CSC/ECE 517 Fall 2007/wiki3 2 dp

Topic

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Find some concise examples that illustrate the principle of programming by contract, and are accessible to a general audience of programmers.

Definition

Programming by contract is a software development approach that enforces the developers to define concrete and verifiable interface specifications for the software code. It is based on the theory of a business contract.

Norms of the contract

The core idea of programming by contract is how components of a software system collaborate with each other on the basis of mutual obligations and benefits. The concept is drawn from the business domain where clients and suppliers agree on a contract that defines certain norms. The corresponding norms that one can associate with a software system are –

•	Preconditions - Certain obligations need to be fulfilled by any module which calls the software 
component are called as  pre-conditions. The module making the call is called the client. The preconditions
are the obligations that the client needs to fulfill and are benefits for the software component, called the 
supplier, as it frees the supplier from having to handle cases outside of the precondition.

•	Postconditions - The supplier in turn, needs to guarantee certain outputs or certain properties on 
exit. These set of properties are called the supplier's postconditions. These postconditions are obligations 
for the supplier, and benefits for the client obtained from calling the supplier module.

•	Class invariant - The supplier needs to maintain a certain property that is assumed to exist on entry 
and is guaranteed on exit, called the class invariant.