Objective-C Fundamentals

64 CHAPTER 3 An introduction to objects
3.4.3 Sending nonexistent messages
With the message send concept, it’s possible to send an object a message that it may not
be able to handle. Unlike in a language such as C++ where a method must be declared
in order for it to be called, requesting that a message be sent is just that: a request.
Because of Objective-C’s dynamic typing, there’s no guarantee that the recipient of
a message will understand how to process it, and no check is made at compile time to
ensure it exists. The following code snippet will successfully compile even though the
NSString class doesn’t implement a method called mainBundle:
NSString *myString = @"Hello World!";
[myString mainBundle];
The best that the Objective-C compiler can do in a situation like this is produce the
following warning during compilation:
'NSString' may not respond to '-mainBundle'
You may be wondering why Objective-C produces only a warning when other languages
are more heavy handed and produce a fatal error during compilation. The
answer is that Objective-C allows objects to dynamically add and remove methods and
also gives them an opportunity to process any unhandled messages. Consequently, the
compiler can never really be sure that a particular object won’t respond to an arbitrary
message without giving it a go at runtime.
If you want to get rid of the compile-time warning, the easiest way is to change the
variable’s data type to id:
id myString = @"Hello World!";
[myString mainBundle];
Alternatively, you could use a typecast:
NSString *myString = @"Hello World";
[(id)myString mainBundle];
Because the id data type can reference an object of any type, the compiler is more
lenient with its warnings. If it didn’t make this concession, it would have to warn about
every message sent to an object typed as id because this type doesn’t explicitly declare
the existence of any methods to the compiler.
It’s important to note that although all the code snippets in this section successfully
compile, they will all fail at runtime because NSString doesn’t handle a message
called mainBundle. If the target object doesn’t define a suitable method, the message
will eventually be rejected, just as addressing an envelope to a nonexistent address will
cause it to become undeliverable. You’ll most likely come across this situation when
you see the following fatal error in the debugger:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: '*** -[NSCFString mainBundle]: unrecognized selector sent to
instance 0x3b0a1e0'