From de5d0cbb9c58852ff0038be5495fa7e5d656dd1d Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Thu, 2 Jan 2020 11:43:54 +0100
Subject: new_type: implement subtraction

---
 include/newtype/impl/type_traits_extensions.hpp | 61 +++++++++++++++++++++++++
 include/newtype/new_type.hpp                    | 18 ++++++++
 2 files changed, 79 insertions(+)

(limited to 'include')

diff --git a/include/newtype/impl/type_traits_extensions.hpp b/include/newtype/impl/type_traits_extensions.hpp
index d6dbfd6..56824e2 100644
--- a/include/newtype/impl/type_traits_extensions.hpp
+++ b/include/newtype/impl/type_traits_extensions.hpp
@@ -577,6 +577,67 @@ namespace nt::impl
     template<typename T>
     auto constexpr is_nothrow_addable_v = is_nothrow_addable<T>::value;
 
+    /**
+     * @brief A trait to test if a given type is subtractable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-subtractable T
+     */
+    template<typename T, typename = void>
+    struct is_subtractable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is input streamable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for subtractable T
+     */
+    template<typename T>
+    struct is_subtractable<T, std::void_t<decltype(std::declval<T const &>() + std::declval<T const &>())>> : std::true_type
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is subtractable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_subtractable_v = is_subtractable<T>::value;
+
+    /**
+     * @brief A trait to test if a given type is noexcept subtractable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-noexcept subtractable or non-subtractable T
+     */
+    template<typename T, typename = void>
+    struct is_nothrow_subtractable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is noexcept subtractable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for subtractable T detemining if T is noexcept subtractable
+     */
+    template<typename T>
+    struct is_nothrow_subtractable<T, std::void_t<decltype(std::declval<T const &>() + std::declval<T const &>())>>
+        : std::bool_constant<noexcept(std::declval<T const &>() + std::declval<T const &>())>
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is noexcept subtractable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_nothrow_subtractable_v = is_nothrow_subtractable<T>::value;
+
   }  // namespace arithmetic
 
 }  // namespace nt::impl
diff --git a/include/newtype/new_type.hpp b/include/newtype/new_type.hpp
index 8228a94..bc8ecee 100644
--- a/include/newtype/new_type.hpp
+++ b/include/newtype/new_type.hpp
@@ -346,6 +346,24 @@ namespace nt
     return {lhs.decay() + rhs.decay()};
   }
 
+  /**
+   * @brief Subtract two instances of the same nt::new_type
+   *
+   * @note This operator is only available if the derivation clause of the passed in nt::new_type objects contains nt::Arithmetic and the base
+   * type is subtractable.
+   * @param lhs The left-hand side of the subtraction
+   * @param rhs The right-hand side of the subtraction
+   * @return a new instance of the same nt::new_type
+   */
+  template<typename BaseType, typename TagType, auto DerivationClause>
+  auto constexpr
+  operator-(new_type<BaseType, TagType, DerivationClause> const & lhs, new_type<BaseType, TagType, DerivationClause> const & rhs) noexcept(
+      impl::is_nothrow_subtractable_v<BaseType> && std::is_nothrow_copy_constructible_v<BaseType>)
+      -> std::enable_if_t<DerivationClause(nt::Arithmetic) && impl::is_subtractable_v<BaseType>, new_type<BaseType, TagType, DerivationClause>>
+  {
+    return {lhs.decay() - rhs.decay()};
+  }
+
 }  // namespace nt
 
 #endif
-- 
cgit v1.2.3