Confused by use of double logical not (!!) operator

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Double Negation in C++ code 14 answers

What is “!!” in C? [duplicate] 7 answers

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It is not as simple as double negation. For example, if you have x == 5 , and then apply two ! operators ( !!x ), it will become 1 - so, it is used for normalizing boolean values in {0, 1} range.

Note that you can use zero as boolean false, and non-zero for boolean true, but you might need to normalize your result into a 0 or 1, and that is when !! is useful.

It is the same as x != 0 ? 1 : 0 x != 0 ? 1 : 0 .

Also, note that this will not be true if foo is not in {0, 1} set:

!!foo == foo

#include <iostream>

using namespace std;

int main()
{
        int foo = 5;

        if(foo == !!foo)
        {
                cout << "foo == !!foo" << endl;
        }
        else
        {
                cout << "foo != !!foo" << endl;
        }



        return 0;
}

Prints foo != !!foo .

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It can be used as shorthand to turn foo into a boolean expression. You might want to turn a non-boolean expression into true or false exclusively for some reason.

foo = !!foo is going to turn foo into 1 if it's non-zero, and leave it at 0 if it already is.

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if foo != 0 , then !!foo == 1 . It is basically a trick to convert to bool.

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