CFD Online Discussion Forums - Multiple inheritance paradigms in OpenFOAM

Merry Christmas everyone,

from time to time, by going through the OF code (v16.12), I see some crazy (to me) inheritance hierarchies combined with multiple inheritance. One example is the class template reactingMixture (src/thermophysicalModels/reactionThermo/mixtures/reactingMixture):

Code:

template<class ThermoType>

class reactingMixture

:

    public speciesTable,

    public autoPtr<chemistryReader<ThermoType>>,

    public multiComponentMixture<ThermoType>,

    public PtrList<Reaction<ThermoType>>

{

    // ...

    // declarations

    // ...

};

I've learned that public inheritance can be understood as "is a" relationship (here reactingMixture<ThermoType> is a autoPtr<chemistryReader<ThermoType>>, for instance). The previously posted code does not seem to fit this principle, as the "is a" criterion does not fit for most of these base classes.

My question: Is there a reason for that (some kind of paradigm I do not know about)? What are the advantages of such design? I thought that usually this kind of problem would be solved through composition. Am I just not seeing the big picture?

Thanks for your answers
RP