converting dot syntax to bracket syntax on a struct - objective-c

Question

I have a property of CGSize:
#property (nonatomic) CGSize descriptionSize;
'
#synthesize descriptionSize = _descriptionSize;
I can access the height through the dot syntax:
self.descriptionSize.height = 35;
but how does this work with the bracket syntax?
[self setDescriptionSize:???];
Looked stupid simple to me, but I can't get the clue. Thanks in advance!

Answer 1

This is one of the pitfalls of dot notation for properties: Those two dots in self.descriptionSize.height look the same but mean very different things. The first is a property accessor which maps to a "get descriptionSize" method, but the second is an old-school struct reference. The first dot returns a CGSize scalar, NOT a pointer to the size value in the object. When the second dot sets the height in that returned CGSize, it's setting a value on the stack instead of changing the value in the object. This is how you have to do it:
CGSize size = self.descriptionSize;
size.height = 35;
self.descriptionSize = size;
…or the equivalent without properties dot notation:
CGSize size = [self descriptionSize];
size.height = 35; // we still use the dot here: size is a struct not an object
[self setDescriptionSize:size];

Answer 2

The implementation of descriptionSize will return a copy of the CGSize struct, so you can't work directly with that and hope it will work. What you need to do is get the whole of the CGSize struct, modify it, and then pass it back in:
CGSize size = [self descriptionSize];
size.height = 35;
[self setDescriptionSize:size];
However given you are working on a property of self and the property isn't an object, which requires memory management, the most efficient way of modifying the size is:
_descriptionSize.height = 35;
However you'd use the former getter/setter approach if:
The object was not self.
You had manually written the setter method to do something as a side-effect of changing the size (for example invalidating bits of the view in order to automatically update the view).

Answer 3

Dot syntax can mean two different things: Either a struct reference (CGSize is a C struct), or an objective-C message send.
Theoretically, if you have a method like - (void)doSomething;, you could call it like this: myObject.doSomething; //bad style. Don't do this. Dot syntax is not meant for calling methods that actually do stuff, other than getting or setting values (although nothing in the language or the IDE is going to stop you).
Synthesizing properties creates accessor methods: - (myType)myProperty and - (void)setMyProperty:(myType)newValue. Here, dot syntax lets you access the getter in the ordinary way (because the getter is an ordinary Objective-C method), and has a special case for the setter: myObject.myProperty = newValue gets translated to [myObject setMyProperty:newValue].
This means you can switch between dot syntax and Objective-C style message sending syntax for properties (and technically for all other parameter-less Objective-C method sends), but you must use dot syntax to access struct members. Structs are not objects, and they know now methods.