Seeing as this is a course header, I think students are supposed to include it and then use most of the standard library that way. I am surprised Stroustrup teaches it that way (it is, in my opinion, still bad practice), but it does mean that he has one less bit of syntax to explain to his students. I don’t think any non-coursework applications (or, even worse, libraries) are supposed to include it, so it’s arguably somewhat acceptable.
I still think it’s very poor style, and that saying that standard things have to be prefixed with std:: would have been better.
solved using namespace std; in header file