Slow performance compared to C++, ideas?

[Steven Schveighoffer]

On Mon, 03 Jun 2013 23:39:25 -0400, Manu <turkeyman at gmail.com> wrote:

> On 4 June 2013 12:50, Steven Schveighoffer <schveiguy at yahoo.com> wrote:
>
>> On Mon, 03 Jun 2013 12:25:11 -0400, Manu <turkeyman at 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.

> 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?

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 :)

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.

> And where is the most likely source of those 1 million new users to  
> migrate
> from? Java?

 From all over the place, I would say.  D seems to be an island of misfit  
programmers.

-Steve