c++ - 如何将 C++ v8 字符串转换为 C 兼容的无符号字符？

Question

typedef unsigned char   DL_UINT8;

--

if (info[0]->IsArray()) {
    v8::Handle<v8::Array> messageFields = v8::Handle<v8::Array>::Cast(info[0]);
    for (unsigned int i = 0; i < messageFields->Length(); i++) {
      v8::Handle<v8::Array> messageField = v8::Handle<v8::Array>::Cast(messageFields->Get(i));

      DL_UINT16 messageFieldPosition = (DL_UINT16)messageField->Get(0)->Uint32Value();
      const v8::String::Utf8Value _messageFieldValue(messageField->Get(1)->ToString());
      // how to convert?
      const DL_UINT8 messageFieldValue = (DL_UINT8)(_messageFieldValue); 

      //(DL_UINT16 iField, const DL_UINT8 *iDataStr)
      (void)DL_ISO8583_MSG_SetField_Str( messageFieldPosition, messageFieldValue, &isoMsg);
    }
}

我需要帮助转换String此行中的值

const DL_UINT8 messageFieldValue = (DL_UINT8)(_messageFieldValue);