Design By Contract - Description

Description

The central idea of DbC is a metaphor on how elements of a software system collaborate with each other on the basis of mutual obligations and benefits. The metaphor comes from business life, where a "client" and a "supplier" agree on a "contract" that defines for example that:

• The supplier must provide a certain product (obligation) and is entitled to expect that the client has paid its fee (benefit).
• The client must pay the fee (obligation) and is entitled to get the product (benefit).
• Both parties must satisfy certain obligations, such as laws and regulations, applying to all contracts.

Similarly, if a routine from a class in object-oriented programming provides a certain functionality, it may:

• Expect a certain condition to be guaranteed on entry by any client module that calls it: the routine's precondition—an obligation for the client, and a benefit for the supplier (the routine itself), as it frees it from having to handle cases outside of the precondition.
• Guarantee a certain property on exit: the routine's postcondition—an obligation for the supplier, and obviously a benefit (the main benefit of calling the routine) for the client.
• Maintain a certain property, assumed on entry and guaranteed on exit: the class invariant.

The contract is the formalization of these obligations and benefits. One could summarize approach by the "three questions" that the designer must repeatedly ask about the contract:

• What does contract expect?
• What does contract guarantee?
• What does contract maintain?

Many languages have facilities to make assertions like these. However, DbC considers these contracts to be so crucial to software correctness that they should be part of the design process. In effect, DbC advocates writing the assertions first.

The notion of a contract extends down to the method/procedure level; the contract for each method will normally contain the following pieces of information:

• Acceptable and unacceptable input values or types, and their meanings
• Return values or types, and their meanings
• Error and exception condition values or types that can occur, and their meanings
• Side effects
• Preconditions
• Postconditions
• Invariants
• (more rarely) Performance guarantees, e.g. for time or space used

Subclasses in an inheritance hierarchy are allowed to weaken preconditions (but not strengthen them) and strengthen postconditions and invariants (but not weaken them). These rules approximate behavioral subtyping.

All class relationships are between client classes and supplier classes. A client class is obliged to make calls to supplier features where the resulting state of the supplier is not violated by the client call. Subsequently, the supplier is obliged to provide a return state and data that does not violate the state requirements of the client. For instance, a supplier data buffer may require that data is present in the buffer when a delete feature is called. Subsequently, the supplier guarantees to the client that when a delete feature finishes its work, the data item will, indeed, be deleted from the buffer. Other design contracts are concepts of "class invariant". The class invariant guarantees (for the local class) that the state of the class will be maintained within specified tolerances at the end of each feature execution.

When using contracts, a supplier should not try to verify that the contract conditions are satisfied; the general idea is that code should "fail hard", with contract verification being the safety net. DbC's "fail hard" property simplifies the debugging of contract behavior, as the intended behaviour of each routine is clearly specified. This distinguishes it markedly from a related practice known as defensive programming, where the supplier is responsible for figuring out what to do when a precondition is broken. More often than not, the supplier throws an exception to inform the client that the precondition has been broken, and in both cases—DbC and defensive programming—the client must figure out how to respond to that. DbC makes the supplier's job easier.

Design by contract also defines criteria for correctness for a software module:

• If the class invariant AND precondition are true before a supplier is called by a client, then the invariant AND the postcondition will be true after the service has been completed.
• When making calls to a supplier, a software module should not violate the supplier's preconditions.

Because the contract conditions should never be violated in program execution, they can be either left in as debugging code or removed from the production version of the code altogether for performance reasons.

Design by contract can also facilitate code reuse, since the contract for each piece of code is fully documented. The contracts for a module can be regarded as a form of software documentation for the behavior of that module.