Computer Science Department
School of Computer Science, Carnegie Mellon University


CZ: Multiple Inheritance Without Diamonds

Donna Malayeri, Jonathan Aldrich

December 2008

Complemented by Computer Science Department
Technical Report CMU-CS-09-153.pdf


Keywords: Multiple inheritance, multiple dispatch, diamond inheritance

Multiple inheritance has long been plagued with the "diamond" inheritance problem, leading to solutions that restrict expressiveness, such as mixins and traits. Instead, we address the diamond problem directly, considering two important difficulties it causes: ensuring a correct semantics for object initializers, and typechecking multiple dispatch in a modular fashion–the latter problem arising even with multiple interface inheritance. We show that previous solutions to these problems are either unsatisfactory or cumbersome, and suggest a novel approach: supporting multiple inheritance but forbidding diamond inheritance. Expressiveness is retained through two features: a "requires" construct that provides a form of subtyping without inheritance (inspired by Scala [29]), and a dynamically-dispatched "super" call similar to that found in traits. Through examples, we illustrate that inheritance diamonds can be eliminated via a combination of "requires" and ordinary inheritance. We provide a sound formal model for our language and demonstrate its modularity and expressiveness.

33 pages

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