Add optional C++17 support.

It provides the same features as the optional C++11 support, but taking
in const string views instead of const string refs.

source/utf8/checked.h

@@ -325,9 +325,11 @@ namespace utf8
 
 } // namespace utf8
 
-#if UTF_CPP_CPLUSPLUS >= 201103L // C++ 11 or later
+#if UTF_CPP_CPLUSPLUS >= 201703L // C++ 17 or later
+#include "cpp17.h"
+#elif UTF_CPP_CPLUSPLUS >= 201103L // C++ 11 or later
 #include "cpp11.h"
-#endif // C++ 11 or later
+#endif
 
 #endif //header guard
 

source/utf8/cpp17.h

@@ -0,0 +1,102 @@
+// Copyright 2020 Nemanja Trifunovic
+
+/*
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+*/
+
+#ifndef UTF8_FOR_CPP_67fa6330_f477_4994_85ef_8d6f9be35ca3
+#define UTF8_FOR_CPP_67fa6330_f477_4994_85ef_8d6f9be35ca3
+
+#include "checked.h"
+#include <string>
+
+namespace utf8
+{
+
+    inline void append(char32_t cp, std::string& s)
+    {
+        append(uint32_t(cp), std::back_inserter(s));
+    }
+
+    inline std::string utf16to8(const std::u16string_view s)
+    {
+        std::string result;
+        utf16to8(s.begin(), s.end(), std::back_inserter(result));
+        return result;
+    }
+
+    inline std::u16string utf8to16(const std::string_view s)
+    {
+        std::u16string result;
+        utf8to16(s.begin(), s.end(), std::back_inserter(result));
+        return result;
+    }
+
+    inline std::string utf32to8(const std::u32string_view s)
+    {
+        std::string result;
+        utf32to8(s.begin(), s.end(), std::back_inserter(result));
+        return result;
+    }
+
+    inline std::u32string utf8to32(const std::string_view s)
+    {
+        std::u32string result;
+        utf8to32(s.begin(), s.end(), std::back_inserter(result));
+        return result;
+    }
+
+    inline std::size_t find_invalid(const std::string_view s)
+    {
+        std::string_view::const_iterator invalid = find_invalid(s.begin(), s.end());
+        return (invalid == s.end()) ? std::string::npos : (invalid - s.begin());
+    }
+
+    inline bool is_valid(const std::string_view s)
+    {
+        return is_valid(s.begin(), s.end());
+    }
+
+    inline std::string replace_invalid(const std::string_view s, char32_t replacement)
+    {
+        std::string result;
+        replace_invalid(s.begin(), s.end(), std::back_inserter(result), replacement);
+        return result;
+    }
+
+    inline std::string replace_invalid(const std::string_view s)
+    {
+        std::string result;
+        replace_invalid(s.begin(), s.end(), std::back_inserter(result));
+        return result;
+    }
+
+    inline bool starts_with_bom(const std::string_view s)
+    {
+        return starts_with_bom(s.begin(), s.end());
+    }
+ 
+} // namespace utf8
+
+#endif // header guard
+