Interface vs Base class

Question

When should I use an interface and when should I use a base class?

Should it always be an interface if I don\'t want to actually define a base implementation of the

Answer 1

Interfaces

• Defines the contract between 2 modules. Cannot have any implementation.
• Most languages allow you to implement multiple interfaces
• Modifying an interface is a breaking change. All implementations need to be recompiled/modified.
• All members are public. Implementations have to implement all members.
• Interfaces help in Decoupling. You can use mock frameworks to mock out anything behind an interface
• Interfaces normally indicate a kind of behavior
• Interface implementations are decoupled / isolated from each other

Base classes

• Allows you to add some default implementation that you get for free by derivation
• Except C++, you can only derive from one class. Even if could from multiple classes, it is usually a bad idea.
• Changing the base class is relatively easy. Derivations do not need to do anything special
• Base classes can declare protected and public functions that can be accessed by derivations
• Abstract Base classes can't be mocked easily like interfaces
• Base classes normally indicate type hierarchy (IS A)
• Class derivations may come to depend on some base behavior (have intricate knowledge of parent implementation). Things can be messy if you make a change to the base implementation for one guy and break the others.