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Design by Contract
A Conversation with Bertrand Meyer, Part II
by Bill Venners
December 8, 2003

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Design by Contract

Bill Venners: Could you give an overview of Design by Contract?

Bertrand Meyer: People who have heard a little bit about contracts often think they are simply a way to put the equivalent of assertions in a program: that is to say, to put in a few checks here and there for conditions that are expected to hold at specific points of execution. Such assertions are basically debugging statements. The assertion code is executed during debugging, and if one of the conditions is found not to hold, execution usually stops with a message. That's one of the things you can do with contracts, but it is only a small subset of the purpose of contracts.

The main purpose of contracts is to help us build better software by organizing the communication between software elements through specifying, as precisely as possible, the mutual obligations and benefits that are involved in those communications. The specifications are called contracts. The underlying observation, which is not particularly deep, is that a software system is made of a number of elements that cooperate with each other. In an object-oriented architecture, these elements might be classes and methods, which I prefer to call routines. The elements might be something else in a different programming model, but let's just assume we are working in an object-oriented context. So we have a software system made of a number of classes, and these classes themselves contain among other things routines that are going to be executed and call each other. So both the architecture of the system and its execution rely on a large set of possible communication channels between the software elements. The metaphor of contracts is used to guarantee that these communications occur not on the basis of vague expectations of services rendered, but on the basis of precise specifications of what these services are going to be.

Typically in such a communication between program elements—for example, when one routine calls another routine—it's like a client/supplier relationship. The calling routine, which we will call the client, is performing some computation, probably for the need of its own client. In order to perform that computation, that is to say, to perform its own service, it needs services from some other software element, typically some other routine. So there is a client/supplier relationship, where the client needs a certain service and the supplier provides that service. And in the basic scheme both the client and the supplier are routines.

For the software and software designer to be able to guarantee any kind of correctness and robustness properties, they must know the precise constraints over such communications. This is where the contract metaphor from business applies to software. Say I'm contracting from you to have a certain business operation performed on my behalf . For example, perhaps you have a part that I can use for a product that I'm manufacturing. So I'm your client, and you're my supplier. In business, we're going to organize our collaboration on the basis of a contract: a precise statement of the mutual obligations and benefits that we'll expect. In software, we're going to do exactly the same thing when we write client and supplier routines.

So we're going, for example, to impose on the client certain obligations as to the kind of original program state that is permissible when the client calls the supplier or the kind of arguments that the client routine passes to the supplier. These are pre-conditions, and they're obligations for the client. In the other direction, we are going to express the conditions that the supplier routine must guarantee to the client, on completion of the supplier's task. That's the post-condition of the contract, specifically, the post-condition of that particular routine. And of course, the post-condition is an obligation for the supplier.

Pre- and post-conditions are symmetric. The pre-condition of the routine is an obligation for the client, because the pre-condition says that before calling a particular routine, the client must satisfy a particular property. For the supplier, the pre-condition is a benefit, because it facilitates the supplier's job by restricting the set of cases that the supplier has to handle. The obligation of the client is a benefit for the supplier. The post-condition is of course an obligation for the supplier, because it describes the job that the supplier has to guarantee to perform. But the post-condition is clearly a benefit for the client, because it describes the result that the client is entitled to expect from the execution of the routine for the benefit of its own problem set.

Before moving on I can describe a very simple example of pre- and post-conditions: a routine that computes the square root of a real number. The pre-condition would say that the real number has to be non-negative, because otherwise the result would be undefined. The post-condition may say that the result returned by the routine is the approximation, within a specified precision, of the exact mathematical square root of the routine.

So pre- and post conditions are two fundamental elements of contracts. The third fundamental element—which is really useful mostly in an object-oriented context, where we have not only routines but at the higher level, classes—is invariants. A class invariant is a condition that applies to an entire class. It describes a consistency property that every instance of the class must satisfy whenever it's observable from the outside. That means that this property, the class invariant, must be satisfied whenever an instance of the class is created. And every exported routine, routines that can be called from the outside of the class, must preserve it. That is to say, assuming the class invariant was satisfied before the routine was called, the routine must ensure that class invariant is again going to be satisfied on exit. It's as if the class invariant is added to the pre- and post-condition of every single exported routine of the class. But more fundamentally, a class invariant is a way to characterize the fundamental consistency and integrity properties of the class and its instances.

For example, if you have a bank account class with a field that represents the current balance and a field that contains a list of all deposits and withdrawals since the opening of the account, then you would have a class invariant that states that the value of the balance field is equal to the total of all the deposit values so far minus the total of all the withdrawal values so far. It's typical that class invariants define properties that indicate how the various constituents of the class, such as balance and deposit/withdrawal lists, maintain consistency with each other.

Pre- and post-conditions and class invariants are not the only elements of contracts. There are also loop invariants and a few others, but pre- and post-conditions and class invariants are really the basic fabric of contracts. In my experience, relying on these notions—that is to say, making sure when you write software that don't just write the implementation, but also write the more abstract properties underlying the implementation in the form of contracts—provides a greatly added software development experience in several respects. It helps ensure correctness in the first place, helps debugging, helps testing, helps ensure inheritance is properly handled, helps managers, provides a quite effective form of documentation, and a few others.

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