The Dependency Inversion Principle

Object Mentor SOLID Design Papers

by Robert C. Martin

This is the third of my Engineering Notebook columns for The C++ Report. The articles that will appear in this column will focus on the use of C++ and OOD, and will address issues of software engineering. I will strive for articles that are pragmatic and directly useful to the software engineer in the trenches. In these articles I will make use of Booch's and Rumbaugh's new unified Modeling Langage (UML Version 0.8) for documenting object oriented designs. The sidebar provides a brief lexicon of this notation.

My last column (Mar, 96) talked about the Liskov Substitution Principle (LSP). This principle, when applied to C++, provides guidance for the use of public inheritance. It states that every function which operates upon a reference or pointer to a base class, should be able to operate upon derivatives of that base class without knowing it. This means that the virtual member functions of derived classes must expect no more than the corresponding member functions of the base class; and should promise no less. It also means that virtual member functions that are present in base classes must also be present in the derived classes; and they must do useful work. When this principle is violated, the functions that operate upon pointers or references to base classes will need to check the type of the actual object to make sure that they can operate upon it properly. This need to check the type violates the Open-Closed Principle (OCP) that we discussed last January.

In this column, we discuss the structural implications of the OCP and the LSP. The structure that results from rigorous use of these principles can be generalized into a principle all by itself. I call it "The Dependency Inversion Principle" (DIP).

A. HIGH LEVEL MODULES SHOULD NOT DEPEND UPON LOW LEVEL MODULES. BOTH SHOULD DEPEND UPON ABSTRACTIONS.
B. ABSTRACTIONS SHOULD NOT DEPEND UPON DETAILS. DETAILS SHOULD DEPEND UPON ABSTRACTIONS.

12 pages, ebook

First published May 1, 1996

About the author

Robert Cecil Martin, commonly called Uncle Bob, is a software engineer, advocate of Agile development methods, and President of Object Mentor Inc. Martin and his team of software consultants use Object-Oriented Design, Patterns, UML, Agile Methodologies, and eXtreme Programming with worldwide clients.

He was Editor in Chief of the C++ Report from 1996 to 1999. He is a featured speaker at international conferences and trade shows.

Reviews

[Keith · Rating 3 out of 5]

Not the most clear elucidation of dependency inversion I've seen. I begin to think that Uncle Bob is unlikely to become my favorite author on Agile methodology. I found Inversion of Control Containers and the Dependency Injection Pattern to be far more useful.