How to get a selector of a class method? - ios

Question

I have the next code where I am getting a pointer to instance method:
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#interface TestClass : NSObject
#end
#implementation TestClass
- (void)someMethod { // This is instance method, it's okay
NSLog(#"Hello from some method!");
}
#end
int main(int argc, const char * argv[]) {
typedef void (*MethodWithoutParams)();
MethodWithoutParams someMethodImplementation =
class_getMethodImplementation([TestClass class], #selector(someMethod));
someMethodImplementation();
return 0;
}
It works pretty good. But if I'd like to get a pointer to class method, it doesn't work:
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#interface TestClass : NSObject
#end
#implementation TestClass
+ (void)someMethod { // This is class method, it doesn't work
NSLog(#"Hello from some method!");
}
#end
int main(int argc, const char * argv[]) {
typedef void (*MethodWithoutParams)();
MethodWithoutParams someMethodImplementation =
class_getMethodImplementation([TestClass class], #selector(someMethod));
someMethodImplementation();
return 0;
}
It doesn't work because it can not look up the method implementation.
I am sure it must work, because it works if I am getting the implementation in such way:
MethodWithoutParams someMethodImplementation =
[TestClass methodForSelector:#selector(someMethod)];
So I looked into NSObject implementation and see the next code:
+ (IMP)methodForSelector:(SEL)sel {
if (!sel) [self doesNotRecognizeSelector:sel];
return object_getMethodImplementation((id)self, sel);
}
- (IMP)methodForSelector:(SEL)sel {
if (!sel) [self doesNotRecognizeSelector:sel];
return object_getMethodImplementation(self, sel);
}
Also object_getMethodImplementation() function has the next implementation:
IMP object_getMethodImplementation(id obj, SEL name)
{
Class cls = (obj ? obj->getIsa() : nil);
return class_getMethodImplementation(cls, name);
}
So the implementation is same for both class method lookup and instance method lookup.
But it doesn't work and I have no idea why. I guess that any type of methods (both class and instance) gonna be located in the dispatch table, and I can get the pointer to any of this methods. But as you see I can't do it.

Answer 1

You need to query the class's metaclass. A class method is an instance method on its metaclass.
Class meta = objc_getMetaClass("TestClass");
SEL sel = #selector(someMethod);
typedef void (*MyMethodImplementation)(Class, SEL);
MyMethodImplementation someMethodImplementation = (MyMethodImplementation)class_getMethodImplementation(meta, sel);
someMethodImplementation([TestClass class], sel);
Note, a method (instance or class) is never "without parameters". It must always be called with at least the receiver (instance or class) and the selector.

Answer 2

I have absolutely no idea about Objective-C runtime. But here is my rant:
object_getMethodImplementation for class methods:
// returns a class method implementation, can't find instance methods
class_getMethodImplementation([TestClass class]->getIsa(),
#selector(classMethod));
which is equivalent to
class_getMethodImplementation(objc_getMetaClass("TestClass"),
#selector(classMethod));
object_getMethodImplementation for instance methods:
// returns an instance method implementation, can't find class methods
class_getMethodImplementation((<TestClass instance>)->getIsa(),
#selector(someMethod));
which is equivalent to:
class_getMethodImplementation([TestClass class],
#selector(someMethod))
So it seems that <TestClass instance>->getIsa() returns a pointer that is equal to what [TestClass class] returns. And I guess now you know what's the difference between a self in a class method and a self in an instance method.
So you should use meta class as #KenThomases suggests.