On 12 Feb 2009, at 1:42 am, Jason Wiggins wrote:
How do I access the ivars of modelObject1 from modelObject2 and vice
versa?
Others have had plenty of useful advice to give on the general
question, but in your original, this stood out for me.
The answer here is: YOU DON'T. ivar stands for "instance variable" and
it should be private to the object that defines them. No other object
should ever access an object's ivars. This is the principle of
encapsulation. What you may have meant is how do I access another
object's *properties*. That's different. An object's properties are
not equivalent to its ivars. Often properties are implemented using
ivars, but they are not the same thing - how a property is implemented
is an internal detail of the object that has that property.
It's important to understand this difference, which is why I'm harping
on about it - terminology matters.
example:
An object has a property 'name' that returns a string. It matters not
to another object how 'name' is generated, it could be:
- (NSString*) name
{
return mName; // return the ivar that holds the name
}
or equally validly:
- (NSString*) name
{
return [NSString stringWithFormat:@"My name is: %@",
[[MyNameHelperClass stringElement] capitalizedString]];
}
In other words, the property is 'name' but it has nothing necessarily
to do with ivars.
To help enforce this, it's usually a good idea to set all ivars of an
object to be private using @private in your class definition. This
forces you to
ensure there are accessors for every one that you need to expose as a
property. Even subclasses are then forced to use the property
accessors and not the ivars (though that is occasionally inconvenient
and not what you want, but those cases are the exception not the rule).
Sorry for the somewhat tangential reply - it's not answering your main
question, but remember, terminology matters. Incorrect use of
terminology (as in conflating 'ivar' with 'property') typically
reveals incorrect or sloppy thinking about concepts.
--Graham
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