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Proxy in C++
Proxy is a structural design pattern that provides an object that acts as a substitute for a real service object used by a client. A proxy receives client requests, does some work (access control, caching, etc.) and then passes the request to a service object.
The proxy object has the same interface as a service, which makes it interchangeable with a real object when passed to a client.
Complexity:
Popularity:
Usage examples: While the Proxy pattern isn’t a frequent guest in most C++ applications, it’s still very handy in some special cases. It’s irreplaceable when you want to add some additional behaviors to an object of some existing class without changing the client code.
Identification: Proxies delegate all of the real work to some other object. Each proxy method should, in the end, refer to a service object unless the proxy is a subclass of a service.
Conceptual Example
This example illustrates the structure of the Proxy design pattern. It focuses on answering these questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
main.cc: Conceptual example
#include <iostream>
/**
* The Subject interface declares common operations for both RealSubject and the
* Proxy. As long as the client works with RealSubject using this interface,
* you'll be able to pass it a proxy instead of a real subject.
*/
class Subject {
public:
virtual void Request() const = 0;
};
/**
* The RealSubject contains some core business logic. Usually, RealSubjects are
* capable of doing some useful work which may also be very slow or sensitive -
* e.g. correcting input data. A Proxy can solve these issues without any
* changes to the RealSubject's code.
*/
class RealSubject : public Subject {
public:
void Request() const override {
std::cout << "RealSubject: Handling request.\n";
}
};
/**
* The Proxy has an interface identical to the RealSubject.
*/
class Proxy : public Subject {
/**
* @var RealSubject
*/
private:
RealSubject *real_subject_;
bool CheckAccess() const {
// Some real checks should go here.
std::cout << "Proxy: Checking access prior to firing a real request.\n";
return true;
}
void LogAccess() const {
std::cout << "Proxy: Logging the time of request.\n";
}
/**
* The Proxy maintains a reference to an object of the RealSubject class. It
* can be either lazy-loaded or passed to the Proxy by the client.
*/
public:
Proxy(RealSubject *real_subject) : real_subject_(new RealSubject(*real_subject)) {
}
~Proxy() {
delete real_subject_;
}
/**
* The most common applications of the Proxy pattern are lazy loading,
* caching, controlling the access, logging, etc. A Proxy can perform one of
* these things and then, depending on the result, pass the execution to the
* same method in a linked RealSubject object.
*/
void Request() const override {
if (this->CheckAccess()) {
this->real_subject_->Request();
this->LogAccess();
}
}
};
/**
* The client code is supposed to work with all objects (both subjects and
* proxies) via the Subject interface in order to support both real subjects and
* proxies. In real life, however, clients mostly work with their real subjects
* directly. In this case, to implement the pattern more easily, you can extend
* your proxy from the real subject's class.
*/
void ClientCode(const Subject &subject) {
// ...
subject.Request();
// ...
}
int main() {
std::cout << "Client: Executing the client code with a real subject:\n";
RealSubject *real_subject = new RealSubject;
ClientCode(*real_subject);
std::cout << "\n";
std::cout << "Client: Executing the same client code with a proxy:\n";
Proxy *proxy = new Proxy(real_subject);
ClientCode(*proxy);
delete real_subject;
delete proxy;
return 0;
}
Output.txt: Execution result
Client: Executing the client code with a real subject:
RealSubject: Handling request.
Client: Executing the same client code with a proxy:
Proxy: Checking access prior to firing a real request.
RealSubject: Handling request.
Proxy: Logging the time of request.
