binary'=='：找不到带有'std :: string'类型左手操作数的运算符（或者没有可接受的转换）

I am writing a template class function comparing std::strings. std::string is the template parameter. My problem is that I can't compare two const string with "==" operator, then I think I create two non-const temporary string variables to performe the comparation, but it still can't compile. Don't know why.I am writing a template class function comparin