可变数据结构模板实例化

Question

The following web page has an example of a variadic data structure (a tuple) link . I have a question on the following code, which is claimed to generate the following class hierarchy. My question is, how is the hierarchy generated? I don't see where the new struct type<Ts...> is generated? Thanks

template <class... Ts> struct tuple {};

template <class T, class... Ts>
struct tuple<T, Ts...> : tuple<Ts...> {
  tuple(T t, Ts... ts) : tuple<Ts...>(ts...), tail(t) {}

  T tail;
};

Class Hierarchy:

tuple<double, uint64_t, const char*> t1(12.2, 42, "big");

struct tuple<double, uint64_t, const char*> : tuple<uint64_t, const char*> {
  double tail;
}

struct tuple<uint64_t, const char*> : tuple<const char*> {
  uint64_t tail;
}

struct tuple<const char*> : tuple {
  const char* tail;
}

struct tuple {
}

Answer 1

There are two class templates here

template <class... Ts> struct tuple {}; #1

and

template <class T, class... Ts>
struct tuple<T, Ts...> : tuple<Ts...> { ... } #2

And what happens in this case is that #2 is considered more specialized so when you have

tuple<double, uint64_t, const char*> 

#2 gets stamped out giving you

struct tuple<double, uint64_t, const char*> : tuple<uint64_t, const char*> 

This keeps happening creating the chain that you have, until you get to

struct tuple<const char*> : tuple {
  const char* tail;
}

and here : tuple has no parameters so the only template it can call is #1 which then generates the end case of

struct tuple {};

Answer 2

Whenever it is impossible to extract a single type out of the type list (that means that this list is empty), the following code takes effect:

template <class... Ts> struct tuple {};

That is an empty struct that becomes the base for the rest of the classes.

If there is at least one type, the compiler divides the list into class T and the rest (one type less in the list):

template <class T, class... Ts>
struct tuple<T, Ts...> : tuple<Ts...> {
  tuple(T t, Ts... ts) : tuple<Ts...>(ts...), tail(t) {}

  T tail;
};