Deriving from Scrollbar

[SimulatedKey]

Hi Jules,

I wanted to create a class deriving from the Scrollbar to add functionality: a method and some data members to occasionally (on change message) load and store data to be visualized in the background of the scrollbar by a custom LookAndFeel.

So two quick questions,

• Would it be possible to have a virtual destructor on the Scrollbar class to make it extensible?

• Would it be possible to have a method to use a custom scrollbar class in viewport?

Thank you!

[jules]

Would it be possible to have a virtual destructor on the Scrollbar class to make it extensible?

Sigh… I see this misunderstanding so often. When any method overrides a virtual method, it becomes virtual by default. You don’t need to keep adding the word ‘virtual’ in every derived class.

Would it be possible to have a method to use a custom scrollbar class in viewport?

Not sure, I don’t like bloating classes with custom stuff like that. But I’m soon going to need to add support for more modern scrollbars (i.e. minimalist ones like on OSX) so may be making some changes that will help you.

[jrlanglois]

When any method overrides a virtual method, it becomes virtual by default. You don’t need to keep adding the word ‘virtual’ in every derived class.

Yes but, shouldn’t the destructor be virtual in a class to consider it a base-class (as SimulatedKey asked in the first question)?

[TheVinn]

@jrlanglois

shouldn’t the destructor be virtual in a class to consider it a base-class?

It is:

virtual ~Component();

[jrlanglois]

juce::Scrollbar’s destructor is not virtual. Derivatives of a juce::Scrollbar would not be castable to the Scrollbar base-class, nor would a juce::Scrollbar* to a derived class.

[TheVinn]

@jrlanglois

juce::Scrollbar’s destructor is not virtual

@jules

When any method overrides a virtual method, it becomes virtual by default.

[jrlanglois]

This is, strangely, news to me, so I decided to look this “C++ feature/rule” up. If you don’t mind me sharing my finding: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf

10.3 Virtual functions

2. If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly
   from Base, a member function vf with the same name, parameter-type-list (8.3.5), and cv-qualification as Base::vf
   is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides103) Base::vf. For
   convenience we say that any virtual function overrides itself. Then in any well-formed class, for each virtual function
   declared in that class or any of its direct or indirect base classes there is a unique final overrider that overrides that
   function and every other overrider of that function. The rules for member lookup (10.2) are used to determine the final
   overrider for a virtual function in the scope of a derived class but ignoring names introduced by using-declarations.

[SimulatedKey]

Ah, right, I forgot that, of course, the scrollbar inherited from Component. Brain mis-fire.

Thanks Jules, will look for changes in the future!

-Trevor