For some reason, whenever I declare a method with package visibility, it becomes non-virtual. Is this normal behavior, or is there a bug in DMD 2.059?

On Thursday, June 14, 2012 00:32:45 BLM768 wrote:
> For some reason, whenever I declare a method with package visibility, it becomes non-virtual. Is this normal behavior, or is there a bug in DMD 2.059?

Only public and protected functions can be virtual. private and package functions are never virtual. This is by design.

- Jonathan M Davis

On Wednesday, 13 June 2012 at 22:48:34 UTC, Jonathan M Davis wrote:
> On Thursday, June 14, 2012 00:32:45 BLM768 wrote:
>> For some reason, whenever I declare a method with package
>> visibility, it becomes non-virtual. Is this normal behavior, or
>> is there a bug in DMD 2.059?
>
> Only public and protected functions can be virtual. private and package
> functions are never virtual. This is by design.
>
> - Jonathan M Davis

That explains it. I kind of wish I'd known that a few hours ago :)
It would be nice if DMD would issue a warning when redefining a non-virtual function in a subclass. I might have to start using "override" more often despite force of habit and dislike of its verboseness. Perhaps "override" should be implied by default since it's by far the most common case. In cases where the programmer really wants to redefine the function non-virtually, there could be a keyword that would handle that. It could reduce extra typing and save beginners hours of stress :)

On Thursday, June 14, 2012 01:07:17 BLM768 wrote:
> On Wednesday, 13 June 2012 at 22:48:34 UTC, Jonathan M Davis
> 
> wrote:
> > On Thursday, June 14, 2012 00:32:45 BLM768 wrote:
> >> For some reason, whenever I declare a method with package visibility, it becomes non-virtual. Is this normal behavior, or is there a bug in DMD 2.059?
> > 
> > Only public and protected functions can be virtual. private and
> > package
> > functions are never virtual. This is by design.
> > 
> > - Jonathan M Davis
> 
> That explains it. I kind of wish I'd known that a few hours ago :) It would be nice if DMD would issue a warning when redefining a non-virtual function in a subclass. I might have to start using "override" more often despite force of habit and dislike of its verboseness. Perhaps "override" should be implied by default since it's by far the most common case. In cases where the programmer really wants to redefine the function non-virtually, there could be a keyword that would handle that. It could reduce extra typing and save beginners hours of stress :)

override will eventually be required when overriding a function. It is already if you compile with -w but not yet all of the time - though since protected isn't virtual and isn't really overriding anything, the compiler doesn't complain if you don't use override with it (though it will if you do, since it's not overriding anything). So, eventually (or now if you use -w), you will _always_ know whether a function is overiding another or not, because it will have to have override if it is and can't have it if it isn't.

- Jonathan M Davis

>
> override will eventually be required when overriding a function. It is already
> if you compile with -w but not yet all of the time - though since protected
> isn't virtual and isn't really overriding anything, the compiler doesn't
> complain if you don't use override with it (though it will if you do, since
> it's not overriding anything). So, eventually (or now if you use -w), you will
> _always_ know whether a function is overiding another or not, because it will
> have to have override if it is and can't have it if it isn't.
>
> - Jonathan M Davis

That's good to know. I'll start using -w from now on.
However, the problem still exists that if a function is meant to override a virtual method, but the method is not actually virtual, and the programmer forgets to type "override," the compiler won't complain and the error will slip through.

On 06/14/2012 01:34 AM, BLM768 wrote:
>
>>
>> override will eventually be required when overriding a function. It is
>> already
>> if you compile with -w but not yet all of the time - though since
>> protected
>> isn't virtual and isn't really overriding anything, the compiler doesn't
>> complain if you don't use override with it (though it will if you do,
>> since
>> it's not overriding anything). So, eventually (or now if you use -w),
>> you will
>> _always_ know whether a function is overiding another or not, because
>> it will
>> have to have override if it is and can't have it if it isn't.
>>
>> - Jonathan M Davis
>
> That's good to know. I'll start using -w from now on.
> However, the problem still exists that if a function is meant to
> override a virtual method, but the method is not actually virtual, and
> the programmer forgets to type "override," the compiler won't complain
> and the error will slip through.
>
>

True, but it will be explicit in the derived class code:
No 'override', no function that is overridden.

On Thursday, June 14, 2012 01:34:42 BLM768 wrote:
> > override will eventually be required when overriding a
> > function. It is already
> > if you compile with -w but not yet all of the time - though
> > since protected
> > isn't virtual and isn't really overriding anything, the
> > compiler doesn't
> > complain if you don't use override with it (though it will if
> > you do, since
> > it's not overriding anything). So, eventually (or now if you
> > use -w), you will
> > _always_ know whether a function is overiding another or not,
> > because it will
> > have to have override if it is and can't have it if it isn't.
> > 
> > - Jonathan M Davis
> 
> That's good to know. I'll start using -w from now on.
> However, the problem still exists that if a function is meant to
> override a virtual method, but the method is not actually
> virtual, and the programmer forgets to type "override," the
> compiler won't complain and the error will slip through.

True enough, but then all it takes is looking at the function to know that it's not overriding anything (because it doesn't have override on it), and naming a function the same in a derived class when it's not virtual generally isn't a good idea, since it's _not_ overriding anything - the same happens with both protected and private. But there's really no fix for that other than making it illegal to have a private or protected function with the same name in derived classes, which then effectively leaks some of the private/protected API of the base classes into the derived classes rather than hiding it. So, it's not like there's really a perfect solution.

IIRC, the docs are quite clear that protected is non-virtual, but it's probably easy to miss.

- Jonathan M Davis

>
> True, but it will be explicit in the derived class code:
> No 'override', no function that is overridden.

However, if a programmer expects it to override, there could be an issue. Imagine a novice D programmer who is not used to using "override" and looks at at the following code:

class Base {
    private:
    void test() {}
}

class Derived: Base {
    private:
    void test() {}
}

He/she would assume (as I did) that Derived.test virtually overrides Base.test because there's clearly no "final" attribute on Base.test. This subtle quirk could cause (as it did in my code) somewhat subtle and very frustrating bugs. It might be good to create a warning in this situation, then have a keyword that tells the compiler, "Yes, I really did mean to redefine a non-virtual function."

I guess that another solution to this whole mess is to just start requiring the use of override; then everyone would be educated and it would be obvious where the bug is in the code I posted. Since we don't want to break code, though, maybe there should be a message prominently displayed on the D homepage telling people to use the -w switch until the requirement is actually enforced for all code. If that requirement is going to be enforced, though, I can't think of a better time than now; otherwise, people are going to write code without "override" and eventually end up making the same mistakes that I did.

On 06/14/2012 01:57 AM, BLM768 wrote:
> I guess that another solution to this whole mess is to just start
> requiring the use of override; then everyone would be educated and it
> would be obvious where the bug is in the code I posted. Since we don't
> want to break code, though, maybe there should be a message prominently
> displayed on the D homepage telling people to use the -w switch until
> the requirement is actually enforced for all code. If that requirement
> is going to be enforced, though, I can't think of a better time than
> now; otherwise, people are going to write code without "override" and
> eventually end up making the same mistakes that I did.
>
>

Yes, this is the plan.