7 Standard conversions [conv]

7.14 Boolean conversions [conv.bool]

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool.
A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.
For direct-initialization, a prvalue of type std​::​nullptr_­t can be converted to a prvalue of type bool; the resulting value is false.