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

a is a pointer. In C++, nullptr is defined to be an invalid pointer. !pointer turns a nullptr pointer into true and a non nullptr pointer into false. !boolean turns true into false and false into true. It will always work.

!(!a) is a useful way to think of it.