Can an Objective-C implementation be defined in a header file and also be imported by multiple source files?

I have a header file that defines the interface and implementation of an Objective-C class.

The Objective-C runtime is per process, so if two clients in different Mach-O images use this header then you’ll get snarky error messages from the Objective-C runtime.

Code Block 
@import Foundation;
#import <objc/objc-runtime.h>
#ifndef Person_h
#define Person_h
static Class Person;
inline void PersonSpeak(id self, SEL _cmd)
{
		NSLog(@"%Hello");
}
inline void CreatePersonClass()
{
		Person = objc_allocateClassPair([NSObject class], "Person", 0);
		class_addMethod(Person, @selector(speak), (IMP)PersonSpeak, "@@");
		objc_registerClassPair(Person);
}
#endif /* Person_h */

Code Block 
@import Foundation;
#import "Person.h"
int main(int argc, const char * argv[])
{
		@autoreleasepool
		{
				CreatePersonClass();
				id person = [[Person alloc] init];
				
				// Loop used to test for runtime errors when multiple instances are running.
				while (true)
				{
						[person performSelector:@selector(speak)];
				}
		}
		return 0;
}

Code Block 
@import Foundation;
#import "Person.h"

The Objective-C runtime is per process, so if two clients in different Mach-O images use this header then you’ll get snarky error messages from the Objective-C runtime.

When I have both targets running at the same time

To see the problem in action you need two Mach-O images that use this technique in the same process. You can do this, for example, by using it in the main app and in a framework that the main app links to.

Code Block 
@import Foundation;
#import "Person.h"
#import "Test.h"
int main(int argc, const char * argv[])
{
		@autoreleasepool
		{
				id person = [[PersonClass() alloc] init];
				//id person = CreatePersonInstance();
				if (person != nil)
						NSLog(@"Main: Person instance created");
				else
						NSLog(@"Main: Error");
				
				Test test;
		}
		return 0;
}

Code Block 
@import Foundation;
#import <objc/objc-runtime.h>
#ifndef Person_h
#define Person_h
// Creates the Person class. If called from outside of Person.h, this only works in the first .mm it is called from.
static Class& PersonClass()
{
		static Class Person;
		if (Person == nil)
		{
				Person = objc_allocateClassPair([NSObject class], "Person", 0);
				
				if (Person != nil)
						objc_registerClassPair(Person);
		}
		
		return Person;
}
// Creates an instance of a Person. Works regardless of which .mm it is called from.
inline id CreatePersonInstance()
{
		Class Person = PersonClass();
		
		if (Person == nil)
				return nil;
		
		return [[Person alloc] init];
}
#endif /* Person_h */

Code Block 
class Test
{
public:
		Test();
};

Code Block 
@import Foundation;
#import "Person.h"
#import "Test.h"
Test::Test()
{
		id person = [[PersonClass() alloc] init];
		//id person = CreatePersonInstance();
		if (person != nil)
				NSLog(@"Test: Person instance created");
		else
				NSLog(@"Test: Error");
}

Code Block 
Main: Person instance created
Test: Error

Code Block 
Test: Person instance created
Main: Error

Code Block 
Main: Person instance created
Test: Person instance created

The project I am working on is a cross-platform collection of libraries. The single-header design makes it easy to manage distribution. It saves us time not compiling different configurations of dll's and lib's for different operating systems. 

It also makes it easy for the end-user to add to their project. We want to make it as simple as possible to get started using our libraries. All they need to do is include the one file, and they are ready to use it.