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This article mainly introduces the relevant knowledge of "how to realize the agent pattern of C++ design pattern". The editor shows you the operation process through an actual case, and the operation method is simple, fast and practical. I hope this article "how to realize the proxy pattern of C++ design pattern" can help you solve the problem.

Agent pattern is the most important pattern in programming, and it is widely used. According to its function, we can directly understand it as middleware or middle layer, such as the middleware of all kinds of software, the middle layer between software and hardware.

Action

Provide a proxy for other objects to control access to this object. This separates the business from the core functions.

Abstract class view

classification

Virtual proxy: creates expensive objects as needed to store real objects that take a long time to instantiate so that they are created only when they are really needed.

Remote proxy: provides local representation of an object in a different address space, thus hiding the fact that an object exists in a different address space. This different address space can be on this machine or in another machine.

Intelligent reference proxy: when a real object is called, the agent handles other things, such as recording the number of calls to this object.

Security agent: also known as a protection agent, used to control access to real objects and, if necessary, to provide different permissions to different callers.

Copy-on-write agent: a virtual agent that defers replication until only when the customer needs it.

Cache proxy: provides temporary storage space for the operation results of a target, so that other customers can share access, which is a bit of a cache.

Firewall proxy: protect objects from users, special cases of security agents.

Synchronization agent: allows several users to access the same object at the same time without conflict.

Classified implementation

Virtual agent

The main purpose of a virtual agent is to achieve latency. Here is an example from [DP] to consider a document editor that can embed graphical objects in a document. Some graphic objects are expensive to create. But opening a document must be quick, so when opening a document, we should avoid creating all expensive objects at once. Here you can use the proxy mode, when you open the document, do not open the drawing object, but open the proxy of the drawing object to replace the real drawing. When you really need to open the drawing, the agent is still responsible for opening it.

/ / abstract class class Image {public: Image (std::string name): m_name (name) {} virtual ~ Image () {} virtual void Show () = 0; / / the function that displays the document: protected: std::string name; / / document name} / / large entity class class BigImage: public Image {public: BigImage (std::string name): Image (name) {} virtual ~ BigImage () {} void Show () {std::cout Show (); delete Image; return 0;}

Remote agent

Remote agents are often seen in communications, such as network, IPC and RPC. Generally, they need to provide customer helper objects and service helper objects, create the same methods for customer helper objects and service objects (such as stub and skeleton in JAVA), and then realize the specific communication between them. For client, stub (actually stub and skeleton work together) is its remote agent.

Intelligent reference agent

The most typical application is the use of smart pointers, relative to pointers, smart pointers are proxies for pointers; the specific implementation of this can refer to the implementation of auto_ptr or share_ptr.

Copy Agent when Writing

Copy uses virtual proxy and reference counting mechanism when writing. When the copy action is delayed until the count changes, it is typically implemented in memory such as std::string. For specific implementation, you can check the source code of string.

Std::string str1 = "fine"; std::string str2 = str1; / / the data address of str2 str1 is the same after execution; str2 [0] = 'wicked; / / the str2 address changes after execution

Other agents

The remaining agents realize their specific applications through the control of access objects, because they involve complex application scenarios and different language architectures, so they will not talk about it here and may be far from the topic.

This is the end of the introduction on "how to realize the Agent pattern of C++ Design pattern". Thank you for your reading. If you want to know more about the industry, you can follow the industry information channel. The editor will update different knowledge points for you every day.

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