std::basic_string<CharT> has a FAT interface... I'll forward most of it to the embedded string.

src/nfc.cc

@@ -554,6 +554,25 @@ std::u32string createNFC(std::u32string nfd)
 }
 
 
+template<>
+IsNFC UTF<char>::isNFC(char c)
+{
+	if( c & 0x80 )
+		throw illegal_utf("Single octet >0x80 is invalid UTF-8");
+	
+	return IsNFC::Yes; // all ASCII characters are valid NFC.
+}
+
+
+template<>
+IsNFC UTF<char16_t>::isNFC(char16_t c)
+{
+	if(NFC_No.count(c)) return IsNFC::No;
+	if(NFC_Maybe.count(c)) return IsNFC::Maybe;
+	return IsNFC::Yes;
+}
+
+
 template<class CharT>
 IsNFC UTF<CharT>::isNFC_quick_check(std::basic_string_view<CharT> s)
 {

src/nfc.hh

@@ -27,7 +27,6 @@ class illegal_utf : public std::runtime_error
 public:
 	illegal_utf(   std::string_view, unsigned position, const std::string& reason);
 	illegal_utf(std::u16string_view, unsigned position, const std::string& reason);
-protected:
 	explicit illegal_utf(const std::string& message);
 };
 
@@ -48,7 +47,11 @@ public:
 	void generate(const char32_t c, OutIter& out);
 	
 
-	/// return No or Maybe, if at least one character with NFC_Quickcheck class is "No" or "Maybe"
+	/// returns the NFC class of a single character
+	static
+	IsNFC isNFC(CharT c);
+
+	/// returns No or Maybe, if at least one character with NFC_Quickcheck class is "No" or "Maybe"
 	/// might throw illegal_utf exception
 	static
 	IsNFC isNFC_quick_check(std::basic_string_view<CharT> s);
@@ -92,6 +95,8 @@ public:
 		/// only forward iterator. Does a backward_iterator make sense in UTF-encoded strings?
 		typedef typename String::const_iterator  const_iterator;
 		
+		static const size_t npos = String::npos;
+		
 		
 		explicit nfc_string(StringView  src);
 		explicit nfc_string(String &&   src);
@@ -126,13 +131,86 @@ public:
 		std::size_t  size()  const noexcept { return s.size();  }
 		bool         empty() const noexcept { return s.empty(); }
 		
+		std::size_t capacity() const noexcept { return s.capacity(); }
+		void reserve(std::size_t new_capacity) { s.reserve(new_capacity); }
+		void shrink_to_fit() { s.shrink_to_fit(); }
+		
+		const_reference operator[](std::size_t ofs) const noexcept { return s[ofs]; }
+		const_reference at(std::size_t ofs) const { return s.at(ofs); }
+		const_reference front() const noexcept { return s.front(); }
+		const_reference back()  const noexcept { return s.back(); }
+		operator StringView()   const noexcept { return StringView{s}; }
+		
 		const_iterator  begin() const noexcept { return s.cbegin(); }
 		const_iterator cbegin() const noexcept { return s.cbegin(); }  /// r/o access only
 		const_iterator  end()   const noexcept { return s.cend(); }
 		const_iterator cend()   const noexcept { return s.cend(); }    /// r/o access only
 		
+		void clear() { s.clear(); }
+		
+		/// I am lazy and delegate all the 10 different insert() overloads directly to s.
+		template<typename... Args>
+		nfc_string& insert(Args&& ...args)
+		{
+			s.insert( std::forward<Args>(args)... );
+			normalize();
+			return *this;
+		}
+		
+		/// delegates all erase() overloads to s.
+		template<typename... Args>
+		nfc_string& erase(Args&& ...args)
+		{
+			s.erase( std::forward<Args>(args)... );
+			normalize();
+			return *this;
+		}
+		
+		nfc_string& push_back(CharT c);
+		
+		/// delegates all 9 append() overloads to s.
+		template<typename... Args>
+		nfc_string& append(Args&& ...args)
+		{
+			s.append( std::forward<Args>(args)... );
+			normalize();
+			return *this;
+		}
+		
+		nfc_string& operator+=(const StringView& s);
+		
+		/// optimization possible to avoid re-normalization in most cases.
+		nfc_string& operator+=(const nfc_string& s);
+		
+		/// optimization possible to avoid re-normalization in most cases.
+		nfc_string& operator+=(CharT c);
+		
+		/// delegates all 9 compare() overloads to s
+		template<typename... Args>
+		int compare(Args&& ...args) const
+		{
+			return s.compare( std::forward<Args>(args)... );
+		}
+		
+		/// stolen from C++20
+		bool starts_with(StringView s) const;
+		
+		/// stolen from C++20
+		bool ends_with(StringView s) const;
+		
+		/// delegates all 5 find() overloads to s
+		template<typename... Args>
+		std::size_t find(Args&& ...args) const
+		{
+			return s.find( std::forward<Args>(args)... );
+		}
+		
+		
 	private:
 		std::basic_string<CharT> s;
+		
+		/// (re-)normalize the content string s.
+		void normalize();
 	};
 };