Make new instance of the template class of a c++ object

Question

So if I have

template <class T>
class Object {
// stuff
};

and I receive an instance of object in a function I want to call the constructor of class T.

void foo(Object object) {
 auto newT = object::T();
}

Is this possible?

Answer 1

Typically the best solution is to template the inner type:

template <class T>
void foo(Object<T> object) {
    T newT;
}

However, sometimes (with more meta-programming) this sort of solution will be more verbose than the alternatives:

---

Option 1: store the template variable in the Object class:

template <class T>
class Object {
    // stuff
public:
    using inner_type = T;
};

Then you can access the template type like so:

template <class Obj>
void foo(Obj object) {
    typename Obj::inner_type newT;
}

---

Option 2: make a type trait (no need to add inner_type to Object):

template <class T>
struct tag {
    using type = T;
};

template <class>
struct inner;

template <template <class> class S, class T>
struct inner <S<T>> : tag<T> {};

template <typename T>
using inner_t = typename inner<T>::type;

Which you can then use like so:

template <class Obj>
void foo(Obj object) {
    inner_t<Obj> newT;
}

---

It's probably best to generalise inner_type to take the first inner argument so that it could handle template types with more arguments, like std::vector (second argument has a default):

template <class>
struct front;

template <template <class, class...> class R, class S, class ... Ts>
struct front <R<S, Ts...>> : tag<S> {};

template <typename T>
using front_t = typename front<T>::type;

Demo