Inheritance of purity

Jonathan M Davis jmdavisProg at
Thu Feb 16 19:23:34 PST 2012

On Thursday, February 16, 2012 18:49:40 Walter Bright wrote:
> Given:
>      class A { void foo() { } }
>      class B : A { override pure void foo() { } }
> This works great, because is covariant with, meaning it can
> "tighten", or place more restrictions, on foo. But:
>      class A { pure void foo() { } }
>      class B : A { override void foo() { } }
> fails, because 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?

No. Absolutely not. I hate the fact that C++ does this with virtual. It makes 
it so that you have to constantly look at the base classes to figure out what's 
virtual and what isn't. It harms maintenance and code understandability. And 
now you want to do that with @safe, pure, nothrow, and const? Yuck.

I can understand wanting to save some typing, but I really think that this 
harms code maintainability. It's the sort of thing that an IDE is good for. It 
does stuff like generate the function signatures for you or fill in the 
attributes that are required but are missing. I grant you that many D 
developers don't use IDEs at this point (at least not for D) and that those 
sort of capabilities are likely to be in their infancy for the IDEs that we 
_do_ have, but I really think that this is the sort of thing that should be 
left up to the IDE. Inferring attribtutes like that is just going to harm code 
maintainibility. It's bad enough that we end up with them not being marked on 
templates due to inferrence, but we _have_ to do it that way, because the 
attributes vary per instantiation. That is _not_ the case with class member 

Please, do _not_ do this.

- Jonathan M Davis

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