C99: 6.5.3.3 p5: The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

C++11 (or a near enough draft): 5.3.1 p9: The operand of the logical negation operator ! is contextually converted to bool (Clause 4); its value is true if the converted operand is false and false otherwise. The type of the result is bool.

and 4.7 p4 on integral conversion: If the source type is bool, the value false is converted to zero and the value true is converted to one.

Both seem to suggest that when you apply !, you end up with something that is 0 or 1 when used as a number, and adding more ! won't get you out of that.

Thanks a lot! And still interested in earlier C/C++ standards, I'm much more likely to encounter them in the wild for long time.

Ah, sorry. Looks like C++03 has the same wording in the same sections as C++11, and ANSI C's Unary arithmetic operators section is numbered 3.3.3.3 and also has the same wording as C99.

Great, thanks!