Add UTF8ToUTF16 and UTF16ToUTF8.

format.cc

@@ -167,6 +167,42 @@ void fmt::internal::ReportUnknownType(char code, const char *type) {
         << static_cast<unsigned>(code) << type));
 }
 
+#ifdef _WIN32
+
+fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
+  int length = MultiByteToWideChar(
+      CP_UTF8, MB_ERR_INVALID_CHARS, s.c_str(), -1, 0, 0);
+  static const char ERROR[] = "cannot convert string from UTF-8 to UTF-16";
+  if (length == 0)
+    ThrowSystemError(GetLastError(), ERROR);
+  buffer_.resize(length);
+  length = MultiByteToWideChar(
+    CP_UTF8, MB_ERR_INVALID_CHARS, s.c_str(), -1, &buffer_[0], length);
+  if (length == 0)
+    ThrowSystemError(GetLastError(), ERROR);
+}
+
+fmt::internal::UTF16ToUTF8::UTF16ToUTF8(fmt::WStringRef s) {
+  if (int error_code = Convert(s)) {
+    ThrowSystemError(GetLastError(),
+        "cannot convert string from UTF-16 to UTF-8");
+  }
+}
+
+int fmt::internal::UTF16ToUTF8::Convert(fmt::WStringRef s) {
+  int length = WideCharToMultiByte(CP_UTF8, 0, s.c_str(), -1, 0, 0, 0, 0);
+  if (length == 0)
+    return GetLastError();
+  buffer_.resize(length);
+  length = WideCharToMultiByte(
+    CP_UTF8, 0, s.c_str(), -1, &buffer_[0], length, 0, 0);
+  if (length == 0)
+    return GetLastError();
+  return 0;
+}
+
+#endif
+
 void fmt::internal::FormatSystemErrorMessage(
     fmt::Writer &out, int error_code, fmt::StringRef message) {
 #ifndef _WIN32

format.h

@@ -458,6 +458,37 @@ template <typename Char, typename T>
 void FormatCustomArg(
   BasicWriter<Char> &w, const void *arg, const FormatSpec &spec);
 
+#ifdef _WIN32
+// A converter from UTF-8 to UTF-16.
+// It is only provided for Windows since other systems use UTF-8.
+class UTF8ToUTF16 {
+ private:
+  Array<wchar_t, INLINE_BUFFER_SIZE> buffer_;
+
+ public:
+  explicit UTF8ToUTF16(StringRef s);
+  operator const wchar_t*() const { return &buffer_[0]; }
+  size_t size() const { return buffer_.size() - 1; }
+};
+
+// A converter from UTF-16 to UTF-8.
+// It is only provided for Windows since other systems use UTF-8.
+class UTF16ToUTF8 {
+ private:
+  Array<char, INLINE_BUFFER_SIZE> buffer_;
+
+ public:
+  UTF16ToUTF8() {}
+  explicit UTF16ToUTF8(WStringRef s);
+  operator const char*() const { return &buffer_[0]; }
+  size_t size() const { return buffer_.size() - 1; }
+
+  // Performs conversion returning a system error code instead of
+  // throwing exception on error.
+  int Convert(WStringRef s);
+};
+#endif
+
 // Formats a system error message writing the output to out.
 void FormatSystemErrorMessage(Writer &out, int error_code, StringRef message);
 
@@ -1507,7 +1538,7 @@ class SystemErrorSink {
 
 /** Throws SystemError with a code and a formatted message. */
 inline Formatter<SystemErrorSink> ThrowSystemError(
-    int error_code, StringRef format) {
+    int error_code, StringRef format = 0) {
   Formatter<SystemErrorSink> f(format, SystemErrorSink(error_code));
   return f;
 }