Design by contract - Wikipedia, the free encyclopedia. A design by contract scheme. Design by contract (Db. C), also known as contract programming, programming by contract and design- by- contract programming, is an approach for designing software.
It prescribes that software designers should define formal, precise and verifiable interface specifications for software components, which extend the ordinary definition of abstract data types with preconditions, postconditions and invariants. These specifications are referred to as . Where this assumption is considered too risky (as in multi- channel client- server or distributed computing) the opposite . Eiffel Software applied for trademark registration for Design by Contract in December 2. December 2. 00. 4.
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This can be summarised by the . However, Db. C considers these contracts to be so crucial to software correctness that they should be part of the design process. In effect, Db. C advocates writing the assertions first. Contracts can be written by code comments, enforced by a test suite, or both, even if there is no special language support for contracts. 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. These rules approximate behavioural subtyping.
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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 .
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 .
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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. Db. C 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. 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.
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Performance implications. Contracts are therefore typically only checked in debug mode during software development.
Later at release, the contract checks are disabled to maximize performance. In many programming languages, contracts are implemented with assert. Asserts are by default compiled away in release mode in C/C++, and similarly deactivated in C#. This effectively eliminates the run- time costs of contracts in release. Relationship to software testing. Rather, it complements external testing with internal self- tests that can be activated both for isolated tests and in production code during a test- phase.
The advantage of internal self- tests is that they can detect errors before they manifest themselves as invalid results observed by the client. This leads to earlier and more specific error detection.
Language support. Loki Library provides a mechanism named Contract. Checker that verifies a class follows design by contract. C# (and other . NET languages), via Code Contracts (a Microsoft Research project integrated into the . NET Framework 4. 0)DELPHI PRISM. A permanent change to Python to support Design by Contracts was proposed in PEP- 3. Ruby, via Brian Mc.
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