Calculator from Bjarne Stroustrup's book

[DaineCloud]

I'm currently reading Bjarne Stroustrup's book:"C++ Programming Language - third edition". In chapter 6, he introduce a calculator program and I don't fully understand the program yet (but soon - I'm still quite the beginner in programming). I have the feeling that this is a RPN calculator, but I'm not sure, that is why I want confirmation whether it is one.

Here is the code from the book: calc.cpp

[oogabooga]

The very first sentence in chapter 6:

Statements and expressions are introduced by presenting a desk calculator program that provides
the four standard arithmetic operations as infix operators on floating-point numbers.

So it's not RPN.