Module-level accessibility

[Jonathan M Davis]

On Monday 04 October 2010 11:26:17 Steven Schveighoffer wrote:
> What possible use case could private methods being polymorphic allow?
> 
> A private method can only be called by the class that contains the
> implementation.  Allowing base classes to call it makes no sense.
> 
> Make the method protected, it gives the desired effect (including for the
> example in the bug report as stated by the original reporter).
> 
> -Steve

It allows for the non-virtual interface pattern. You declare a final public 
function on the base clase which calls a private virtual one (which is probably 
abstract). Derived classes then override the private virtual method. This allows 
the base class to enforce things about the private virtual method - for instance 
pre and post conditions on the public final method would then always apply to the 
private virtual method. Only the base class can actually call the private 
virtual method, but it still allows for the derived classes to override its 
functionality. It's a useful idiom which D doens't currently support but which 
TDPL specifically discusses and claims that D supports.

Take at look at these for more info:

http://www.gotw.ca/publications/mill18.htm
http://en.wikibooks.org/wiki/More_C++_Idioms/Non-Virtual_Interface

- Jonathan M Davis