Make new instance of the template class of a c object-CodePudding

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?

CodePudding user response：

No, you create an instance of a class template by specializing the template. You do this by putting the type you want to use in angle brackets:

void foo(Object<int> object) {
    //  auto newobj = Object<int>(); this will work
    Object<int> newobj;               // but this has less cruft
}

template <class U>
void foo(Object<U> object) {
    //  auto newobj = Object<U>();
    Object<U> newobj  {object};
}

Remember that the symbol T does not exist outside the template definition. To get a "real" Object you have to put in an actual type. I chose int but you will probably use something else.

Of course, this will only work if the stuff contains a corresponding constructor:

template <class T>
class Object {
   // stuff
   public:
     Object(); // often implicit but sometimes not
     Object(Object<T> const &i) = default;
   // more stuff
};

CodePudding user response：

One solution is to store the template variable in the Object class (this only takes up memory at compile-time):

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;
}

If you don't want to add inner_type to Object then you could make the following type trait:

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 = inner<T>;

Which you can then use like so:

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