Interfaces vs. abstract classes

[sap]

Interfaces vs. abstract classes

[marcellourbani]

IMHO the main argument is protocol enforcement.
An interface can't possibly inherit anything.
A virtual class is one click away from becoming instantiable. And is relatively easy to use code which was not supposed to.

As an example, I create an abstract class and derive an child for real usage and one for unit testing.
Then someone either:

• adds some SQL in the virtual class -> my tests now are database dependent and maybe even alter it
• adds an utility method as public (perhaps so it can be tested) -> interface pollution

Both problems are easy to avoid if everyone knows and follows the rules, but it's a big if

If I have code I want to share between implementations I'd rather put it in another class which then I call from the real implementation, but I don't want parent relations between my test and real classes

[bjoern-jueliger-sap]

Was fixed with #75