Fun with ARC and exceptions

arc-exception-funhouse.m

/*

The desired output of each test case is:
<MyException 0xPTR>{ TestName, retain count 1 } created.
<MyException 0xPTR>{ TestName, retain count 1 } caught.
<MyException 0xPTR>{ TestName, retain count 1 } dealloced.

For a fun time, try to predict the actual output for all six test cases!


UPDATE: I’ve been pointed at the Clang ARC internals documentation
(http://clang.llvm.org/docs/AutomaticReferenceCounting.html#misc.exceptions),
which say that the compiler doesn’t clean up after exceptions unless
-fobjc-arc-exceptions is passed. This is not mentioned in the Transitioning
to ARC Release Notes. With the flag, the ConvenienceConstructor variants all
work, and the AllocInit variants all crash.

FURTHER UPDATE: Documentation enhancement request: http://www.openradar.me/10329810

*/


#import <Foundation/Foundation.h>

@interface MyException: NSObject

+ (id) exceptionWithLabel:(NSString *)label;
- (id) initWithLabel:(NSString *)label;

@end


// Test cases

static void DirectThrowConvenienceConstructorTest(NSString *name)
{
	@throw [MyException exceptionWithLabel:name];
}


static void DirectThrowAllocInitTest(NSString *name)
{
	@throw [[MyException alloc] initWithLabel:name];
}


static void IndirectThrow(id exception) __attribute__((noreturn, noinline));
static void IndirectThrow(id exception)
{
	@throw exception;
}


static void IndirectThrowConvenienceConstructorTest(NSString *name)
{
	IndirectThrow([MyException exceptionWithLabel:name]);
}


static void IndirectThrowAllocInitTest(NSString *name)
{
	IndirectThrow([[MyException alloc] initWithLabel:name]);
}


static void AutoreleasingThrow(__autoreleasing id exception) __attribute__((noreturn, noinline));
static void AutoreleasingThrow(__autoreleasing id exception)
{
	@throw exception;
}


static void AutoreleasingThrowConvenienceConstructorTest(NSString *name)
{
	AutoreleasingThrow([MyException exceptionWithLabel:name]);
}


static void AutoreleasingThrowAllocInitTest(NSString *name)
{
	AutoreleasingThrow([[MyException alloc] initWithLabel:name]);
}




// Test rig

extern void _NSSetLogCStringFunction(void (*)(const char *string, unsigned length, BOOL withSyslogBanner));
static void PrintNSLogMessage(const char *string, unsigned length, BOOL withSyslogBanner)
{
	puts(string);
}


static void RunTest_(void (*testCase)(NSString *name), NSString *name)
{
	NSLog(@"Running test %@", name);
	__weak MyException *weakException;
	
	@autoreleasepool
	{
		@try
		{
			testCase(name);
		}
		@catch (MyException *exception)
		{
			weakException = exception;
			NSLog(@"%@ caught.", exception);
		}
	}
	
	if (weakException != nil)
	{
		NSLog(@"%@ appears to have leaked.", weakException);
	}
	
	NSLog(@"\n");
}


#define RunTest(NAME) RunTest_(NAME, @#NAME)


int main (int argc, const char * argv[])
{
	// Avoid annoying log headers.
	_NSSetLogCStringFunction(PrintNSLogMessage);
	
	@autoreleasepool
	{
		RunTest(IndirectThrowConvenienceConstructorTest);
		RunTest(IndirectThrowAllocInitTest);
		RunTest(AutoreleasingThrowConvenienceConstructorTest);
		RunTest(AutoreleasingThrowAllocInitTest);
		
		RunTest(DirectThrowConvenienceConstructorTest);
		RunTest(DirectThrowAllocInitTest);
	}
}


@implementation MyException
{
	NSString *_label;
}

+ (id) exceptionWithLabel:(NSString *)label
{
	id result = [[self alloc] initWithLabel:label];
	
	// Compiling the exception class separately with ARC off made no difference.
#if !__has_extension(objc_arc)
	[result autorelease];
#endif
	return result;
}


- (id) initWithLabel:(NSString *)label
{
	if ((self = [super init]))
	{
		_label = label;
		NSLog(@"%@ created.", self);
	}
	return self;
}


- (void) dealloc
{
	NSLog(@"%@ dealloced.", self);
	_label = [NSString stringWithFormat:@"ZOMBIE - was %@", _label];
	
#if !__has_extension(objc_arc)
	[super dealloc];
#endif
}


- (NSString *) description
{
	return [NSString stringWithFormat:@"<%@ %p>{ %@, retain count %lu }", self.class, self, _label, CFGetRetainCount((__bridge CFTypeRef)self)];
}

@end