Passing references in c++ - c++

Question

I am writing a program for overloading "[ ] " operator on arrays .Here is my code
/ / A safe array example.
#include <iostream>
#include <cstdlib>
using namespace std;
class atype {
int a[3];
public:
atype(int i, int j, int k) {
a[0] = i;
a[1] = j;
a[2] = k;
}
int &operator[](int i);
};
// Provide range checking for atype.
int &atype::operator[](int i)
{
if(i<0 || i> 2) {
cout << "Boundary Error\n";
exit(1);
}
return a[i];
}
int main()
{
atype ob(1, 2, 3);
cout << ob[1]; // displays 2
cout << " ";
ob[1] = 25; // [] appears on left
cout << ob[1]; // displays 25
ob[3] = 44; // generates runtime error, 3 out-of-range
return 0;
}
In the class we are declaring as
int &operator[](int i);
and outside the class it is defined as
int &atype::operator[](int i)
It should be int atype::&operator[](int i)
but it is giving me error.
1>c:\users\abc\documents\visual studio 2010\projects\[]overl\[]overl\[]overl.cpp(17): error C2589: '&' : illegal token on right side of '::'
1>c:\users\abc\documents\visual studio 2010\projects\[]overl\[]overl\[]overl.cpp(17): warning C4091: '' : ignored on left of 'int' when no variable is declared
1>c:\users\abc\documents\visual studio 2010\projects\[]overl\[]overl\[]overl.cpp(17): error C2143: syntax error : missing ';' before '::'
1>c:\users\abc\documents\visual studio 2010\projects\[]overl\[]overl\[]overl.cpp(17): error C2059: syntax error : '::'
1>c:\users\abc\documents\visual studio 2010\projects\[]overl\[]overl\[]overl.cpp(18): error C2143: syntax error : missing ';' before '{'
1>c:\users\abc\documents\visual studio 2010\projects\[]overl\[]overl\[]overl.cpp(18): error C2447: '{' : missing function header (old-style formal list?)
But when I tried int &atype::operator[](int i)
it worked.Could anybody explain that are we passing reference to class or operator[](int i)

Answer 1

The general syntax is
<return type> class-name :: function-name (argument-list)
In your case the return type is a reference to an integer i.e. int &
int &atype::operator[](int i) is only the correct syntax

Answer 2

Your operator[] returns a reference to the element requested by index. In this case your array is of int type so it will be a reference-to-an-int, ie: int& as a return value.