C++ Virtual Functions – Why No ‘Pure’ Keyword?

The technical term for a class with a pure virtual function is abstract class, but that terms also applies if the class has additional non-pure virtual functions.

Often the term Interface is used in C++ to describe an abstract class like Printable, that describes a service that can be provided by a range of different, otherwise unrelated classes. This corresponds to the use of the interface keyword in C#/Java.

I have never encountered the term 'protocol' in connection with a C++ class

The question is not how many meanings it has, but whether or not it's reserved.

When Microsoft adds new features to C#, it sometimes need to add new keywords. This can compromise backward compatibility, since it invalidates old code that declares identifiers with the same name. To prevent this, Microsoft makes these new keywords contextual - they only serve as keywords when placed in specific places, places where you can't normally put identifiers.

So - async is a contextual keyword because older code might have declared a variable named async.

new was there from day one, so it doesn't have this problem - if your code declared a variable named new it wouldn't work even in the first version of C#. Therefore, there is no backwards compatibility issue here and new can be a regular keyword.