Design pattern--a singleton mode C + + implementation

Single-instance mode C + + implementation

1 Description:

Singleton mode, also known as single-piece mode.

Definition: Ensure that a class has only one instance, and instantiate it itself and provide it to the entire system.

2 Concrete class Diagram Description:

2.1 STARTUML Internal generation pattern class diagram

This class diagram generates C + + code backwards by startuml

Class Singleton {
Public
Static singleton* Instance ();

Protected

Singleton ();

Private
Static Singleton *instance;
};

singleton* singleton::instance ()

{

if (instance==null)

Instance = new Singleton ();

return instance;

}

3. Detailed:

Based on the definition of the singleton pattern, the simplest way to achieve this requirement is to

① defines a global static object for use by the system.

② single-instance mode to achieve the requirements.

Note: These two parties are able to meet the demand, but, think of 6 principles: dependency inversion, interface isolation. Our design should be to rely on the interface, not the object, so long pass Diao ① bar.

4 Advantages and disadvantages of the single-case model:

Advantages: one instance reduces memory overhead, reduces system overhead, avoids multiple uses of a resource, sets global access points, optimizes and contributes resources.

Cons: No excuses, poor extensibility; not conducive to testing; there is a conflict with the single principle of responsibility, and the singleton model blends the "want Singleton" with the business logic in a class.

5 Usage Scenarios:

An environment that requires a unique serial number to be generated;

Share data or share access points;

Creating an object consumes too much resources, such as accessing the IO and database resources;

You need to define a large number of static constants and static methods (tool classes)

6 Thread safety issues

eg. when two threads run to if (instance==null), a thread-safety issue may occur

Improved resolution

Class Singleton {
Public
Static singleton* Instance ();

Protected

Singleton ();

Private
Static Singleton *instance;
};

The singleton is initialized directly, and no longer is judged in the method instance.

singleton* singleton::instance = new Singleton;

singleton* singleton::instance ()

{

return instance;

}

7 The expansion of the singleton mode--Multiple cases mode.

Class Singleton {
Public
Static singleton* Instance ();

Protected

Singleton ();

Private

Static Enum{max = 100};

static int curnum = 0;

Static List<singleton *>instance_list;

};

singleton* singleton::instance ()

{

Random random = new random ();

int count = Random.nextint (Max); Get random number

Return Instance_list.get (count);//random removal of an instance to use

}

8 questions

The initialization of a list in a multi-instance pattern is being tangled up, and finding a way to do so will add up.

Design pattern--a singleton mode C + + implementation