Question

For as far as I know, a method is compiled into a function like this:

+(NSArray *)arrayWithObject:(id)object;
// becomes
NSArray *_c_NSArray_arrayWithObject_(id object);

Is it possible to get the name of the function of a selector so I can pass the method as an argument to a C function, or is this not possible?

Answer 1

Not quite — they look like:

id method(id, SEL, ...); 

So an Objective-C method (whether a class or instance method) expects to take the instance of the class it is being called on (which will be the metaclass in the case of a class method) and the selector that triggered it.

This type is defined as ‘IMP’ by objc.h:

typedef id          (*IMP)(id, SEL, ...); 

You can get an IMP pointer either by using the C function class_getMethodImplementation or by using the +instanceMethodForSelector: (or -methodForSelector:, if you want to allow for a particular instance possibly having had its methods rearranged at runtime) method on anything that descends from NSObject.

So, e.g.

#import <objc/objc.h>

id someObject = ...something...;
IMP functionPointerToMethod = [someObject methodForSelector:@selector(method)];

functionPointerToMethod(someObject, @selector(method));

Or:

IMP functionPointerToMethod = [someObject methodForSelector:@selector(method:)];
functionPointerToMethod(someObject, @selector(method:), argument);

Or even:

IMP functionPointerToMethod = class_getMethodImplementation([someObject class],
                                                            @selector(method:));

functionPointerToMethod(someObject, @selector(method:), argument);

Of course, in all this what you’re doing is removing Objective-C’s normal dynamic dispatch from the loop. So if you subsequently add or rearrange methods on a class then your C function pointer will become out of date.

Class methods are accessible by much the same means, but via the metaclass, e.g.

// to do the same as [NSString alloc], not that you ever would...
IMP functionPointerToAlloc = [[NSString class] methodForSelector:@selector(alloc)];
functionPointerToMethod([NSString class], @selector(alloc));