Custom C++17 type traits

MyTypeTraits.hpp

#pragma once

#include <type_traits>

namespace jjh
{
// Custom because 'custom' is true
class CustomClass1
{
public:
    static constexpr bool custom = true;
};

// Not custom because 'custom' is false
class CustomClass2
{
public:
    static constexpr bool custom = false;
};

// Custom because it inherits from a class that has 'custom'
class CustomClass3 : public CustomClass1
{
};

// Not custom because it does not have the 'custom' boolean at all
class CustomClass4
{
};

// is_custom implementation
namespace // anonymous
{
    template <typename T, typename = void> struct is_custom_base : std::false_type {};

    template <typename T>
    struct is_custom_base<T, std::enable_if_t<T::custom>> : std::true_type {};
}

// exposes is_custom and removes const/volatile to allow any combination
template <typename T> struct is_custom : is_custom_base<std::remove_cv_t<T>> {};

// helper
template <typename T> inline constexpr bool is_custom_v = is_custom<T>::value;

// is_small implementation
namespace // anonymous
{
    template <typename T, typename = void>
    struct is_small_base : std::false_type {};

    template <typename T>
    struct is_small_base<T, std::enable_if_t<sizeof(T) == 1>> : std::true_type {};
}

// exposes is_small and removes const/volatile to allow any combination
template <typename T> struct is_small : is_small_base<std::remove_cv_t<T>> {};

// helper
template <typename T> inline constexpr bool is_small_v = is_small<T>::value;
} // namespace jjh