Inheritance of purity

Bruno Medeiros brunodomedeiros+dng at
Thu Feb 23 09:04:30 PST 2012

On 17/02/2012 05:08, Kapps wrote:
> On Friday, 17 February 2012 at 03:24:50 UTC, Jonathan M Davis wrote:
>> 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
>> functions.
>> Please, do _not_ do this.
>> - Jonathan M Davis
> In the situation where the IDE writes it for you, said IDE will help you
> only when you write the code.
> In the situation where the IDE tells you what they are (through
> something like hovering over it), it will help you no matter who writes
> the code. It is also significantly easier to implement, particularly
> taking into consideration things like style, comments, etc.

Exactly. If one is worried about having to look at the base classes, 
it's quite easy to check that info when you are using an IDE - for 
example, with a hover over the overriding function which lists all the 
parameters and attributes, and documentation too.

Bruno Medeiros - Software Engineer

More information about the Digitalmars-d mailing list