How to resolve 'unrecognized selector sent to instance'?

Question

In the AppDelegate, I'm alloc'ing an instance defined in a static library. This instance has an NSString property set a "copy". When I access the string property on this instance, the app crashes with 'unrecognized selector sent to instance'. Xcode provides a code hint for the property, which means it is known in the calling app. The particular class is compiled into the static library target. What am I missing?

Adding some code.

//static library  //ClassA.h @interface ClassA : NSObject { ... NSString *downloadUrl; } @property(nonatomic, copy) NSString *downloadUrl;  //ClassA.m @synthesize downloadUrl; 

In the calling app's appDelegate.

//app delegate header file @interface myApp : NSObject <UIApplicationDelegate> { ClassA *classA; } @property (nonatomic, retain) ClassA *classA;  //app delegate .m file @synthesize classA;  - (void)applicationDidFinishLaunching:(UIApplication *)application { classA = [[ClassA alloc] init]; //exception occurs here.  downloadUrl is of type NSCFNumber classA.downloadUrl = @"http://www.abc.com/"; ...} 

Other classes in the app will get a reference to the delegate and call classA.downloadUrl.

Answer 1

1) Is the synthesize within @implementation block?

2) Should you refer to self.classA = [[ClassA alloc] init]; and self.classA.downloadUrl = @"..." instead of plain classA?

3) In your myApp.m file you need to import ClassA.h, when it's missing it will default to a number, or pointer? (in C variables default to int if not found by compiler):

#import "ClassA.h".

Answer 2

Set flag -ObjC in Other linker Flag in your Project setting... (Not in the static library project but the project you that is using static library...) And make sure that in Project setting Configuration is set to All Configuration