Extended Type Design: further examples

[Andrei Alexandrescu (See Website For Email)]

Derek Parnell wrote:
> On Mon, 19 Mar 2007 17:50:28 -0700, Andrei Alexandrescu (See Website For
> Email) wrote:
> 
>> Derek Parnell wrote:
>>> Hmmm ... so the difference between 'const' and 'invariant' is that const is
>>> only effective for one level deep, but 'invariant' is effective for all
>>> levels, right?
>> Nonono. Const means: it might have originated as a variable, and 
>> somebody, somewhere, might hold a mutating reference to this and call it 
>> "my precious".
>>
>> Invariant is: nobody ever holds a mutating reference to it.
> 
> Yeah ... but that's not what I was talking about. But be that as it may, it
> seems you are saying that the *only* difference between 'const' and
> 'invariant' is that 'invariant' means that every write attempt (direct and
> indirect) is denied, but 'const' only prevents direct write attempts.
> 
> By 'direct' I mean explicitly using the symbol in the statement, and not
> another symbol that happens to contain the same address of the data.
> 
> 
> But I was talking earlier about the situation where something contains
> reference types or structs, and what effect 'const' and 'invariant' have on
> the things indirectly accessible via member data.
> 
>  struct Foo
>  {
>     char[] s;
>  }
> 
>  void Func(const Foo fc, invariant Foo fi)
>  {
>     fc.s = "abc"; // fails ???

Works. Understand that fc is the private copy owned by Func.

>     fc.s[0] = 'a'; // okay ???

Fails. fc would change bits not belonging to itself, and "const" makes 
those bits unchangeable.

>     fi.s = "abc"; // fails ???

Works. Modifies Func's own private copy of fi.

>     fi.s[0] = 'a'; // fails ???

Fails.

>  }
> 
>  Foo a;
>  Foo b;
>  Func(a,b); // fails ???

Fails. Can't bind modifiable b to an invariant Foo.

>  const Foo ac;
>  const Foo bc;
>  Func(ac,bc); // fails ???

Fails. Can't bind const b (which conservatively may be aliased to a 
modifiable view) to an invariant Foo.

>  invariant Foo ai;
>  invariant Foo bi;
>  Func(ai,bi); // fails ???

Works. No problem to bind ai to a const Foo.

>  Func(ac,bi); // okay ???

Perfect.


Andrei