Returning reference to struct - c++

Question

Using overloaded [] operator , we can set a value of element inside container. E.g
class Example{
public:
Example(){
arr = new int[10];
}
int operator [] ( unsigned int i) const{
return arr[i];
}
int & operator [] ( unsigned int i){
return arr[i];
}
private:
int *arr[];
}
So using
Example a;
a[0] = 0;
a[1] = 1;
a[2] = 2;
We can set the value of elements. Is it possible to do something like this with struct? If we didnt have array of int but vector of structs. e.g
struct Test{
int value;
int index;
Test( int a , int b){
value = a;
index = b;
}
}
class Example{
public:
Example(){
}
int operator [] ( unsigned int i) const{
return a[i].value;
}
Test & operator [] ( unsigned int i){
Test a(0, i );
one.push_back(a);
return a -> value;
}
private:
vector<Test> a;
}
Example a;
a[0] = 0;
a[1] = 1;
a[2] = 2;
How can i change the the value property of returned struct same way as changing/setting value using typical int container?

Answer 1

Yes, it is possible.
The code has an error: return a -> value; returns an int, whereas the function operator[](unsigned int i) says it will return a reference to Test. You could instead return one[one.length()-1] so that you can say example[0]->value = 1;, where example is of type Example?
The statement example[0] is a call to operator[](unsigned int i) which returns a reference to one[0], whose type is a reference to Test.
It is perfectly valid to define operator[] to return an int, and implement it the way you did above. But that's highly unintuitive; I would certainly find it very confusing because the operator [] usually returns an element of a container, not a field of an element of a container. Better to define a separate function with self-explanatory name for that, for example int& getValue(unsigned int); for the value field and int& getIndex(unsigned int) for the index field.

Answer 2

#include <vector>
using namespace std;
struct Test {
int value;
int index;
Test(int a, int b) {
value = a;
index = b;
}
};
class Example {
public:
Example() {}
int operator [] (unsigned int i) const {
return _a[i].value;
}
int & operator [] (unsigned int i) {
Test a(0, i);
_a.push_back(a);
return _a[i].value;
}
private:
vector<Test> _a;
};
int main() {
Example a;
a[0] = 0;
a[1] = 1;
a[2] = 2;
return 0;
}