What is a Proxy Class in C++? Why it is created and where it is useful?
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 only want to contain binary digits (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 throw if we say something like a[1] = 42
, but that isn't possible because that operator only sees the index of 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 || n < 0) {
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.