How to check type equality between non-template class and template class?

Question

Suppose I have

class A {};

template<typename T, typename U> class Wrapper {};

I'm trying to check if the wrapper's first inner type is A.

typeCheck(new A(), new Wrapper<A,B>);  // expect true
typeCheck(new A(), new Wrapper<C,B>);  // expect false

What's the best approach to do it?

I've tried template partial specialization but did not have luck.

Here is the code I tried:

template<typename T> struct get_inner {
  using type = T;
}

template<typename T, typename U>
struct get_inner<Wrapper<T,U>> {
  using type = T;
}

std::cout<< typeid(get_inner<decltype(new Wrapper<A,B>)>::type);

The console shows the same wrapper type instead of the inner type A. Is there anything mistake here?

The type of `new Wrapper` is `Wrapper *`, not `Wrapper `. Do you have [a good book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list)? — molbdnilo, Mar 03 '23 at 06:35

JeJo · Accepted Answer · 2023-03-03T07:27:50.397

The console shows the same wrapper type instead of the inner type A. Is there anything mistake here?

The type of new Wrapper<A,B> is a pointer to the Wrapper<A,B>, not Wrapper itself. Therefore, it doesn't suit to your specialization, and chose the first base template, where type is equal to the passed type, which is Wrapper<A,B>*.

Instead, you should have written

typename get_inner<Wrapper<A,B>>::type;

Note that, the decltype is also unnecessary because you are passing the template type itself already.

Since you are trying to see check the type at compile time, I would suggest using std::is_same as well. For example

std::cout << std::boolalpha
          << std::is_same_v<typename get_inner<Wrapper<A,B>>::type, A>;

See a demo

What's the best approach to do it?

Well, there are many ways!

One way is to do, like std::is_same_v, in standard type traits, you can write traits for this directly:

#include <type_traits>  // std::false_type, std::true_type

template<typename T> // base template
struct is_inner_type : std::false_type{};

template<typename T, typename U> // specialization
struct is_inner_type<Wrapper<T,U>> {
  static constexpr bool value = std::is_same_v<A, T>;
};
// variable template for convenience
template<typename ClassType>
inline constexpr bool is_inner_type_v = is_inner_type<ClassType>::value;

And you would use it like

static_assert(!is_inner_type_v<Wrapper<int, float>>);
static_assert(is_inner_type_v<Wrapper<A, float>>);

See a demo

However, that is not generic enough to change the type to be checked. There, I would suggest something similar to your original implementation.

// traits for finding the T & U type!
template <typename T> struct Class;
template <template<typename A, typename B> class Type,  typename A, typename B>
struct Class<Type<A,B>> {
    using T = A;
    // using U = B; // if needed
};
template <typename Type> using  class_t = typename Class<Type>::T;
// template <typename Type> using  class_u = typename Class<Type>::U; // if needed

// variable template for convenience
template<typename ClassType, typename TypeToCheck>
inline constexpr bool is_inner_type_v = std::is_same_v<class_t<ClassType>, TypeToCheck>;

I used, template template class specialization, in case you want to make it more generic and use it for other similar class templates.

Actually, I'm comparing fields of two classes, so there will be different type A. Is it still the best approach to do it? — LunaticJape, Mar 03 '23 at 06:44
Really appreciated. I'm still leaning C++, would you mind explaining the logic of your code? Or why mine does not work? Thanks. — LunaticJape, Mar 03 '23 at 07:09

How to check type equality between non-template class and template class?

1 Answers1