D mixins

[Tom S]

janderson wrote:
> I'm trying to explain to a friend on msn what mixins are.  Can you
> provide some good examples, that would be difficult otherwise?
> 
> -Joel

They're a convenient tool for inserting parametrized code into scopes - 
in some cases this can be partially accomplished in C++ through the 
(ab)use of multiple inheritance. For instance:

template <typename SomePolicy>
struct Foo : SomePolicy {
};

could be coded in D as:

struct Foo(SomePolicy) {
     mixin SomePolicy;
}

It doesn't seem like a big win in this case, but when the Policy must 
use the Foo struct for some reason, the C++ approach would be to 
specialize the policy on the struct's type, like
struct Foo : SomePolicy<Foo>

while in D, the mixin just knows about the struct's type because mixing 
it in means that it becomes a part of the struct's declaration, so for 
instance typeof(this) inside a mixin will work just like in the struct 
itself.

D further benefits from the fact that mixins can be easily named - it 
helps avoid naming conflicts.


--
Tomasz Stachowiak