{"id":13499,"date":"2025-06-02T21:58:11","date_gmt":"2025-06-02T19:58:11","guid":{"rendered":"https:\/\/monodes.com\/predaelli\/?p=13499"},"modified":"2025-06-02T21:58:11","modified_gmt":"2025-06-02T19:58:11","slug":"design-by-contract-dbc-v-test-driven-design-tdd-wolands-cat","status":"publish","type":"post","link":"https:\/\/monodes.com\/predaelli\/2025\/06\/02\/design-by-contract-dbc-v-test-driven-design-tdd-wolands-cat\/","title":{"rendered":"Design-by-Contract (DbC) v Test-Driven Design (TDD) | Woland’s cat"},"content":{"rendered":"

Design-by-Contract (DbC) v Test-Driven Design (TDD) from\u00a0 Woland’s cat<\/a><\/em><\/p>\n

shortly: DbC = contracts + tests<\/strong><\/p>\n

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Design-by-Contract (DbC) v Test-Driven Design\u00a0(TDD)<\/a><\/h2>\n
Posted on<\/span> 04\/03\/2021<\/span><\/a> by<\/span> wolandscat<\/a><\/span><\/div>\n
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A software contract in the Eiffel language<\/figcaption><\/figure>\n

Another bit of software engineering knowledge from my archive relates to two well-known formal quality methods used in software development. This is from a presentation made at ETH Zurich in 2010.<\/p>\n

<\/span>The background to this is my use of the Eiffel language for 25 years, in which Design by Contract (DbC) figures strongly. Its creator, Bertrand Meyer (currently provost Chair of Software Engineering at Schaffhausen Institute of Technology, Switzerland) coined the term, and developed DbC from earlier formulations such as in the Z language, Djikstra etc.<\/p>\n

For those not familiar with \u2018contracts\u2019, the full form consists of routine pre- and post-conditions and class invariants. An example of a contract is shown below:<\/p>\n

fill<\/em> is<\/strong>\n        -- Fill tank with liquid\n    require<\/strong>\n        in_valve.open\n        out_valve.closed<\/em>\n    deferred<\/strong>\n        -- i.e., no implementation\n    ensure<\/strong>\n        in_valve.closed\n        out_valve.closed\n        is_full<\/em>\n    end<\/strong><\/pre>\n
The contract consists of the require and ensure predicates. The Eiffel website has a full explanation<\/a>.<\/p>\n
DbC was built into Eiffel (since 1988), and has some native supported in some languages like Ada 2012, Clojure, and Kotlin. Many other languages have had additional frameworks or tools created to add contracts, e.g. Java, C#, Groovy and so on.<\/p>\n
However, it has to be said that the culture of contract-oriented has not become mainstream, which those of us who use them routinely find strange \u2013 they catch so many errors in testing that it is clear they greatly add to the quality of software.<\/p>\n
Some experts think that Test-Driven Design (TDD) is a better alternative, since can be used to force developers to prove that each function they write passes an appropriate test, before they write more code. Personally, although I think routine use of unit testing via built-in frameworks is useful, TDD doesn\u2019t replace DbC, and indeed, it can lead some developers to think their software, if passing all its tests, is \u2018done\u2019.<\/p>\n
Understanding the difference between DbC and TDD is instructive to understanding why TDD, although very useful, doesn\u2019t tell you if your software is correct.<\/p>\n
Here\u2019s the basic difference:<\/p>\n