Question

Objective-C noob here.

Why would this:

NSString *myString = [NSString alloc];
[myString initWithFormat:@"%f", storedNumber];  

results in the following exception -length only defined for abstract class. Define -[NSPlaceholderString length]!

When this works just fine:

NSString *myString = [[NSString alloc] initWithFormat:@"%f", storedNumber];

I would think that the latter is merely a contraction of the former (but I’m obviously wrong, at least according to the compiler).

Answer 1

Because -initWithFormat: is returning an object that’s different from the one returned by +alloc, i.e., an object that’s different from the one pointed by myString. That’s the reason why you should always couple +alloc with -init….

This situation is common in class clusters such as NSString. +alloc returns a generic string object, then -initWithFormat: decides upon a concrete subclass of NSString, deallocates the current object created by +alloc, creates a new object from a concrete subclass of NSString, and then returns this new object.