Re^2: The Pretty Good Idea of Demeter (Was: Definition of Law of Demeter)

To me, the a->b->c->d->foo() example just tells me that this needs to be factored out into a routine somewhere. Putting it in a's class is not necessarily the right thing, perhaps it just needs to be in a function local to the module that uses 108 times (of course if those 108 uses are spread all over then it probably should be in a's class).

Choosing the correct class to put it in may be difficult. It may be useful to call x->b->c->d->foo() for lots of different xs that don't necessarily share a base class and adding in a mixin class to all those classes may not be not possible.

Many examples of LoD are really about removing accessors. I think it's better to replace a->mother->brother->child->foo() with a->cousin->foo(). Do not replace it with a->foo_the_cousin() because chances are you'll want to call other methods on the cousin and also because it's not much of a saving in terms of typing (in this case it's not a saving at all).

So my sugestion for a better LoD is Consider replacing chains of accessor methods with a single method

Comment on Re^2: The Pretty Good Idea of Demeter (Was: Definition of Law of Demeter) Select or Download Code

Replies are listed 'Best First'.
Re^3: The Pretty Good Idea of Demeter (Was: Definition of Law of Demeter) by Aristotle (Chancellor) on Jan 17, 2005 at 22:30 UTC
Depends. In other cases it might still be appropriate to replace `a->b->foo` with `a->foo`. The question is: does the caller need to be aware of the has-a relationship, or is he really just requesting a service from object a? In that case, the presence of object b would be an implementation detail, and shouldn't be exposed just because b is also an object. The lesson taught by Demeter is really don't expose your inner objects without good reason. I believe what came to be called the Law was formed in the context of larger class frameworks. In this environment, I would guess, people are likely not to think about distinguishing between public and private class relationships. A reminder is then necessary that relationships are part of the published interface of a class no less than methods and properties are, and so require equal consideration. Makeshifts last the longest.	[reply] [d/l] [select]
Re^3: The Pretty Good Idea of Demeter (Was: Definition of Law of Demeter) by jplindstrom (Monsignor) on Jan 16, 2005 at 14:52 UTC
I like your example with mother->brother->child and cousin. Not because it reduces the amount of keystrokes, but because it adds abstraction and meaning. This may be the real gift of the LoD, that when applied properly it's an opportunity to clarify the intentions and the meaning of the relationship between mother, brother, and child. /J	[reply]