操作员重载和设置值

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

1. See Sly_TheKing's answer (applying index operator to pointer).
2. 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