iOS, Use struct in Objective C, struct values not assignable from super view

Question

I am trying to use a struct to store three values as a unit so to speak. I am getting an error: "Expression not assignable" when I try to assign values to the struct's values from the object's super view.

Anyone know why this is?

in my class's .h file I have defined the struct and a property

@interface MyClass : UIView

{
    struct customStruct {
        float a;
        float b;
        float c;
    };

}

@property (assign, nonatomic) struct customStruct myStruct;

from the super view I try to assign a value and I get an error: "Expression not assignable"

object.myStruct.a = someValue;

Solution 1

Try this:

struct customStruct aStruct = object.myStruct;
aStruct.a = someValue;
object.myStruct = aStruct

It is exactly the same situation as not being able to do this:

view.frame.size.width = aWidthValue;

BTW declaring a struct inside a class interface seems like a very bad style. This is much cleaner:

typedef struct { 
    float a;
    float b;
    float c;
} customStruct;

@interface MyClass : UIView

@property (assign, nonatomic) customStruct myStruct;

Solution 2

This is because object.myStruct returns a copy of your structure member and there is no point of changing member a of that copy.

You should do get the entire struct change the member and then set the struct member again (using the get/set synthesized methods)

Solution 3

Define it outside/before interface scope:

struct LevelPath{
    int theme;
    int level;
};

//here goes your interface line
@interface BNGameViewController : UIViewController{
}
//create propertee here
@property (nonatomic, assign) struct LevelPath levelPath;

Solution 4

To add on to the earlier suggestions, I'd suggest to not store your struct as a property, but a member variable. E.g:

@interface MyClass () {

    CGPoint myPoint;

}

Then in your class you can assign new values to that struct directly:

myPoint.x = 100.0;

instead of when it's a @property, which is read-only:

myPoint = CGPointMake(100.0, myPoint.y);

Given that this isn't an NSObject subclass, unless you are overwriting the getter/setter methods, I don't see the purpose of making it an @property anymore, since that is mostly useful for ARC to help with your objects' memory management. But please correct me if I'm wrong on that point (ouch).

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