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Multiple to singleinheritancetransformation

June 15th, 2004

M. Dao, M. Huchard, T. Libourel,

A. Pons, J. Villerd

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Multiple inheritance: a new birth?

s OO programming languages and MI:Q"good" implementation in not so popular languages: Eiffel, CLOS, …Q"bad" implementation in once popular languages: C++, …?Q"no" implementation in today popular languages: Java (MI interfaces though), C#, …?

s Not much excitement about MI lately, but…

s …the modelling era has begun!QWhen you model, for instance in UML, you can use MI (almost) without moderation

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MI to SI transformation

s High-level MI models may eventually need to be transformed into SI models ("popular" OO languages, for instance)

s Two possible approaches (among others?)Q"combinatorial": cut inheritance links to obtain SI

–need to duplicate class properties–how to choose those inheritance links

–based on inheritance metrics (Roume 02)

Q"semantic": classify MI situations and transform MI into SI

–use UML annotations to precise different MI situations–apply the "most appropriate" inheritance transformation

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UML inheritance annotations

s Discriminators allow to group inheritance links that share a same semantic

s Constraints on subsets of instances of a class: complete, disjoint, incomplete, overlapping

s We propose new constraints:Qconcurrent (special case of overlapping)Qsuccessive, exclusive (special case of disjoint)QcombinedQalternative

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UML inheritance annotations

Employee

SalariedEmployee

ExemptEmployee

HourlyEmployee

VestedEmployee

UnvestedEmployee

:pension:status

VestedExemptEmployee

{disjoint,incomplete} {disjoint,complete}

DiscriminatorsConstraints

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MI transformations

s DuplicationQcut one inheritance link and duplicate formerly inherited properties in the subclass

s Nested generalizationsQcreate a class for each possible MI situation

s Direct linkQmake the leaf class directly inherit from root class

s Role aggregationQreplace one MI link by an aggregation

s Class mergeQreplace all classes with one

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Example of transformation

Employee

SalariedEmployee

ExemptEmployee

HourlyEmployee

VestedEmployee

UnvestedEmployee

:pension:status

VestedExemptEmployee

{disjoint,incomplete} {disjoint,complete}

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The right transformation for the right inheritance situation

s Our proposition:Qelaborate a list of possible MI "situations" based on UML extended annotations and other criteria (size, symmetry, etc.)Qfor each situation, propose a suitable MI transformation

s ExamplesQrole aggregation transformation might be used with complete constraint: the structure will not evolveQdirect link transformation might be used when symmetry is important for example in the case of the alternative constraint

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Perspectives

s elaborate a list of MI situations as complete as possibleQa situation might be a subsituation of another: classification

s try and find a most appropriate MI transformation for each situation

s difficulties:–be able to automatically detect MI situations–different MI situations may overlap–order of transformations?

s use of a model transformation language?

s study an "hybrid" approach (combinatorial and semantic)?