Operator overloading and setting values - c++

Question

I created a Container class and used the new keyword in combination with pointers to learn how it works and how I can use it.
template<typename T>
class Container {
private:
T value;
public:
Container(T value) {
this->value = value;
}
Container() {
}
virtual ~Container() {
}
T getValue() {
return value;
}
void setValue(T value) {
this->value = value;
}
void operator[](T value) {
this->value = value;
}
};
int main() {
std::vector<Container<int>*> arr(10);
for (int i = 0; i < 10; i++) {
Container<int> *a = new Container<int>;
a->setValue(i);
// a[i];
arr[i] = a;
std::cout << arr[i]->getValue() << std::endl;
delete a;
}
return 0;
}
The [] operator has the same code as setValue(), but it only prints the numbers from 0 to 9 if I use a->setValue(i) and with using a[i] it prints just a random number. Why?

Answer 1

See Sly_TheKing's answer (applying index operator to pointer).
The index operator is intended to access values at a specific offset to some reference. It should accept a signed or unsigned integer value and return some specific value. So the operator, to be valid, should look like this:
T& operator[](size_t index)
{
return value;
}
Actually, as you do not have anything you could apply an index to (the only valid index in your case would be 0 – or from another point of view, with above implementation, any index would return the same value, so &a[0] == &a[1] would apply - which might be syntactically correct, but violates the semantics of an index operator...), dereferencing operators would be more appropriate:
T& operator*() { return value; }
T& operator->() { return value; }
Possibly, you could add an assignment operator, too (would replace setValue):
Container& operator=(T const& value) { this->value = value; return *this; }

Answer 2

In line
Container<int> *a = new Container<int>;
you initialize a as pointer, so with this line
a[i];
you just access some memory with address stored in a and offset i * sizeof(container<int>)
So the correct usage would be
std::vector<Container<int>*> arr(10);
for (int i = 0; i < 10; i++) {
Container<int> *a = new Container<int>;
(*a)[i];
arr[i] = a;
std::cout << arr[i]->getValue() << std::endl;
delete a;
}
With (*a)[i]; you access operator[] you wrote in your class