April 11, 2011, 6:44 a.m.
posted by madinteger
Prefer to define and inherit from abstract interfaces. An abstract interface is an abstract class made up entirely of (pure) virtual functions and having no state (member data) and usually no member function implementations. Note that avoiding state in abstract interfaces simplifies the entire hierarchy design (see [Meyers96] for examples).
Respecting the DIP implies that hierarchies should be rooted in abstract classes, not concrete classes. (See Item 35.) The abstract base classes must worry about defining functionality, not about implementing it. Put another way: Push policy up and implementation down.
The Dependency Inversion Principle has three fundamental design benefits:
The related Law of Second Chances states: "The most important thing to get right is the interface. Everything else can be fixed later. Get the interface wrong, and you may never be allowed to fix it." [Sutter04]
Typically, choose a public virtual destructor to enable polymorphic deletion (per Item 50), unless you use an object broker such as COM or CORBA that uses an alternate memory management mechanism.
Be wary about using multiple inheritance of classes that are not abstract interfaces. Designs that use multiple inheritance can be very expressive, but are harder to get right and easier to get wrong. In particular, state management is particularly hard in designs using multiple inheritance.