Add window support for Windows.

src/core/StringUtils.cpp

@@ -2,6 +2,10 @@
 #include <locale>
 #include <algorithm>
 
+#ifdef _WIN32
+#include "Windows.h"
+#endif
+
 bool StringUtils::IsAlphaNumeric(char c)
 {
     std::locale loc;
@@ -21,3 +25,23 @@ std::string StringUtils::ToLower(const std::string& s)
             [](unsigned char c){ return std::tolower(c); });
     return ret;
 }
+
+std::string StringUtils::convert(const std::wstring& input)
+{
+    if (input.empty())
+    {
+        return std::string();
+    }
+
+#ifdef _WIN32
+    const auto size = ::WideCharToMultiByte(CP_UTF8, 0, &input[0], 
+        (int)input.size(), nullptr, 0, nullptr, nullptr);
+
+    std::string result(size, 0);
+    ::WideCharToMultiByte(CP_UTF8, 0, &input[0], (int)input.size(), 
+        &result[0], size, nullptr, nullptr);
+    return result;
+#else
+    throw std::logic_error("Not implemented");
+#endif
+}