Defining double exclamation?

Question

I understand what a double exclamation mark does (or I think I understand) but I am not sure how it is defined on a random object. For example in the code snippet below:

Answer 1

!! is not a single token in C++ and simply resolves to applying the ! operator twice.

As a is a pointer and not an object of class type the ! cannot be overloaded. It is defined to return true if a is a null pointer and false otherwise.

The second application of ! simply negates the result of the first !.

The expression !!a is equivalent to a != 0.