Inheritance of purity

[Andrei Alexandrescu]

On 2/16/12 8:49 PM, Walter Bright wrote:
> Given:
>
> class A { void foo() { } }
> class B : A { override pure void foo() { } }
>
> This works great, because B.foo is covariant with A.foo, meaning it can
> "tighten", or place more restrictions, on foo. But:
>
> class A { pure void foo() { } }
> class B : A { override void foo() { } }
>
> fails, because B.foo tries to loosen the requirements, and so is not
> covariant.
>
> Where this gets annoying is when the qualifiers on the base class
> function have to be repeated on all its overrides. I ran headlong into
> this when experimenting with making the member functions of class Object
> pure.
>
> So it occurred to me that an overriding function could *inherit* the
> qualifiers from the overridden function. The qualifiers of the
> overriding function would be the "tightest" of its explicit qualifiers
> and its overridden function qualifiers. It turns out that most functions
> are naturally pure, so this greatly eases things and eliminates annoying
> typing.
>
> I want do to this for @safe, pure, nothrow, and even const.
>
> I think it is semantically sound, as well. The overriding function body
> will be semantically checked against this tightest set of qualifiers.
>
> What do you think?

I thought about this for a while and seems to work well. The maintenance 
scenarios have already been discussed (people add or remove some 
attribute or qualifier) and I don't see ways in which things become 
inadvertently broken.

The const qualifier is a bit different because it allows overloading. 
Attention must be paid there so only the appropriate overload is 
overridden.

Congratulations Walter for a great idea. Inference is definitely the way 
to go.


Andrei