CSC/ECE 517 Fall 2011/ch18 6a sc

Programming by Contract

Programming by contract

Programming by Contract or Design by Contract (DbC) was first introduced by Bertrand Meyer, the creator of Eiffel programming language. Although Eiffel has implemented assertions as built in DbC support, the concepts can be applied in any language. It uses pre-conditions and post-conditions to document or programmatically assert the change in state caused by a piece of a program. (1)

History and Background

Programming by Contract or Design by Contract (DbC) has its roots in work on formal verification, formal specification and Hoare logic. The original contributions includes:

• A clear metaphor to guide the design process
• The application to inheritance, in particular a formalism for redefinition and dynamic binding
• The application to exception handling
• The connection with automatic software documentation

Methodology

Programming by Contract creates a contract between the software developer and software user - in Meyer's terms the supplier and the consumer.

Before enters a method or routine, a pre-condition that must be satisfied by the consumer of the routine. Each routine ends with post-conditions which the supplier guarantees to be true (if and only if the preconditions were met). Also, each class has an invariant which must be satisfied after any changes to an object represented by the class. In the other words, the invariant guarantees the object is in a valid state. (2)

References

1. Cunningham & Cunningham, Inc., Design by Contract
2. University of North Carolina
http://en.wikipedia.org/wiki/Design_by_contract
http://www.cs.unc.edu/~stotts/204/contract.html
http://www.cs.usfca.edu/~parrt/course/601/lectures/programming.by.contract.html