singleton macro iOS

singleton macro

//from Urban Airship iOS SDK "1.1.2"


#define SINGLETON_INTERFACE(CLASSNAME)                                                      \
+ (CLASSNAME*)shared;                                                                       \

#define SINGLETON_IMPLEMENTATION(CLASSNAME)                                                 \
                                                                                            \
static CLASSNAME* g_shared##CLASSNAME = nil;                                                \
\
+ (CLASSNAME*)shared                                                                        \
{                                                                                           \
static dispatch_once_t sharedOncePredicate##CLASSNAME;                                                 \
\
dispatch_once(&sharedOncePredicate##CLASSNAME, ^{                                                      \
g_shared##CLASSNAME = [[self alloc] init];                                                  \
});                                                                                         \
return g_shared##CLASSNAME;                                                                 \
}                                                                                           \
\
+ (id)allocWithZone:(NSZone*)zone                                                           \
{                                                                                           \
static dispatch_once_t allocOncePredicate##CLASSNAME;                                                  \
dispatch_once(&allocOncePredicate##CLASSNAME, ^{                                                       \
if (g_shared##CLASSNAME == nil) {                                                           \
g_shared##CLASSNAME = [super allocWithZone:zone];                                           \
}                                                                                           \
});                                                                                         \
return g_shared##CLASSNAME;                                                                 \
}                                                                                           \
\
- (id)copyWithZone:(NSZone*)zone                                                            \
{                                                                                           \
return self;                                                                                \
}                                                                                           \