ruby-nil-messaging-objc.m

//
//  Created by Damien DeVille on 6/23/13.
//  Copyright (c) 2013 Damien DeVille. All rights reserved.
//
 
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
 
extern id _objc_setNilReceiver(id newNilReceiver);
 
static id _createNilCatcherObject(void)
{
	Class NilCatcher = objc_allocateClassPair([NSObject class], "NilCatcher", 0);
	
	NSMethodSignature * (^methodSignatureForSelectorBlock)(id, SEL) = ^ NSMethodSignature * (id _block, SEL selector) {
		/*
			We will not actually use the method signature in forwardInvocation so any signature will do it.
		 */
		return [NSObject instanceMethodSignatureForSelector:@selector(description)];
	};
	IMP methodSignatureForSelectorIMP = imp_implementationWithBlock(methodSignatureForSelectorBlock);
	Method methodSignatureForSelectorMethod = class_getClassMethod([NSObject class], @selector(methodSignatureForSelector:));
	class_addMethod(NilCatcher, @selector(methodSignatureForSelector:), methodSignatureForSelectorIMP, method_getTypeEncoding(methodSignatureForSelectorMethod));
	
	void (^forwardInvocationBlock)(id, NSInvocation *) = ^ void (id _block, NSInvocation * invocation) {
		@throw [NSException exceptionWithName:NSInternalInconsistencyException reason:[NSString stringWithFormat:@"Attempting to message %s to nil", sel_getName([invocation selector])] userInfo:nil];
	};
	IMP forwardInvocationIMP = imp_implementationWithBlock(forwardInvocationBlock);
	Method forwardInvocationMethod = class_getClassMethod([NSObject class], @selector(forwardInvocation:));
	class_addMethod(NilCatcher, @selector(forwardInvocation:), forwardInvocationIMP, method_getTypeEncoding(forwardInvocationMethod));
	
	return [NilCatcher new];
}
 
int main(int argc, const char **argv)
{
	@autoreleasepool {
		id nilCatcher = _createNilCatcherObject();
		_objc_setNilReceiver(nilCatcher);
		
		[(id)nil isEqualToString:@"Cat"];
	}
	return 0;
}