从self创建NSMutableString时出现NSString + Extension警告

Question

in an iphone application I created and extension class to the NSString.

NSString+Extensions.m class

In one of the methods I need to convert the string to NSMutableString.

I tried to use this:

NSMutableString * stringToManipulate = [NSMutableString stringWithString:self];

But it is giving a warning:

Incompatible pointer types sending 'const Class' to parameter of type 'NSString *'

To my knowlede self is a reference to the string I called the method on, right? so why shouldn't it be of type NSString*? knowing that I can use the usual NSString methods on self.

Any idea on this issue?

Thanks

Answer 1

Check whether the method is declared as a class method (with the + symbol) or an instance method (with the - symbol). If its the first case, the compiler warning is normal. Turn the method to an instance one then

Answer 2

In Objective-C, when you call self within a class method, it references the class object. For an NSString class method, calling self is the same as calling [NSString class].

So, your line is the same as :

NSMutableString * stringToManipulate = [NSMutableString stringWithString:[self class]];

If you change your method from a class one (+ symbol) to an object one (- symbol), it will work but you must call it differently (from an NSString object).

If you want further explanations, can you post the entire method declaration ? You can also check the NSObject protocol reference for more informations on the +class method : https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html

Hope this will help,