Linker cannot find a class that should be there

I have a project that uses the Core Telephony framework. Recently my code stopped working on a CTCarrier category, the linker complains that it can’t find the CTCarrier class:

Undefined symbols:
 开发者_运维问答 "_OBJC_CLASS_$_CTCarrier", referenced from:
      l_OBJC_$_CATEGORY_CTCarrier_$_Foo in CTTests.o
ld: symbol(s) not found

This is a sample code that triggers the error above:

#import <CoreTelephony/CTCarrier.h>

@interface CTCarrier (Foo)
- (void) doFoo;
@end

@implementation CTCarrier (Foo)
- (void) doFoo {}
@end

If I change the category to class extension, the code suddenly builds:

#import <CoreTelephony/CTCarrier.h>

@interface CTCarrier ()
- (void) doFoo;
@end

@implementation CTCarrier
- (void) doFoo {}
@end

What’s going on? Sample code on GitHub.

There is a bug in 4.2 that doesn't allow the direct creation of a CTCarrier object, the proper way to access CTCarrier is via the CTTelephonyNetworkInfo object like so:

#import <CoreTelephony/CTTelephonyNetworkInfo.h>
#import <CoreTelephony/CTCarrier.h>

CTTelephonyNetworkInfo *telephony = [[CTTelephonyNetworkInfo alloc] init];
CTCarrier *carrier = telephony.subscriberCellularProvider;
[telephony release];

In the first example you don't really are implementing CTCarrier class but only add a method to it. The categories provide a way to add methods to an already defined implementation.