fixed convert_wstring_to_mbstring as well

Author: pfeatherstone

dlib/unicode/unicode.cpp

@@ -114,11 +114,19 @@ namespace dlib
 
     const std::string convert_wstring_to_mbstring(const std::wstring &src)
     {
-        using namespace std;
-        std::string str;
-        str.resize((src.length() + 1) * MB_CUR_MAX);
-        wcstombs(&str[0], src.c_str(), str.size());
-        return std::string(&str[0]);
+        std::mbstate_t st{};
+        const wchar_t* p = src.c_str();
+
+        // Compute length
+        size_t n = std::wcsrtombs(nullptr, &p, 0, &st);
+        if (n == static_cast<std::size_t>(-1)) throw std::runtime_error("Invalid wide sequence / locale mismatch");
+
+        // Convert
+        std::string out(n, '\0');
+        st = std::mbstate_t{};
+        n  = std::wcsrtombs(out.data(), &p, out.size(), &st);
+        if (n == static_cast<std::size_t>(-1)) throw std::runtime_error("Conversion failed");
+        return out;
     }
 
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