Array subscript operator

/* masfiq 8th december, 2016*/


#include<iostream>
#include<cstring>
#include<cstdlib>
using namespace std;

class Demo{
    int a[];
public :
    int& operator [] (int idx){
        return a[idx];
    }
};

int main(){
    Demo demo;
    demo[2] = 10;
    cout<<demo[2];

}
output :
10