Example of an NSValue subclass with a return value on getValue:

gistfile1.m

#import <Foundation/Foundation.h>

@interface RSTLValue : NSValue
- (BOOL)getValue:(void *)value expectedSize:(size_t)expectedSize;
@end

struct Foo {
	NSUInteger i;
	char * bar;
};

int main(int argc, const char * argv[])
{
	@autoreleasepool {
		struct Foo foo;
		foo.i = 1;
		foo.bar = "string";
		RSTLValue *value = [[RSTLValue alloc] initWithBytes:&foo objCType:@encode(struct Foo)];
		struct Foo foo2;
		if ([value getValue:&foo2 expectedSize:sizeof(struct Foo)])
		{
			NSLog(@"%li %s\n", foo2.i, foo2.bar);
		}
		else
		{
			NSLog(@"Fail");
		}
		value = [[RSTLValue alloc] init];
		struct Foo foo3;
		if ([value getValue:&foo3 expectedSize:sizeof(struct Foo)])
		{
			NSLog(@"%li %s\n", foo3.i, foo3.bar);
		}
		else
		{
			NSLog(@"Fail");
		}
	}
    return 0;
}

@implementation RSTLValue
{
	NSValue *_realValue;
	size_t _expectedSize;
}

- (instancetype)init
{
	self = [super init];
	if (self)
	{
		_expectedSize = 0;
	}
	return self;
}

- (id)initWithBytes:(const void *)value objCType:(const char *)type
{
	self = [self init];
	if (self)
	{
		_realValue = [[NSValue alloc] initWithBytes:value objCType:type];
		NSUInteger size;
		NSUInteger align;
		//const char * retval = 
		NSGetSizeAndAlignment(type, &size, &align);
		_expectedSize = size;
	}
	return self;
}

- (BOOL)getValue:(void *)value expectedSize:(size_t)expectedSize
{
	if (_realValue == nil)
	{
		return NO;
	}
	if (_expectedSize != expectedSize)
	{
		return NO;
	}
	[_realValue getValue:value];
	return YES;
}

- (void)getValue:(void *)value
{
	@throw [NSException exceptionWithName:@"" reason:@"" userInfo:nil];
}

- (id)forwardingTargetForSelector:(SEL)aSelector
{
	return _realValue;
}

@end