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

There is no "!!" operator, so in fact, the statement is equivalent to:

hasSolution = !(!a);

So first, operator!() is called on expression "a", then another operator!() is called on the result. In the case of our code, "a" is a pointer to Assignement. C++ defines a special case for using operator!() on a pointer type: it returns a bool which is true if the pointer is null and false otherwise. In short, the same as the expression (a == 0). Calling operator!() on the result of (!a), which is a bool, simply reverts the result, i.e. returns true if (!a) is false and false if (!a) is true.

So in conclusion, (!!a) return the same result as (a != 0). This is because "a" is a pointer.