about the new const regime

[Steven Schveighoffer]

"Daniel919" wrote
>> The difference is :
>>
>> string s = "bla";
>> s = "abc";     // compile, you can reassign to an invariant array with 
>> another invariant array, but you can't change it partially.
>> s[0] = 'x'; //error as you said
>
> But the same is true for a const(char)[].
> const(char)[] s = "bla";
> s = "abc";
> s[0] = 'x'; //error
>
> const char[] s = "bla";
> s = "abc" //error
>
> invariant char[] s = "bla";
> s = "abc" //error

The difference is subtle.  invariant means "nothing can change this".  const 
means "I can't change this".

So for a const array, for instance:

char[] s = "bla".dup;
const(char)[] c = s; // ok
invariant(char)[] i = s; // error
invariant(char)[] i2 = c; // error

This is an error because since you could potentially change i by changing s, 
you can't assign s to i or c to i without an explicit cast.  The explicit 
cast tells the compiler you are ensuring that s will no longer be changed by 
anything:

invariant(char)[] i3 = cast(invariant(char)[])s; // ok

This is ok, but you can no longer change s.  If you do, the behavior is 
undefined.

Do you see the difference now?  In most cases, if you are initializing a 
const variable with a literal, or a new() statement, you can change the 
modifier to invariant because logically, nothing can change that variable. 
It would be nice if the compiler automatically did this for you, but I don't 
think this is the case today.

-Steve