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Design pattern: Twin

Description: This design pattern provides an alternative for the use of multiple inheritance, based on defining two (or more) tightly coupled instances of the parent class whose protocol we want to inherit, hence the pattern name. Each instance refers to the other and is responsible for its parent protocol, forwarding to its twin other messages.

This pattern can be used with both classes and prototypes.

This pattern is not described in the GoF book. See e.g. the following Wikipedia page for details and references:

https://en.wikipedia.org/wiki/Twin_pattern

Logtalk supports multiple inheritance (and also multiple instantiation). But its support for categories often allows better solutions. This pattern provides a third alternative that can be preferable in some cases. When compared with a category-based solution, a key difference is that, with categories, each category protocol adds to the protocol of the object (importing the categories) while with this pattern each object must use message forwarding to its twin object (and thus one level of indirection). Note that this pattern implies a circular reference between the twin objects, which prevents some static binding optimizations.