static class and singleton

问题:

Isn't a class with all static members/methods a kind of singleton design pattern? Is there any disadvantage in particular of having such classes? A detailed explanation would help.

回答1:

This kind of class is known as a monostate - it is somewhat different from a singleton.

Why use a monostate rather than a singleton? In their original paper on the pattern, Bell & Crawford suggest three reasonns (paraphrased by me):

• More natural access syntax
• singleton lacks a name
• easier to inherit from

I must admit, I don't find any of these particularly compelling. On the other hand, the monostate is definitely no worse than the singleton.

回答2:

Robert C. Martin wrote an article some times ago about the differences between the mono state pattern and the singleton pattern.

回答3:

Consider a family of Logging classes. They all implement "LogMessage(message, file, line_number). Some send messages to stderr, some send email to a set of developers, some increment the count of the particular message in a message-frequency table, some route to /dev/null. At runtime, the program checks its argument vector, registry, or environment variables for which Logging technique to use and instantiates the Logging Singleton with an object from a suitable class, possibly loading an end-user-supplied DLL to do so. That functionality is tough to duplicate with a pure static Singleton.

回答4:

class with all static members/methods a kind of singleton design pattern

Class - not pattern. When we talk about classes we can say class implements pattern.

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Static functions - is not member functions, they are similar on global functions. Maybe you don't need any class?

Quote from wikipedia:

In software engineering, the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object.

By this definition your implementation is not singleton implementation - you don't use common idea One (or several in extended definition) instance of class.

But sometimes (not always) usage of class with all static functions and singleton pattern - not have meaningful difference.

回答5:

For a singleton all constructors have to be private, so that you can access only through a function. But you're pretty close to it.