Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

Proxy: fronting for another object

If we implement Proxy using the above diagram, it looks like this:

//: C10:ProxyDemo.cpp
// Simple demonstration of the Proxy pattern.
#include <iostream>
using namespace std;
class ProxyBase {
virtual void f() = 0;
virtual void g() = 0;
virtual void h() = 0;
virtual ~ProxyBase() {}
class Implementation : public ProxyBase {
void f() { cout << "Implementation.f()" << endl; }
void g() { cout << "Implementation.g()" << endl; }
void h() { cout << "Implementation.h()" << endl; }
class Proxy : public ProxyBase {
ProxyBase* implementation;
Proxy() { implementation = new Implementation(); }
~Proxy() { delete implementation; }
// Forward calls to the implementation:
void f() { implementation->f(); }
void g() { implementation->g(); }
void h() { implementation->h(); }
int main() {
Proxy p;
} ///:~

In some cases, Implementation doesn t need the same interface as Proxy as long as Proxy is somehow speaking for the Implementation class and referring function calls to it, then the basic idea is satisfied (note that this statement is at odds with the definition for Proxy in GoF). However, with a common interface you are able to do a drop-in replacement of the proxy into the client code the client code is written to talk to the original object, and it doesn t need to be changed in order to accept the proxy (This is probably the key issue with Proxy). In addition, Implementation is forced, through the common interface, to fulfill all the functions that Proxy needs to call.

The difference between Proxy and State is in the problems that are solved. The common uses for Proxy as described in GoF are:

1.      Remote proxy. This proxies for an object in a different address space. This is implemented by some remote object technologies.

2.      Virtual proxy. This provides lazy initialization to create expensive objects on demand.

3.      Protection proxy. Used when you don t want the client programmer to have full access to the proxied object.

4.      Smart reference. To add additional actions when the proxied object is accessed. Reference counting is an example: this keeps track of the number of references that are held for a particular object, in order to implement the copy-on-write idiom and prevent object aliasing.[142] A simpler example is counting the calls to a particular function.

Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire