Question

I have a Singleton class called Constants and I need to set some app wide constants in there. Id like these constants to be readonly. So I did the following in the Constants.h file

 @interface Constants : NSObject
 {

 }

 @property (nonatomic, readonly)double U_LAT;
 @property (nonatomic, readonly)double U_LNG;

Then in my .m fileI got this method

 -(id)init
 {
     self = [super init];

     self.U_LAT = 49.2765;
     self.U_LNG = -123.2177;

     return self;
 }

I get this error from this code:

 Assignment to readonly property

Can I not initialize my readonly variable in the init method? If not how do I initialize them?

Answer 1

self.propertyName = val; is the same as [self setPropertyName:val]; — it requires a setter method to exist. Read-only properties don’t have setter methods.

You can set the ivar which backs the property directly, however:

-(id)init
 {
     self = [super init];

     _U_LAT = 49.2765;
     _U_LNG = -123.2177;

     return self;
 }

The ivar’s name will be the property name prefixed with an underscore if you are allowing the properties to be synthesized automatically. If you have an explicit @synthesize propName;, the ivar will have the same name. You can also create a variable with any name you like, again using a synthesize statement: @synthesize cat = dog;

It’s also possible for the property to be read-only publicly, but be writeable by the class; this involves either declaring a setter method in a class extension or redeclaring the property in the extension.