基本照搬这里的代码：
http://blog.csdn.net/kindazrael/article/details/7917863
改善了对其风格和缩进，减少了代码量,对齐风格是emacs的
#define SYNTHESIZE_SINGLETON_FOR_CLASS(classname) \
static classname *shared##classname = nil; \
\
+ (classname *)shared##classname{ \
@synchronized(self){ \
if (shared##classname == nil){ \
shared##classname = [[self alloc] init]; \
} \
} \
\
return shared##classname; \
} \
\
+ (id)allocWithZone:(NSZone *)zone{ \
@synchronized(self){ \
if (shared##classname == nil){ \
shared##classname = [super allocWithZone:zone]; \
return shared##classname; \
} \
} \
\
return nil; \
} \
\
- (id)copyWithZone:(NSZone *)zone{ \
return self; \
}