Gaudi Interfaces

Gaudi component classes (Algorithms, Services, Tools, etc) are concrete implementations of an abstract Interface class.

What are interface classes?

Interface classes define a set of methods and imply certain behavior that concrete implementations must provide.

How are they beneficial?

Factoring the code this way leads to some practical benefits.

Reduced compilation dependencies
User code accesses a component object only through the interface class. This means their code must only "#include" the interface class header file. This header is unlikely to ever change and thus user code need not be recompiled when the concrete implementation code changes.
Central registration
interface implementations register themselves so they can be looked up by name by other code. This further reduces coupling between codes and lends support to configuration mechanisms.
Support scripting
because interfaces provide well a defined collection of methods their concrete implementations can be easily accessible from Python. One needs only to make the interface accessible and all implementations can then be accessed.
Faster casting
A concrete component may implement one or more interfaces. Standard C++ would require a dynamic_cast to go from one interface to another. Gaudi interfaces have a faster mechanism (queryInterface) that can use static casts and which ends up being faster. [In practice this is not a huge benefit, but is one nonetheless].