How to call a method without an instantiated object

Question

This is an objective-c question.

I would like to call a method in an object, but there is no instantiation of the object. Is this possible?

The method I want to call is not a class method.

Answer 1

Unless the method is static you will not be able to do this. static routines in Objective-C will be prepended with a +. For example NSObject provides these two routines (among many):

+ (id)alloc; // static - an NSObject instance is not required
- (NSString*)description; // nonstatic - an NSObject instance is required

One would make the respective calls like so:

NSObject* result = [NSObject alloc];
NSString* desc = [result description];

Answer 2

You can't call a method on an object that doesn't exist. But you can call a method on a class even if you have no instantiated objects of that class. (That's what alloc is in @fbrereton's answer -- a class method).

Class methods are declared and defined with a + instead of a -, are called on the class rather than the instance, and cannot access self or any instance variables in the class (for reasons that should be obvious).

Answer 3

You can indeed invoke an instance method without an instance, provided it is functionally a class method (that is, it accesses no instance variables). Here's an example:

/*
Compile with:
gcc -framework Foundation inst_method_without_inst.m -o inst_method_without_inst
*/
#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface Foo : NSObject {
}
- (BOOL)doSomethingWithThis:(NSString *)this;
@end

@implementation Foo
- (BOOL)doSomethingWithThis:(NSString *)this {
  NSLog(@"Look, it's this: %@", this);
  return YES;
}
@end

typedef BOOL (*my_sel_t)(id, SEL, NSString *);
int
main(void) {
  Class cls = [Foo class];
  SEL my_sel = @selector(doSomethingWithThis:);
  Method m = class_getInstanceMethod(cls, my_sel);
  // You could also use +[NSObject instanceMethodForSelector:] to get |m|,
  // since |cls| is a kind of NSObject.
  my_sel_t f = (my_sel_t)method_getImplementation(m);
  BOOL result = f(nil, my_sel, @"Hello from an instanceless instance method invocation!");
  NSLog(@"result: %d", (int)result);
  return EXIT_SUCCESS;
}

You could get it to work even if the instance method accesses instance variables by allocating memory for it to work with (using either +alloc or class_getInstanceSize() plus malloc()) and passing a pointer to that memory as the first id argument to the implementation instead of nil.

While this is entertaining as an exercise, I can't think of a good reason to not just instantiate the class and use the standard messaging syntax and compiler support. In fact, the only reason we couldn't just do [(Foo *)nil doSomethingWithThis:@"BOO!"] here is that objc_msgSend() special-cases messages to nil with the result that NO is returned and nothing happens.

Answer 4

Perhaps you just want a plain-old C function. If you don't want a class method, and you don't want an instance method, that appears to be your only option. Don't be afraid of using C functions in Objective-C. Every technique has its place.

Answer 5

Sorry to nit-pick Chris' terminology, but we don't call a method on an object in Objective-C, we send a message to an object. When you send a message, the runtime will look up the appropriate method and call it. The distinction matters.