const question/suggestion

[Myron Alexander]

Craig Black wrote:
> OK, I think I'm starting to grasp the subtle differences between const, 
> final, and invariant.  It seems to me that to have three keywords is 
> unnecessary.  Perhaps const and final could be merged into one?
> 
> From my understanding, the only difference between const and final is that 
> local final fields can be initialized in a constructor, right?  Couldn't 
> that just be the default behavior of a local const field?  Then we could get 
> rid of final and have only two keywords.  Or am I missing something?  IMO, 
> two keywords is way less confusing than three.
> 
> Another question.  Since invariant data should always be invariant, does it 
> make sense to be able to cast non-invariant data to invariant?  The compiler 
> will think that it is invariant when it isn't.
> 
> -Craig 
> 
> 

Craig, const and final have two very different roles. Const ensures that 
the value referenced/pointed to by the reference/pointer cannot be 
changed. In other words, if an object reference is bound to a const 
variable, then the object is immutable when referenced via that variable.

Final is different in that it prevents a variable from having another 
value bound to it. In other words, if you have a reference variable, 
once initialised, you cannot assign a new reference to the variable.

Examples:

SomeObject is a mutable class, IE it's value can be modified.

auto mutvar = new SomeObject ();
mutvar.changesomething() -- ok

const var = new SomeObject ();
var.changesomething() -- fails

Now for final:

final finvar = new SomeObject (); -- ok
finvar.changesomething() -- ok
...
finvar = new SomeObject (); -- fails

Regards,

Myron.