Does a method which I check for with respondsToSelector have to actually exist?
What if I only define it in the interface part and fail to implement it? I'm looking at a poor-man's virtual function in Objective-C.
+1
A:
respondsToSelector will return NO, since the selector isn't callable at run-time. The interface part only affects compilation.
dorianj
2010-04-18 05:44:34
+4
A:
First, yes the method actually has to exist for the check to succeed in the context you describe. respondsToSelector: will return NO if the method is not implemented.
More importantly, I think you mean a poor man's pure virtual function in Objective-C. All instance methods are "virtual" in Objective-C; since method lookup is done a run-time, the subclass' implementation will always be used, even from a pointer of the superclass type. In Objective-C, there is no such thing as a pure virtual base class. You can often achieve what you want by either using a @protocol to define an API or using a base class that provides an implementation that throws an NSNotImplementedException as its body. Subclasses would obviously have to override the implementation, making it effectively pure virtual.
Barry Wark
2010-04-18 06:17:18
Might want to edit the first line. Technically speaking, the methods does not have to exist. After all `respondsToSelector:` sole purpose is to determine whether a method does exist or not. It works fine with non-existant methods, it just returns no. I think you answer makes sense in the assumed context of the parent but for future readers you might want to make the distinction explicit.
TechZen
2010-04-18 13:31:55
@TechZen Good point. Edited.
Barry Wark
2010-04-18 19:59:23
+1
A:
Given that calling respondsToSelector: only makes sense when you don’t know whether a method exists, it’s not entirely clear what you mean.
If you mean, does some implementation of a method with the specified selector have to exist somewhere, the answer is no. Selectors merely represent names of methods. The @selector directive doesn’t reference any aspect of any method implementation.
Ahruman
2010-04-18 10:55:40