Why private methods cant be virtual?

claptrap clap at trap.com
Tue Sep 22 21:16:21 UTC 2020

On Tuesday, 22 September 2020 at 13:19:10 UTC, Daniel Kozak wrote:
> On Tue, Sep 22, 2020 at 3:05 PM claptrap via 
> Digitalmars-d-learn < digitalmars-d-learn at puremagic.com> wrote:
>> The thread title is...
>> "Why private methods cant be virtual?"
>> IE Not...
>> "how do I override private functions in a non-polymorphic 
>> manner."
>> And what you suggest wont work because I was asking about 
>> virtual functions, so I specifically want polymorphism. And 
>> FWIW it's no big deal I can just use protected, i wasn't 
>> looking for a solution, I was looking for an explanation as to 
>> why it was done that way. But apparently there is none.
> And I did not try to show solution. It was just an answer to 
> this part of your response:
> So final private functions can be overriden? It seems not, but 
> the sentence is definitely confusing if not just plain wrong.
> So the reason why there is this:
> "Functions marked as final may not be overridden in a derived
>  class, unless they are also private"
> Is because with private methods final keyword has no meaning.

Its not that final has no meaning for private methods, but that 
final has no meaning for non-virtual methods, and private methods 
happen to be non-virtual. IE. It's a side effect of making 
private methods non-virtual, not a direct effect of them being 

And lets be honest, overriding a virtual method is a different 
thing to "overriding" or rather hiding a non virtual one.

It's mistake to use the same terminology for both cases.

> And there is a reason "Why private methods cant be virtual?".
> It is because it would break existing code.

Why would it break existing code?

> And because  private methods are final it makes them fast.
>  And yes compiler probably could findout that method could be 
> made non-virtual but I am not sure how easy is this and how it 
> would slow down compilation times

Testing it out on compiler explorer it seems neither LDC or DMD 
de-virtualize a simple case, a class with one method, no decedent 

So maybe the docs shouldn't say that it does so.

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