add path conversion functions

include/nana/filesystem/filesystem.hpp

@@ -162,6 +162,10 @@ namespace filesystem
 		const value_type*c_str() const;
 		const string_type& native() const;
 		operator string_type() const;
+
+		std::string string() const;
+		std::wstring wstring() const;
+		std::string u8string() const;
 	private:
 		void _m_assign(const std::string& source_utf8);
 		void _m_assign(const std::wstring& source);

source/filesystem/filesystem.cpp

@@ -173,6 +173,21 @@ namespace nana {	namespace experimental {
 			return native();
 		}
 
+		std::string path::string() const
+		{
+			return to_osmbstr(to_utf8(pathstr_));
+		}
+
+		std::wstring path::wstring() const
+		{
+			return to_wstring(pathstr_);
+		}
+
+		std::string path::u8string() const
+		{
+			return to_utf8(pathstr_);
+		}
+
 		void path::_m_assign(const std::string& source_utf8)
 		{
 #if defined(NANA_WINDOWS)

source/paint/detail/image_bmp.hpp

@@ -84,7 +84,7 @@ namespace nana{	namespace paint
 
 			bool open(const nana::experimental::filesystem::path& filename) override
 			{
-				std::ifstream ifs(filename.c_str(), std::ios::binary);
+				std::ifstream ifs(filename.string(), std::ios::binary);
 				if(ifs)
 				{
 					ifs.seekg(0, std::ios::end);

source/paint/image.cpp

@@ -263,7 +263,7 @@ namespace paint
 				//Check for BMP
 				if (!helper)
 				{
-					std::ifstream ifs(file.c_str(), std::ios::binary);
+					std::ifstream ifs(file.string(), std::ios::binary);
 					if (ifs)
 					{
 						unsigned short meta = 0;