How Can Proxy Classes Enforce Data Integrity in C ?

Exploring Proxy Class in C

A proxy class is a fundamental programming technique in C that enables the creation of a modified interface to another class, known as the target class. This concept proves invaluable in various programming scenarios.

Consider the example of an array designed to store only binary digits (0 and 1). In a naive attempt, we might define an array with an index operator as shown below:

struct BinaryArray {
    int mArray[10];
    int &operator[](int i);
};

However, this approach presents a flaw: there's no way to enforce that only binary digits are stored in the array. To overcome this, we can employ a proxy class. A proxy acts as an intermediary between the user and the target class, providing a customized interface with restricted access to its members.

In our binary digit array example, we can create a proxy class called BitProxy that intercepts the assignments made through the index operator:

struct BitProxy {
    BitProxy(int &r) : mPtr(&r) {}
    void operator=(int n) {
        if (n != 0 && n != 1) {
            throw "Invalid binary digit";
        }
        *mPtr = n;
    }

private:
    int *mPtr;
};

By modifying the array class to return a BitProxy object in its index operator, we effectively limit the range of values that can be stored in the array:

struct BinaryArray {
    int mArray[10];
    BitProxy operator[](int i) {
        return BitProxy(mArray[i]);
    }
};

Now, when we attempt to assign non-binary digits to the array, the BitProxy ensures the integrity of the array by throwing an exception. This simple example illustrates the power of proxy classes in providing fine-grained control over class interfaces.

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