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Why optional properties in a class are represented as “has a” relationship?


In page 40, the class AddressViewController define:

   public var address: Address? //Model 

For me, this is not a strong reference to the model, so it should be “use” or “depends” or “delegates to”.

At being optional it could be injected when needed. even further, it could be a protocol so you are not going to programming to an instance but a protocol.

Please, could someone explain it? Thanks.


@andreskwan Thanks very much for your question!

In Computer Science/Object Oriented Programming terminology, there are two types of relationships that are defined:

  1. “Is a” relationship. This would refer to inheritance, and “type” of a particular object.
  2. “Has a” relationship. This refers to properties that an object contains

It is for this reason the address variable would be referred to as a “has a” relationship. Moreover, these terms are quite old, and very generic, so they predate Optionals. I believe the term “delegates to” is an incorrect reference to this property, since “delegate” has a very specific meaning in iOS.

The class HAS an address property. Whether it actually USES it (because it is nil) is another matter. The fact that a property can be optional should not negate the fact that it is a property belonging to a particular class. An example would be a bank account that belongs to an individual. This bank account is allowed to carry an amount of $0.00. The fact that the account is allowed to have no money does not negate the fact that it is still a bank account, which belongs to a particular person. :slight_smile:

I hope this helps!

All the best!

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@jrg.developer Can you please help with this when you get a chance? Thank you - much appreciated! :]

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Thanks for your question. :smile:

All in all, the reply from @syedfa is pretty spot on! Specifically,

It doesn’t matter whether the property is optional. If a class “has a” strong property (see note later about “uses” for weak), you nearly always show this as a solid, “has a” line.

By the way, this IS a strong reference, but it’s an optional type. That is, AddressViewController owns an Address, but it may not yet be set and be nil instead.

There is a “uses” relationship too — shown as a dashed line with solid pointer — but this often is used to indicate either (1) the relationship is to a weak property or (2) there isn’t a property relationship but rather it’s a type that’s “used,” such as a method input parameter but not held onto.

Since there is ambiguity, “uses” relationships are typically annotated for what they mean, via text next to the line.

Make sense? :smile:

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Yes, it does. Thank you.