Calculator from Bjarne Stroustrup's book

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: Attachment 11953

The very first sentence in chapter 6:

Quote:

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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.

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So it's not RPN.