@property and @synthesize: why both?

Question

I have been climbing the learning curve of X-code for about two months now. I understand the purpose of the @property/@synthesize directives, but it seems that it is a bit redundant to always declare the properties in the .h file, and then synthesize them in the .m file.

Why does the compiler need both directives? If they are always used together, isn't one of them technically redundant?

Thanks in advance for any insights on this subject.

John Doner

Answer 1

Well, you can't use @synthesize in category implementations, so that might be one reason.

Answer 2

It's so that you can split implementation and declaration. Seems pretty neat to me.

Answer 3

The @property (...) foo keyword is used inside your class definition. You don't need to use @synthesize keyword in the implementation file if you provide appropriate getters/setters.

Example:

@property int SomeInt;

---

-(int)SomeInt {
  return _someInt;
}

-(void)setSomeInt:(int)newValue {
  _someInt = newValue;
}

Or you can use either @synthesize foo to inform the compiler to generate getters/setters for you or @dynamic to inform the compiler that these methods will be available at runtime - both in the implementation file.

There's actually more magic behind properties in Objective-C, read up on them at Apple Reference Library.