Question

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Double Negation in C++ code.

I am working with production code where I have run across statements like this a few times:

Class.func(!!notABool);

The first couple of times I dismissed it as a programmer quirk (maybe to emphasize that it is a conditional statement rather than a number being passed into func?) but I have run across several statements that use the above and now I am wondering whether it actually makes a difference or not. In most cases notABool is a number(int, float, double… I have seen all 3) My initial guess was that it is akin to typing:

Class.func((bool)notABool);

but I am not entirely sure?

Answer 1

For primitive types, yes, it’s essentially equivalent to:

!(notABool != 0)

which in turn is equivalent to:

(bool)notABool

For non-primitive types, it will be a compiler error, unless you’ve overloaded operator!, in which case, it might do anything.