Multiple inheritance and a broken super() chain

[Chris Angelico]

On Wed, 5 Jul 2023 at 10:31, Greg Ewing via Python-list
<python-list at python.org> wrote:
>
> On 5/07/23 10:33 am, Alan Gauld wrote:
> > (*) C++ is the odd one out because it doesn't have GC, but then
> > neither does it have an Object superclass so very often MI in C++
> > does not involve creating diamonds! And especially if the MI
> > style is mixin based.
>
> Even if all your mixins have empty constructors, in C++ there
> is still a diamond problem if they have any data members, because
> you end up with multiple copies of them.
>
> But C++ has the concept of virtual base classes, which avoids the
> diamond problem, albeit at the expense of making you explicitly
> call all the base class constructors in your ancestry.

Yeah, non-virtual MI in C++ is basically composition with a shorthand
for calling non-conflicting methods or accessing non-conflicting data
members. Point of random interest: Pike actually allows that sort of
"composition MI" but will give you back an array of all parents when
you seek a superclass's method, giving a very elegant syntax for MI.

inherit Thing1;
inherit Thing2;

void method() {
    ::method(); //this is actually calling an array of two methods
}

Python's way of doing it requires that every class choose to cooperate
in the MI and then be aware that they are all operating on the same
object. Pike's and C++'s can sometimes be used as composition in
disguise, but in general, MI does require proper cooperation.

ChrisA