Proxy en C++
Proxy es un patrón de diseño estructural que proporciona un objeto que actúa como sustituto de un objeto de servicio real utilizado por un cliente. Un proxy recibe solicitudes del cliente, realiza parte del trabajo (control de acceso, almacenamiento en caché, etc.) y después pasa la solicitud a un objeto de servicio.
El objeto proxy tiene la misma interfaz que un servicio, lo que lo hace intercambiable con un objeto real cuando se pasa a un cliente.
Complejidad:
Popularidad:
Ejemplos de uso: Aunque el patrón Proxy no es un invitado habitual en la mayoría de aplicaciones C++, resulta de mucha utilidad en algunos casos especiales. Es insustituible cuando queremos añadir algunos comportamientos adicionales a un objeto de una clase existente sin cambiar el código cliente.
Identificación: Los proxies delegan todo el trabajo real a otro objeto. Cada método proxy debe, al final, referirse a un objeto de servicio, a no ser que el proxy sea una subclase de un servicio.
Ejemplo conceptual
Este ejemplo ilustra la estructura del patrón de diseño Proxy. Se centra en responder las siguientes preguntas:
- ¿De qué clases se compone?
- ¿Qué papeles juegan esas clases?
- ¿De qué forma se relacionan los elementos del patrón?
main.cc: Ejemplo conceptual
#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: Resultado de la ejecución
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.
