Setting values in a custom struct as property in Objective-C [duplicate]

Question

I have class A with a header that looks something like this:

typedef struct {
    int x;
    int y;
} Position;

@interface ClassA : NSObject

@property Position currentPosition;

@end

And I try to assign individual values of the position struct from the property in another class like this:

ClassA * classA = [ClassA new];
classA.currentPosition.x = 10;

Which gives an error "expression is not assignable" and won't compile.

I can set it like this:

ClassA * classA = [ClassA new];
Position position = {
    .x = 1,
    .y = 2
};

classA.currentPosition = position;

And I can even alter individual "properties" of position variable like this:

ClassA * classA = [ClassA new];
Position position = {
    .x = 1,
    .y = 2
};

// WORKS
position.x = 4;    

// DOESN'T WORK
// classA.currentPosition.x = 4;

classA.currentPosition = position;

Why can't I set values individually when they are a property?

Answer 1

This expression:

classA.currentPosition

returns a temporary copy of your struct, not the struct itself. The compiler error is telling you that you can't assign a value to some member of that temporary copy (because it's an rvalue, technically). But you don't want to assign a value to that member anyway, because it would just disappear along with the struct itself.

So why are you only getting a copy of the struct in the first place?

Because

@property Position currentPosition

is actually just shorthand for:

-(Position)currentPosition;
-(void)setCurrentPosition(Position value);

and in C-family languages, the first line (the getter) indicates that it's returning a Position struct by-value, or as a copy.

You could make your own accessor that returns a reference, but you probably shouldn't. This isn't a common idiom in Objective-C -- at least not in this context -- and you should generally try to stick with common idioms for a language.

Instead, you should use position like the following;

Position pos = classA.position;
pos.x = 4;
classA.position = pos;

Lastly, if you really want to be able to set currentPosition using the syntax you originally desired, while maintaing Objective-C idioms, you could just make Position a class rather than a struct. Then, the property can return a Position * and the rest of the syntax would work. Make sure to initialize the pointer in your init function (or when appropriate).

Answer 2

Properties don't work for C structs.

You can do it like:

@property Position *currentPosition;

Basically, using a pointer.

Now you actually need to initialize that pointer so:

- (id) init {
    self = [super init];
    if(self){
        self.currentPosition = malloc(sizeof(Position));
    }
    return self;
}

Then, don't forget to use arrow notation, since you're dealing with a pointer:

classA.currentPosition->x = 5;

And don't forget to free the memory you requested!

-(void)dealloc{
    free(self.currentPosition);
}