Question

If I have overloaded operator bool(). Do I need to overload operator !() too? When and why. Thanks for help.

Answer 1

You should also implement operator!() if you want a developer to be able to say !myobject where myobject is an instance of your class.

Section 13.3.1.2 specifies that when applying a unary operator to an object of user-defined type

the built-in candidates include all of the candidate operator functions defined
in 13.6 that, compared to the given operator,

• have the same operator name, and
• accept the same number of operands, and
• accept operand types to which the given operand or operands can be converted according to
  13.3.3.1, and
• do not have the same parameter-type-list as any non-template non-member candidate.

So the compiler may use the built-in bool operator!(bool) and your user-defined conversion, but only when your operator bool() is implicitly callable. operator bool() is almost always made explicit to avoid its use in arbitrary integer contexts. Multiple user-defined conversions could also create ambiguity among built-in candidate operators as chris mentioned in a comment.

So it’s best to just define operator!() yourself.