3 files changed, 40 insertions, 14 deletions
diff --git a/include/unicode/type_traits.h b/include/unicode/type_traits.h
index 3ee1d82..c3507e7 100644
--- a/include/unicode/type_traits.h
+++ b/include/unicode/type_traits.h
@@ -1,6 +1,6 @@
 #pragma once
 
-#include "utf.h"
+#include "utf_fwd.h"
 
 #include <string>
 #include <type_traits>
diff --git a/include/unicode/utf.h b/include/unicode/utf.h
index dd504a7..81e8f2b 100644
--- a/include/unicode/utf.h
+++ b/include/unicode/utf.h
@@ -1,5 +1,8 @@
 #pragma once
 
+#include "utf_fwd.h"
+#include "type_traits.h"
+
 #include <list>
 #include <string>
 #include <stdexcept>
@@ -37,7 +40,7 @@ namespace unicode::detail {
          (... && is_utf8_followup_byte(bytes)); // left fold for linear evaluation from left to right
  }
 
- template<typename T, typename std::enable_if_t<(sizeof(T) == 1), bool> = true>
+ template<typename T, typename std::enable_if_t<is_utf_8_v<T>, bool> = true>
  inline bool validate_utf(const std::basic_string<T>& s)
  {
   int i{};
@@ -78,7 +81,7 @@ namespace unicode::detail {
   }
  }
 
- template<typename T, typename std::enable_if_t<(sizeof(T) == 2), bool> = true>
+ template<typename T, typename std::enable_if_t<is_utf_16_v<T>, bool> = true>
  inline bool validate_utf(const std::basic_string<T>& s)
  {
   int i{};
@@ -95,7 +98,7 @@ namespace unicode::detail {
   return true;
  }
 
- template<typename T, typename std::enable_if_t<(sizeof(T) == 4), bool> = true>
+ template<typename T, typename std::enable_if_t<is_utf_32_v<T>, bool> = true>
  inline bool validate_utf(const std::basic_string<T>& s)
  {
   for (auto i: s)
@@ -135,10 +138,10 @@ namespace unicode::detail {
    return decode_utf8_leading_byte<sequence_length>(b) | decode_utf8_followup_byte(bytes...);
  }
 
- template<typename T, typename Container=std::basic_string<T>>
+ template<typename T, typename Container>
  struct utf_iterator
  {
-  static_assert(sizeof(T) == 1 || sizeof(T) == 2 || sizeof(T) == 4);
+  static_assert(is_utf_8_v<T> || is_utf_16_v<T> || is_utf_32_v<T>);
 
   typedef T value_type;
   typedef char32_t internal_type;
@@ -199,13 +202,13 @@ namespace unicode::detail {
    }
   }
 
-  template<class X = value_type, typename std::enable_if_t<(sizeof(X) == 1), bool> = true>
+  template<class X = value_type, typename std::enable_if_t<is_utf_8_v<X>, bool> = true>
   inline internal_type calculate_value()
   {
    return calculate_utf8_value(static_cast<utf8_t>(get_code_unit<0>()));
   }
 
-  template<class X = value_type, typename std::enable_if_t<(sizeof(X) == 2), bool> = true>
+  template<class X = value_type, typename std::enable_if_t<is_utf_16_v<X>, bool> = true>
   inline internal_type calculate_value()
   {
    char16_t unit0 {static_cast<char16_t>(get_code_unit<0>())};
@@ -226,7 +229,7 @@ namespace unicode::detail {
    }
   }
 
-  template<class X = value_type, typename std::enable_if_t<(sizeof(X) == 4), bool> = true>
+  template<class X = value_type, typename std::enable_if_t<is_utf_32_v<X>, bool> = true>
   inline internal_type calculate_value()
   {
    internal_type result {static_cast<internal_type>(get_code_unit<0>())};
@@ -296,10 +299,10 @@ namespace unicode::detail {
    return utf8_trailing_byte<m - n - 1, From, To>(value);
  }
 
- template<typename T, typename Container=std::basic_string<T>>
+ template<typename T, typename Container>
  struct utf_back_insert_iterator
  {
-  static_assert(sizeof(T) == 1 || sizeof(T) == 2 || sizeof(T) == 4);
+  static_assert(is_utf_8_v<T> || is_utf_16_v<T> || is_utf_32_v<T>);
 
   typedef T value_type;
   typedef char32_t internal_type;
@@ -341,7 +344,7 @@ namespace unicode::detail {
    }
   }
 
-  template<class X = value_type, typename std::enable_if_t<(sizeof(X) == 1), bool> = true>
+  template<class X = value_type, typename std::enable_if_t<is_utf_8_v<X>, bool> = true>
   inline void append_utf(const internal_type& value)
   {
    using Y = internal_type;
@@ -357,7 +360,7 @@ namespace unicode::detail {
    }
   }
 
-  template<class X = value_type, typename std::enable_if_t<(sizeof(X) == 2), bool> = true>
+  template<class X = value_type, typename std::enable_if_t<is_utf_16_v<X>, bool> = true>
   inline void append_utf(const internal_type& value)
   {
    if (value <= 0xFFFF) { // expect value to be already valid Unicode values (checked in input iterator)
@@ -368,7 +371,7 @@ namespace unicode::detail {
    }
   }
 
-  template<class X = value_type, typename std::enable_if_t<(sizeof(X) == 4), bool> = true>
+  template<class X = value_type, typename std::enable_if_t<is_utf_32_v<X>, bool> = true>
   inline void append_utf(const internal_type& value)
   {
    // expect value to be already valid Unicode values (checked in input iterator)
diff --git a/include/unicode/utf_fwd.h b/include/unicode/utf_fwd.h
new file mode 100644
index 0000000..f3f6c52
--- /dev/null
+++ b/include/unicode/utf_fwd.h
@@ -0,0 +1,23 @@
+#pragma once
+
+// Forward declarations
+
+#include <string>
+
+namespace unicode::detail {
+
+ template<typename T, typename Container=std::basic_string<T>>
+ struct utf_iterator;
+
+ template<typename T, typename Container=std::basic_string<T>>
+ struct utf_back_insert_iterator;
+
+} // namespace unicode::detail
+
+namespace unicode {
+
+ template<typename InputIt, typename OutputIt>
+ struct UTF;
+
+} // namespace unicode
+