From bb58e65e6596a2ffac2e85e9fda1e8568aa9b4b1 Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Tue, 25 Feb 2020 20:57:06 +0100
Subject: new_type: implement cbegin

---
 include/newtype/impl/type_traits_extensions.hpp | 43 +++++++++++++++++++++++--
 include/newtype/new_type.hpp                    | 20 ++++++++++++
 2 files changed, 61 insertions(+), 2 deletions(-)

(limited to 'include')

diff --git a/include/newtype/impl/type_traits_extensions.hpp b/include/newtype/impl/type_traits_extensions.hpp
index 80ba5b0..ee33730 100644
--- a/include/newtype/impl/type_traits_extensions.hpp
+++ b/include/newtype/impl/type_traits_extensions.hpp
@@ -1051,7 +1051,7 @@ namespace nt::impl
 
   }  // namespace std_support
 
-  inline namespace iterable
+  inline namespace iterable_begin
   {
     template<typename T, typename = void>
     struct has_free_begin : std::false_type
@@ -1100,7 +1100,46 @@ namespace nt::impl
 
     template<typename T>
     auto constexpr has_begin_v = has_begin<T>::value;
-  }  // namespace iterable
+  }  // namespace iterable_begin
+
+  inline namespace iterable_cbegin
+  {
+    template<typename T, typename = void>
+    struct has_free_cbegin : std::false_type
+    {
+    };
+
+    template<typename T>
+    struct has_free_cbegin<T, std::void_t<decltype(cbegin(std::declval<T const &>()))>>
+        : std::is_same<typename T::const_iterator, std::remove_cvref_t<decltype(cbegin(std::declval<T const &>()))>>
+    {
+    };
+
+    template<typename T>
+    auto constexpr has_free_cbegin_v = has_free_cbegin<T>::value;
+
+    template<typename T, typename = void>
+    struct has_member_cbegin : std::false_type
+    {
+    };
+
+    template<typename T>
+    struct has_member_cbegin<T, std::void_t<decltype(std::declval<T const &>().cbegin())>>
+        : std::is_same<typename T::const_iterator, decltype(std::declval<T const &>().cbegin())>
+    {
+    };
+
+    template<typename T>
+    auto constexpr has_member_cbegin_v = has_member_cbegin<T>::value;
+
+    template<typename T>
+    struct has_cbegin : std::disjunction<has_free_cbegin<T>, has_member_cbegin<T>>
+    {
+    };
+
+    template<typename T>
+    auto constexpr has_cbegin_v = has_cbegin<T>::value;
+  }  // namespace iterable_cbegin
 
 }  // namespace nt::impl
 
diff --git a/include/newtype/new_type.hpp b/include/newtype/new_type.hpp
index 0db01b2..474ac88 100644
--- a/include/newtype/new_type.hpp
+++ b/include/newtype/new_type.hpp
@@ -71,6 +71,11 @@ namespace nt
         -> std::enable_if_t<DerivationClauseV(nt::Iterable) && impl::has_free_begin_v<BaseTypeT>,
                             typename new_type<BaseTypeT, TagTypeT, DerivationClauseV>::const_iterator>;
 
+    template<typename BaseTypeT, typename TagTypeT, auto DerivationClauseV>
+    auto constexpr friend cbegin(new_type<BaseTypeT, TagTypeT, DerivationClauseV> const & obj)
+        -> std::enable_if_t<DerivationClauseV(nt::Iterable) && impl::has_free_cbegin_v<BaseTypeT const>,
+                            typename new_type<BaseTypeT, TagTypeT, DerivationClauseV>::const_iterator>;
+
     using super = impl::new_type_move_assignment<BaseType, TagType>;
 
   public:
@@ -198,6 +203,13 @@ namespace nt
     {
       return this->m_value.begin();
     }
+
+    template<typename NewType = new_type>
+    auto constexpr cbegin() const -> std::enable_if_t<NewType::derivation_clause(nt::Iterable) && impl::has_member_cbegin_v<BaseType const>,
+                                                      typename NewType::const_iterator>
+    {
+      return this->m_value.cbegin();
+    }
   };
 
   /// Equality Comparison Operators
@@ -476,6 +488,14 @@ namespace nt
     return begin(obj.m_value);
   }
 
+  template<typename BaseType, typename TagType, auto DerivationClause>
+  auto constexpr cbegin(new_type<BaseType, TagType, DerivationClause> const & obj)
+      -> std::enable_if_t<DerivationClause(nt::Iterable) && impl::has_free_cbegin_v<BaseType const>,
+                          typename new_type<BaseType, TagType, DerivationClause>::const_iterator>
+  {
+    return cbegin(obj.m_value);
+  }
+
 }  // namespace nt
 
 namespace std
-- 
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