SqlServer中解析字符串的表值函数

经典Singletons

Sometimes you want to override the default way in which Ruby creates objects. As an

example, let’s look at our jukebox. Because we’ll have many jukeboxes, spread all over

the country, we want to make maintenance as easy as possible. Part of the requirement

is to log everything that happens to a jukebox: the songs played, the money received,

the strange fluids poured into it, and so on. Because we want to reserve the network

bandwidth for music, we’ll store these log files locally. This means we’ll need a class

that handles logging. However, we want only one logging object per jukebox, and we

want that object to be shared among all the other objects that use it.

Enter the Singleton pattern, documented in Design Patterns [GHJV95]. We’ll arrange

things so that the only way to create a logging object is to call MyLogger.create, and

we’ll ensure that only one logging object is ever created.

class MyLogger

private_class_method :new

@@logger = nil

def MyLogger.create

@@logger = new unless @@logger

@@logger

end

end

By making MyLogger’s new method private, we prevent anyone from creating a logging

object using the conventional constructor. Instead, we provide a class method,

MyLogger.create. This method uses the class variable @@logger to keep a reference

to a single instance of the logger, returning that instance every time it is called.3 We

can check this by looking at the object identifiers the method returns.