What is Proxy Class in C++?

Here we will see what is the proxy class in C++. The Proxy class is basically the Proxy design pattern. In this pattern an object provides a modified interface for another class. Let us see one example.

In this example, we want to make an array class, that can store only binary values [0, 1]. This is the first try.

Example Code

class BinArray {    int arr[10];    int & operator[](int i) {       //Put some code here    } };

In this code, there is no condition checking. But we want the operator[] to complain if we put something like arr[1] = 98. But this is not possible, because it is checking the index not the value. Now we will try to solve this using proxy pattern.

Example Code

#include <iostream> using namespace std; class ProxyPat {    private:       int * my_ptr;    public:       ProxyPat(int& r) : my_ptr(&r) {       }       void operator = (int n) {          if (n > 1) {             throw "This is not a binary digit";          }          *my_ptr = n;       } }; class BinaryArray {    private:       int binArray[10];    public:       ProxyPat operator[](int i) {          return ProxyPat(binArray[i]);       }       int item_at_pos(int i) {          return binArray[i];       } }; int main() {    BinaryArray a;    try {       a[0] = 1; // No exception       cout << a.item_at_pos(0) << endl;    }    catch (const char * e) {       cout << e << endl;    }    try {       a[1] = 98; // Throws exception       cout << a.item_at_pos(1) << endl;    }    catch (const char * e) {       cout << e << endl;    } }

Output

1 This is not a binary digit