Make string::UTF16toUTF8 and UTF16toUTF8 Windows-only, and use

WideCharToMultiByte/MultiByteToWideChar instead of C++11.

include/string.hpp

@@ -12,9 +12,6 @@ namespace rack {
 namespace string {
 
 
-/** Performs a Unicode string conversion from UTF-16 to UTF-8. */
-std::string UTF16toUTF8(const std::u16string& s);
-std::u16string UTF8toUTF16(const std::string& s);
 /** Converts a `printf()` format string and optional arguments into a std::string.
 Remember that "%s" must reference a `char *`, so use `.c_str()` for `std::string`s, otherwise you might get binary garbage.
 */
@@ -89,5 +86,13 @@ struct CaseInsensitiveCompare {
 };
 
 
+#if defined ARCH_WIN
+/** Performs a Unicode string conversion from UTF-16 to UTF-8.
+These are only defined on Windows because the implementation uses Windows' API, and conversion is not needed on other OS's (since everything on Mac and Linux is UTF-8).
+*/
+std::string UTF16toUTF8(const std::u16string& s);
+std::u16string UTF8toUTF16(const std::string& s);
+#endif
+
 } // namespace string
 } // namespace rack

src/string.cpp

@@ -1,10 +1,12 @@
-#include <locale> // for wstring_convert
-#include <codecvt> // for codecvt_utf8_utf16
 #include <cctype> // for tolower and toupper
 #include <algorithm> // for transform
 #include <libgen.h> // for dirname and basename
 #include <zlib.h>
 
+#if defined ARCH_WIN
+	#include <windows.h> // for MultiByteToWideChar
+#endif
+
 #include <string.hpp>
 
 
@@ -12,18 +14,6 @@ namespace rack {
 namespace string {
 
 
-std::string UTF16toUTF8(const std::u16string& s) {
-	std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> converter;
-	return converter.to_bytes(s);
-}
-
-
-std::u16string UTF8toUTF16(const std::string& s) {
-	std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> converter;
-	return converter.from_bytes(s);
-}
-
-
 std::string f(const char* format, ...) {
 	va_list args;
 	va_start(args, format);
@@ -299,5 +289,36 @@ bool CaseInsensitiveCompare::operator()(const std::string& a, const std::string&
 }
 
 
+#if defined ARCH_WIN
+std::string UTF16toUTF8(const std::u16string& sU16) {
+	if (sU16.empty())
+		return "";
+	// Compute length of output buffer
+	int len = WideCharToMultiByte(CP_UTF8, 0, (wchar_t*) &sU16[0], sU16.size(), NULL, 0, NULL, NULL);
+	assert(len > 0);
+	std::string s;
+	// Allocate enough space for null character
+	s.resize(len);
+	len = WideCharToMultiByte(CP_UTF8, 0, (wchar_t*) &sU16[0], sU16.size(), &s[0], len, 0, 0);
+	assert(len > 0);
+	return s;
+}
+
+
+std::u16string UTF8toUTF16(const std::string& s) {
+	if (s.empty())
+		return u"";
+	// Compute length of output buffer
+	int len = MultiByteToWideChar(CP_UTF8, 0, &s[0], s.size(), NULL, 0);
+	assert(len > 0);
+	std::u16string sU16;
+	// Allocate enough space for null character
+	sU16.resize(len);
+	len = MultiByteToWideChar(CP_UTF8, 0, &s[0], s.size(), (wchar_t*) &sU16[0], len);
+	assert(len > 0);
+	return sU16;
+}
+#endif
+
 } // namespace string
 } // namespace rack