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principles:invariant_avoidance_principle [2013-03-11 16:30] christianprinciples:invariant_avoidance_principle [2021-10-18 21:53] (current) – +++ restored +++ christian
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-====== Invariant Avoidance Principle ======+====== Invariant Avoidance Principle (IAP) ======
  
 ===== Variants and Alternative Names ===== ===== Variants and Alternative Names =====
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 ===== Context ===== ===== Context =====
 /* fill in contexts here: */ /* fill in contexts here: */
-  * [[contexts:Object-Oriented Design]]  +  * [[contexts:Object-Oriented Design]]
  
 ===== Principle Statement ===== ===== Principle Statement =====
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 Methods typically have preconditions. Something that has to be true prior to invoking the method so it can work properly. Typical cases are parameters that may not be ''null'' or have to be in a certain range. A solution is better the fewer preconditions there are. Methods typically have preconditions. Something that has to be true prior to invoking the method so it can work properly. Typical cases are parameters that may not be ''null'' or have to be in a certain range. A solution is better the fewer preconditions there are.
  
-Furthermore there are (class) invariants, i.e. conditions that have to be true in all observable states during the whole lifetime of an object. Typical invariants are attributes that may not be ''null'' or have to be in a certain range, lists that have to contain certain objects with certain properties, etc. A solution better the fewer invariants there are.+Furthermore there are (class) invariants, i.e. conditions that have to be true in all observable states during the whole lifetime of an object. Typical invariants are attributes that may not be ''null'' or have to be in a certain range, lists that have to contain certain objects with certain properties, etc. A solution is better the fewer invariants there are.
  
 While preconditions and invariants are absolutely necessary, introducing further ones comes at a certain cost. While preconditions and invariants are absolutely necessary, introducing further ones comes at a certain cost.
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 ===== Origin ===== ===== Origin =====
  
-This principle is newly introduced here.+{{page>resources:A Principle Language for Object-Oriented Design#reference}}
  
  
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   * [[Fail Fast]] (FF): Breaking an invariant is a defect. And in such a case the software should fail fast.   * [[Fail Fast]] (FF): Breaking an invariant is a defect. And in such a case the software should fail fast.
   * [[Don't Repeat Yourself]] (DRY): Duplication of information, like having the same data in different representations or like caching values, creates invariants. So an invariant sometimes is a hidden DRY violation.   * [[Don't Repeat Yourself]] (DRY): Duplication of information, like having the same data in different representations or like caching values, creates invariants. So an invariant sometimes is a hidden DRY violation.
 +  * [[Low Coupling]] (LC): One type of precondition is that a specific method has to be called prior to another one. This also results in a temporal coupling.
  
 ==== Principle Collections ==== ==== Principle Collections ====
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 ==== Example 3: C++ References ==== ==== Example 3: C++ References ====
  
-Sompare the following two methods:+Compare the following two methods:
  
 <code c++> <code c++>
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 /* Choose one of the following and comment out the rest: */ /* Choose one of the following and comment out the rest: */
 /*[[wiki:Stub]]*/ /*[[wiki:Stub]]*/
-[[wiki:Incomplete]] +/*[[wiki:Incomplete]]*
-/*[[wiki:Complete]]*/+[[wiki:Complete]]
  
 ===== Further Reading ===== ===== Further Reading =====
  
 +  * {{page>resources:A Principle Language for Object-Oriented Design#reference}}
 +
 +===== Discussion =====
 +
 +Discuss this wiki article and the principle on the corresponding [[talk:principles:Invariant Avoidance Principle|talk page]].
  
principles/invariant_avoidance_principle.1363015843.txt.gz · Last modified: 2013-05-20 12:46 (external edit)