How to get first element of typelist in C -11-CodePudding

I am following C templates the complete guide and trying to get the first element from a typelist.

The following compiles:

#include <bits/stdc  .h>

using namespace std;

template <typename... Elements>
class Typelist;

using SignedIntegralTypes =
        Typelist<signed char, short, int, long, long long>;

template <typename List>
class HeadT;

template <typename Head, typename... Tail>
class HeadT<Typelist<Head, Tail...>> {
public:
    using Type = Head;
};

template <typename List>
using Head = typename HeadT<Typelist<List>>::Type;


int main() {
    static_assert(is_same<Head<SignedIntegralTypes>, SignedIntegralTypes>::value, "");
}

Head<SignedIntegralTypes> produces SignedIntegralTypes. I would expect it to produce signed char. Why is this happening? How do I fix it?

CodePudding user response：

Let's deconstruct all the templates, one step at a time.

Head<SignedIntegralTypes>

Ok, now let's take the definition of what Head is:

template <typename List>
using Head = typename HeadT<Typelist<List>>::Type;

Since SignedIntegralTypes is the template parameter, that's what List becomes here. So this becomes:

typename HeadT<Typelist<SignedIntegralTypes>>::Type

And since SignedIntegralTypes is, itself a:

using SignedIntegralTypes =
        Typelist<signed char, short, int, long, long long>;

The full class becomes:

typename HeadT<Typelist<Typelist<signed char, short, int, long, long long>>>::Type

And if you work out the result of the specialization, the first type in the Typelist is

Typelist<signed char, short, int, long, long long>

And that's what Type gets aliased to. A.k.a. a SignedIntegralTypes.

And you fix this, to get the intended result, simply by fixing the Head alias:

template <typename List>
using Head = typename HeadT<List>::Type;