On Mon, 03 Jun 2013 12:25:11 -0400, Manu <turkeyman@gmail.com> wrote:
You won't break every single method, they already went through that
recently when override was made a requirement.
It will only break the base declarations, which are far less numerous.
Coming off the sidelines:
1. I think in the general case, virtual by default is fine. In code that
is not performance-critical, it's not a big deal to have virtual functions,
and it's usually more useful to have them virtual. I've experienced plenty
of times with C++ where I had to go back and 'virtualize' a function. Any
time you change that, you must recompile everything, it's not a simple
change. It's painful either way. To me, this is simply a matter of
preference. I understand that it's difficult to go from virtual to final,
but in practice, breakage happens rarely, and will be loud with the new
override requirements.
I agree that in the general case, it's 'fine', but I still don't see how
it's a significant advantage. I'm not sure what the loss is, but I can see
clear benefits to being explicit from an API point of view about what is
safe to override, and implicitly, how the API is intended to be used.
Can you see my point about general correctness? How can a class be correct
if everything can be overridden, but it wasn't designed for it, and
certainly never been tested?
Since when is that on the base class author? Doctor, I overrode this class, and it doesn't work. Well, then don't override it :)
Also there is the possibility that a class that isn't designed from the start to be overridden. But overriding one or two methods works, and has no adverse effects. Then it is a happy accident. And it even enables designs that take advantage of this default, like mock objects. I would point out that in Objective-C, ALL methods are virtual, even class methods and properties. It seems to work fine there.
What I'm really trying to say is, when final is the default, and you really should have made some method virtual (but didn't), then you have to pay for it later when you update the base class.
When virtual is the default, and you really wanted it to be final (but didn't do that), then you have to pay for it later when you update the base class. There is no way that is advantageous to *everyone*.
2. I think your background may bias your opinions :) We aren't all
working on making lightning fast bare-metal game code.
Of course it does. But what I'm trying to do is show the relative merits of
one default vs the other. I may be biased, but I feel I've presented a fair
few advantages to final-by-default, and I still don't know what the
advantages to virtual-by-default are, other than people who don't care
about the matter feel it's an inconvenience to type 'virtual:'. But that
inconvenience is going to be forced upon one party either way, so the
choice needs to be based on relative merits.
It's advantageous to a particular style of coding. If you know everything is virtual by default, then you write code expecting that. Like mock objects. Or extending a class simply to change one method, even when you weren't expecting that to be part of the design originally.
I look at making methods final specifically for optimization. It doesn't occur to me that the fact that it's overridable is a "leak" in the API, it's at your own peril if you want to extend a class that I didn't intend to be extendable. Like changing/upgrading engine parts in a car.
3. It sucks to have to finalize all but N methods. In other words, we
need a virtual *keyword* to go back to virtual-land. Then, one can put
final: at the top of the class declaration, and virtualize a few methods.
This shouldn't be allowed for final classes though.
The thing that irks me about that is that most classes aren't base classes,
and most methods are trivial accessors and properties... why cater to the
minority case?
I think it is unfair to say most classes are not base classes. This would mean most classes are marked as final. I don't think they are. One of the main reasons to use classes in the first place is for extendability.
Essentially, making virtual the default enables the *extender* to determine whether it's a good base class, when the original author doesn't care.
I think classes fall into 3 categories:
1. Declared a base class (abstract)
2. Declared NOT a base class (final)
3. Don't care.
I'd say most classes fall in category 3. For that, I think having virtual by default isn't a hindrance, it's simply giving the most flexibility to the user.
The losses are that if category 3 were simply always final, some other anti-Manu who wanted to extend everything has to contact all the original authors to get them to change their classes to virtual :)It also doesn't really address the problem where programmers just won't do
that. Libraries suffer, I'm still inventing wheels 10 years from now, and
I'm wasting time tracking down slip ups.
What are the relative losses to the if it were geared the other way?
BTW, did you know you can extend a base class and simply make the extension final, and now all the methods on that derived class become non-virtual calls? Much easier to do than making the original base virtual (Note I haven't tested this to verify, but if not, it should be changed in the compiler).
My one real experience on this was with dcollections. I had not declared
anything final, and I realized I was paying a performance penalty for it.
I then made all the classes final, and nobody complained.
The userbase of a library will grow with time. Andrei wants a million D
users, that's a lot more opportunities to break peoples code and gather
complaints.
Surely it's best to consider these sorts of changes sooner than later?
I think it vastly depends on the intent of the code. If your classes simply don't lend themselves to extending, then making them final is a non-issue.>From all over the place, I would say. D seems to be an island of misfit programmers.
And where is the most likely source of those 1 million new users to migrate
from? Java?
-Steve