Decouple package modules with package interfaces

[Jonathan M Davis]

On Tuesday, September 24, 2013 08:54:42 simendsjo wrote:
> I just came across a problem where I either has a cyclic
> dependency, pass around three delegates, implement an interface
> or somehow hack some communication between two modules.
> 
> A simple way would be to add an interface, but interfaces require
> implementations to be public. The methods should be package
> protected though.
> 
> It would have been nice if the implementation for an interface
> wouldn't have to be more visible than the interface itself.
> 
> If an interface is "package", it cannot escape that package, so
> why would the implementation required to be public? The same goes
> for "private".
> 
> Should I add an enhancement request for this, or does it have
> problems I'm not seeing?

Only public and protected are virtual. private and package are never virtual. 
This is by design. As such, it is impossible for an interface to be anything 
but public or protected and actually work.

There has been some discussion of making it so that non-virtual is the 
default, in which case, we'd have the virtual keyword, and it would be 
required to make a function virtual (similar to C++). If we ever actually go 
that route, then it may become possible to make package and private functions 
virtual (or it may not - that will depend on what's decided at that point), 
but it's definitely not going to happen if we don't go that route.

So, if you want to use interfaces, you need to be using public or protected to 
implement them. package and private are not an option.

- Jonathan M Davis