Friends access to an arbitrary class?

Bill Baxter dnewsgroup at
Sun Jul 8 20:39:13 PDT 2007

Ald wrote:
> Suppose I have a hierarchy of classes, with base class Human.
> Each Human has a reference to a class of (super) type Algorithm, which contains the implementation of the Human's behavior.
> How can I give every Algorithm (and derivative) access to every private method and member to any Human (or derivative) that holds reference to that Algorithm?
> I tried inheriting from nested classes and from classes in the same module.

Being in the same module (i.e. same file) should work.  If FunkyHuman 
and CoolAlgorithm are in the same module CoolAlgorithm can access 
anything in FunkyHuman.  Are you saying you tried that and it didn't work?

Anyway, you might be barking up the wrong tree.  Having a member that 
knows how to manipulate every private attribute of it's owner sounds to 
me more like a use case for a mixin.  I.e. reuse Algorithms by making 
them an actual part of the classes you want them to act on.

template CoolAlgorithm()
    void act_cool() {
       mode = COOL;
class FunkyHuman
   mixin CoolAlgorithm;  // FunkyHuman now has an act_cool() method

   int mode = UNCOOL;

You might even be able to make the mixin an alias template parameter so 
you can create FunkyHumans with different algorithms (don't recall if 
that works actually...)

class FunkyHuman(alias AlgoMixin)
    mixin AlgoMixin;

If you need to unplug and replace Algorithms at runtime, then it seems 
like you need to reconsider whether the parts of Human they need access 
to are really private.  If external objects need to access it, then it 
sounds more like it's part of the public interface.


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