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:

Assignment *a;
if (!getAssignment(query, a))
   return false;
hasSolution = !!a;

if (!a)
   return true;

How do I know what value will the double exclamation mark result in ? In other words does it always convert to true ? false ? or can you define a behavior for it such as executing a method to determine the result (how does the object know how to act in this situation) ? I am bit confused about this piece of code due to all these exclamation stuff going on.. Any explanation is appreciated.

Hope I was clear and thanks.

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.