Fifo<>::Element is missing a virtual destructor

[Norman Feske]

Hi Stefan,

> This won't compile because Fifo<>::Element is missing a virtual destructor.
> 
> Is there any reason for this? What do you think about adding the virtual
> destructor to Fifo<>::Element?

there are two reasons, efficiency and design:

* By adding a virtual destructor to 'Fifo<>::Element', each data type
  organized in a fifo would forcibly contain a vtable regardless of
  whether it contains virtual methods or not.

* The 'Element' type is merely needed as a place for the fifo's
  metadata. It is a technicality with no meaningful interface for
  anything except the 'Fifo' implementation. Hence, by publicly
  inheriting the "interface" of 'Fifo<>::Element', you'd pollute
  the interface of the derived class, adding complexity with no
  benefit.

The solution is the use private instead of public inheritance, thereby
excluding 'Fifo<>::Element' from the interface of the derived class.
Since the derived type cannot be casted to the 'Fifo<>::Element' type
now, the virtual destructor is not needed. This makes the compiler happy.

The only remaining problem is that the 'Fifo<>' needs of course access
to the 'Fifo<>::Element' interface. This can be granted by making the
'Fifo' a friend of the derived class.

For an example, take a look here:


https://github.com/genodelabs/genode/blob/master/repos/base/src/lib/ldso/include/linker.h#L138

Alternatively, you may consider the use of 'Fifo_element', which avoids
inheritance altogether.

BTW, the same pattern is at work for 'Genode::List'. So you can find
many more examples by grep'ing for "friend.*List".

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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