21 Mixin Chain

Consider that some $mixins are defined (the implementations are not shown):

$mixin M1<typename T> { [implementation] };
$mixin M2 { [implementation] };
$mixin M3 { [implementation] };
$mixin M4 { [implementation] };

In the following example, the mixin chain of class C refers to the ordered list of mixins [M1<int> M2 M3 M4 M1<float> M3].

$class C mixin [M1<int> M2 M3 M4 M1<float> M3] {};

Note that a given mixin can appear more than once in a mixin chain.

[M1<int> M2 M3 M4 M1<float> M3] can be regarded as a convenient shorthand for M3<M1<float,M4<M3<M2<M1<int,Base<C>>>>>>>, where Base<C> is some system defined template class instantiated with C that "seeds" the mixin chain.

The final class C is passed into the base of the mixin chain to implicitly support the curiously recurring template pattern for each mixin in the chain.

Mixins which in turn have mixin chains

A mixin can itself declare that it inherits from a mixin chain. For example

$mixin M5 mixin [M1<char> M2] {};

Anonymous mixins in the mixin chain

A mixin chain can directly contain anonymous mixins.