How do I get the template type of a given element at runtime in C++?

Question

I'm designing a simple Array class with the capability of holding any type of object, like a vector that can hold multiple types of data in one object. (This is for learning purposes.)

I have an empty base class called Container:

class Container {};

And a templatized subclass called Object:

template <class T>
class Object : public Container {
    T& object;
public:
    Object(T& obj = nullptr) : object(obj) {}
};

I have an Array class which holds a vector of pointers to Containers which I use to hold Objects:

class Array {
    std::vector<Container *> vec;
public:
    template <class T>
    void add_element(const T&);
    auto get_element(int);
};

add_element stores elements into Objects and puts them into vec:

template <class T>
void Array::add_element(const T& element)
{
    vec.push_back(new Object<T>(element));
}

get_element removes the element from it's Object and passes it back to the caller. This is where my problem lies. In order to remove the element from the Object, I need to know what type of Object it is:

auto Array::get_element(int i)
{
    return (Object</* ??? */> *)vec[i])->object;
}

Is there some way for me to find out what sort of object I'm storing?

Edit: since people are claiming that this is not possible, how about this. Is there some way of actually storing type information inside of a class? (I know you can do that in ruby). If I could do that, I could store the return type of get_element in each Object.

Answer 1

I was thinking something along the lines of this template for your get_element.

template <class T>
T& Array::get_element(int i, T &t)
{
    Object<T> *o = dynamic_cast<Object<T> *>(vec[i]);
    if (o == 0) throw std::invalid_argument;
    return t = o->object;
}

Array arr;
double f;
int c;

arr.get_element(0, f);
arr.get_element(1, c);

But alternatively, you could use this:

template <class T>
T& Array::get_element(int i)
{
    Object<T> *o = dynamic_cast<Object<T> *>(vec[i]);
    if (o == 0) throw std::invalid_argument;
    return o->object;
}

f = arr.get_element<double>(0);
c = arr.get_element<int>(1);

Answer 2

The return type of a function like Array::get_element must be determined at compile time. Since class Array does not know what types of containers it stores at compile time, the only option is to just return the base Container* pointer. Then users of that code can use virtual functions and/or dynamic_cast to deal with those generalized containers.