fix support for utf8 logging under win32

example/example.cpp

@@ -14,5 +14,5 @@ spdlog::logger *get_logger();
 int main(int, char *[])
 {    
     auto *l = get_logger();
-    l->info("HE LO ", "GA");    		   
+    l->info(L"HEllo {}", L"HGFS");
 }
\ No newline at end of file

include/spdlog/details/os.h

@@ -79,6 +79,11 @@ bool is_color_terminal() SPDLOG_NOEXCEPT;
 // Detrmine if the terminal attached
 // Source: https://github.com/agauniyal/rang/
 bool in_terminal(FILE *file) SPDLOG_NOEXCEPT;
+
+#if defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT) && defined(_WIN32)
+void wbuf_to_utf8buf(const fmt::wmemory_buffer &wbuf, fmt::memory_buffer &target);
+#endif
+
 } // namespace os
 } // namespace details
 } // namespace spdlog

src/logger.cpp

@@ -172,4 +172,4 @@ void spdlog::logger::err_handler_(const std::string &msg)
         std::strftime(date_buf, sizeof(date_buf), "%Y-%m-%d %H:%M:%S", &tm_time);
         fmt::print(stderr, "[*** LOG ERROR ***] [{}] [{}] {}\n", date_buf, name(), msg);
     }
-}
+}
\ No newline at end of file

src/os.cpp

@@ -397,6 +397,30 @@ SPDLOG_INLINE bool in_terminal(FILE *file) SPDLOG_NOEXCEPT
     return isatty(fileno(file)) != 0;
 #endif
 }
+
+#if defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT) && defined(_WIN32)
+SPDLOG_INLINE void wbuf_to_utf8buf(const fmt::wmemory_buffer &wbuf, fmt::memory_buffer &target)
+{
+    int wbuf_size = static_cast<int>(wbuf.size());
+    if (wbuf_size == 0)
+    {
+        return;
+    }
+
+    auto result_size = ::WideCharToMultiByte(CP_UTF8, 0, wbuf.data(), wbuf_size, NULL, 0, NULL, NULL);
+
+    if (result_size > 0)
+    {
+        target.resize(result_size);
+        ::WideCharToMultiByte(CP_UTF8, 0, wbuf.data(), wbuf_size, &target.data()[0], result_size, NULL, NULL);
+    }
+    else
+    {
+        throw spdlog::spdlog_ex(fmt::format("WideCharToMultiByte failed. Last error: {}", ::GetLastError()));
+    }
+}
+#endif // SPDLOG_WCHAR_TO_UTF8_SUPPORT) && _WIN32
+
 } // namespace os
 } // namespace details
 } // namespace spdlog