Defining double exclamation?

c++ boolean logical-operators
Cemre picture Cemre · Jul 7, 2012 · Viewed 14k times · Source

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

Jeffery Thomas picture Jeffery Thomas · Jul 7, 2012

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.

Related questions

Is there any difference between && and & with bool(s)?

In C++, is there any difference between doing && (logical) and & (bitwise) between bool(s)? bool val1 = foo(); bool val2 = bar(); bool case1 = val1 & val2; bool case2 = val1 && val2; Are case1 and case2 identical or …

Read More
What is the printf format specifier for bool?

Since ANSI C99 there is _Bool or bool via stdbool.h. But is there also a printf format specifier for bool? I mean something like in that pseudo code: bool x = true; printf("%B\n", x); which would print: true

Read More
Easiest way to flip a boolean value?

I just want to flip a boolean based on what it already is. If it's true - make it false. If it's false - make it true. Here is my code excerpt: switch(wParam) { case VK_F11: if (flipVal == true) { …

Read More