operator[] overloading
<p>There is a class like this:</p>

<pre><code>class X {
    public:
        ...
    private:
        int changeable[3];
        int unchangeable[3];
};    
</code></pre>

<p>And this is its desirable usage:</p>

<pre><code>X x;
x[0] = 1;  // here changeable[0] should be used
a = x[0];  // here unchangeable[0] should be used
</code></pre>

<p>Is there any way of defining <code>operator[]</code> in <code>class X</code> to implement that?</p>
