Extended Type Design: further examples

[Andrei Alexandrescu (See Website For Email)]

Sean Kelly wrote:
> Andrei Alexandrescu (See Website For Email) wrote:
>>
>> By the way, a couple of days ago I've had an idea. Many people here ask
>> for a "string" alias because char[] is a tad hard to type and uneasy on
>> the eyes. Probably a good candidate would be:
>>
>> alias invariant char[] string;
>>
>> The resulting type is pretty damn slick:
> ...
>> 3. The ~= operator works (somewhat surprisingly).
> 
> I assume this is because a char[] is actually a struct containing a 
> pointer, so 'invariant' allows that pointer to be reassigned--it simply 
> protects the referenced string?  This is the sticking point for me. 

That is correct. For an understanding of a ~= b, think of it as a = a ~ 
b. Do I modify the stuff indirectly referenced by "a"? No, Sir. I just 
rebind a to a larger entity that embeds a copy of its former self.

In practice, the concatenation will not always produce a copy. The 
implementation will work "as if" you always produce a copy.

> Assume we have this:
> 
>     struct Foo
>     {
>         int  x;
>         int* y;
>     }
> 
>     const Foo foo; // is this legal?

Yes.

>     foo.x  = 1;    // this is allowed

Yes.

>     *foo.y = 2;    // this is not allowed

It's not allowed.

> Is that correct, or do 'const' and 'invariant' truly only apply to pure 
> reference types?

No. They apply to any type that embeds references.

"final" applies to all types and guards the bits of the symbol itself. 
"final const" guards symbol bits + referenced bits, and "final 
invariant" is as opposed to change as the 19th century British education 
system. (Did I get that right?)

>> 4. It's very, very rare that you want to modify some random character in
>> a string, and when you do, use a char[] and then copy it back into a
>> string, or rebuild the string from slices!
> 
> This may be true for a string, but for arrays in general I think this is 
> fairly common.

Yes. That's why arrays will stay in place :o).


Andrei