Metaclass composition using mixin-based inheritance

In the context of meta-programming and re ective languages, classes can be treated as full edged objects which are instances of other classes named metaclasses. Meta-classes have proved to be useful for dening new class properties. Examples of such properties are lazy memory allocation, multiple inheritance, having a single in-stance... A class with some property can be obtained by instantiating a metaclass which implements the desired property. However, instantiation allows assigning to a class only properties dened by a single metaclass. A composition mechanism is needed in order to reuse properties dened by dierent metaclasses and assign them to a given class. This composition should be performed without breaking class-metaclass compatibility. The compatibility issue arises when a class is coupled to its metaclass. So, when composing metaclasses, we need to take care of such coupling in order to avoid run-time exceptions. In this paper, we explore the use of mixin-based inheritance to perform metaclass composition. Mixin-based inheritance is an interesting alternative to both single and multiple inheritance. As opposite to single inheritance, it allows code reuse among dierent class hierarchies. Contrary to multiple inheritance, it allows developers to explicitly specify the desired behavior through explicit linearisation. Our proposal is to dene and compose reusable class properties by introducing mixins at the metaclass level. We demonstrate that this introduction can be done eciently and without altering compatibility