const/invariant

[Steven Schveighoffer]

"Denton Cockburn" wrote
> given a class:
>
> class C {}
>
> what's the difference between:
>
> const C c = new C;
> and
> invariant C c = new C;
>
> From what I can see, neither are modifiable.  So what's different
> (D2.008).

const class references can be used to call const member functions, but 
cannot call normal member functions or invariant member functions.

invariant class references can be used to call const or invariant member 
functions, but not normal member functions.

normal class references can call normal member functions and const member 
functions, but not invariant member functions.

In your example, there is no other non-const reference of c, so technically, 
it is invariant because nothing outside of the c reference can ever change 
it.  Therefore, the second declaration is more accurate and gives you more 
ability (you can now call invariant functions).

if you had something like:

C c = new C;
const C c2 = c;
invariant C c3 = cast(invariant)c; // this is bad

Now, c2 cannot be declared invariant because something using the c reference 
could modify the data.  c2 is like a read only view of c, it can look but 
cannot touch.  c3 is bad because before you cast to invariant, you must 
ensure that no other references to the same data can modify the class.

Anticipating your next question: what is the difference between invariant 
and const functions?  Code-wise, nothing.  However, the compiler is free to 
make better optimizations knowing that none of the members of the class will 
change.

-Steve