参考出处：https://blog.csdn.net/FinalCreed/article/details/71037420
string string2hex(const string& str)
{
string temp;
stringstream ss;
string result;
for (int i = 0; i < str.size(); i++)
{
ss << hex << int(str[i]) << " ";
ss >> temp;
result += temp;
}
return result;
}