typeTrait.hpp

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#pragma once
 
#include <type_traits>
#include "indep/common/typedef.hpp"
 
namespace by {
 
    template <typename T> struct typeTrait {
        typedef T Org;
        typedef T& Ref;
        typedef T* Ptr;
 
        static T ret() { return T{}; } // return default value
 
        static bool isNul(T) { return false; }
 
        static constexpr nbool is_ptr = false;
        static constexpr nbool is_ref = false;
        static constexpr nbool is_like_ptr = is_ptr;
    };
 
    template <typename T> struct typeTrait<T&> {
        typedef T Org;
        typedef T& Ref;
        typedef T* Ptr;
 
        static nbool isNul(const T& it) { return true; }
 
        static T& ret() {
            static T* dummy = nullptr;
            return *dummy;
        }
 
        static constexpr nbool is_ptr = false;
        static constexpr nbool is_ref = true;
        static constexpr nbool is_like_ptr = is_ptr;
    };
 
    template <> struct typeTrait<void> {
        static void ret() {}
 
        static bool isNul(void) { return false; }
 
        static constexpr nbool is_ptr = false;
        static constexpr nbool is_ref = false;
        static constexpr nbool is_like_ptr = is_ptr;
    };
 
    template <typename T> struct typeTrait<T*> {
        typedef T Org;
        typedef T& Ref;
        typedef T* Ptr;
 
        static nbool isNul(const T* it) { return it == nullptr; }
 
        static T* ret() { return nullptr; }
 
        static constexpr nbool is_ptr = true;
        static constexpr nbool is_ref = false;
        static constexpr nbool is_like_ptr = is_ptr;
    };
 
    template <> struct typeTrait<void*> {
        typedef void Org;
        typedef void* Ptr;
 
        static nbool isNul(const void* it) { return it == nullptr; }
 
        static void* ret() { return nullptr; }
 
        static constexpr nbool is_ptr = true;
        static constexpr nbool is_ref = false;
        static constexpr nbool is_like_ptr = is_ptr;
    };
 
    template <typename T> auto nul(T&& it) -> decltype(typeTrait<std::remove_reference_t<std::decay_t<T>>>::isNul(it)) {
        return typeTrait<std::remove_reference_t<std::decay_t<T>>>::isNul(std::forward<T>(it));
    }
} // namespace by