thing in order to assign to its 
member.

If my understanding is correct I am not sure how invariant classes would 
be useful as they could only have invariant fields and as such would 
seem to be sort of a collection of constants.

Undoubtedly however, I will be corrected =P