What is Proxy Class in C++

Question

I wanted to know What is Proxy Class in C++, why it is created and where it is useful.

Thank a lot in advance.

Answer 1

Proxy (among many other meanings) is a design pattern -- see wikipedia for excellent coverage (not intensely C++ specific, of course).

Answer 2

A proxy is a class that provides a modified interface to another class. Here is an example - suppose we have an array class that we we only want to be able to contain the binary digist 1 or 0. Here is a first try:

struct array1 {
    int mArray[10];
    int & operator[]( int i) {
      /// what to put here
    }
}; `

We want operator[] to complain if we say something like a[1] = 42, but that isn't possible because the operator only sees the index of into the array, not the value being stored.

We can solve this using a proxy:

#include <iostream>
using namespace std;;

struct aproxy {
    aproxy( int & r ) : mPtr( & r ) {}
    void operator = ( int n ) {
     if ( n > 1 ) {
      throw "not binary digit";
     }
     *mPtr = n;
    }
    int * mPtr;
};

struct array {
    int mArray[10];
    aproxy operator[]( int i) {
     return aproxy( mArray[i] );
    }
};

int main() {
    try {
     array a;
     a[0] = 1; // ok
     a[0] = 42;      // throws exception
    }
    catch( const char * e ) {
     cout << e << endl;
    }
}

The proxy class now does our checking for a binary digit and we make the array's operator[] return an instance of the proxy which has limited access to the array's internals.

Answer 3

A proxy class allows you to hide the private data of a class from clients of the class.

Providing clients of your class with a proxy class that knows only the public interface to your class enables the clients to use your class's services without giving the client access to your class's implementation details.