Why The Use Of an operand of type bool is forbidden to the 'operator++' in C++.

Question

I Mean Why increment and decrement operator does not works on Bool values although they work fine when written as x+1 or x-1. Might be a silly question but I'm just curious bout it.thnks

Answer 1

When we do x+1 or x-1 where x is Boolean, implicit widening conversion occurs and the result of the evaluation would be an integer value instead of a boolean. If x++ is allowed to be executed on a boolean, the program would attempt to increment the value, and then coerce it to boolean. The latter result wouldn't be what you want. (As has been explained by Satania)
A simpler explanation would be that operator++ simply isn't overloaded for boolean data type.

Answer 2

[meta] Tracking the deprecation (where the deprecated behavior is described as "bool++ sets the boolean to true");
I did not find the original reason it was deprecated (from 1998), but the final action reason (actually finishing the deprecation) seems to be that they:
* ... started in C++98 (decrement never worked anyway; found notes that it wasn't definable)
* ... added std::exchange for a rare use case to help bool++ get deprecated (C++14)
* ... waited nearly 20 years and finally just wanted to finish (C++17)
Action mention in 1998 C++ Standard, but the paper is hard to find (withdrawn):
http://www.codingstandard.com/rule/1-3-1-do-not-use-the-increment-operator-on-a-variable-of-type-bool/
https://www.iso.org/standard/25845.html
Library Working Group (LWG) explanation of C++17 plans:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0002r1.html
Links at the bottom of that page:
"Core Issue 1653" (big page! -- short message from Bjarne Stroustrup 2013-Apr that bool++ was already deprecated
"N3668 exchange() utility function", included to address a rare use case for bool++

Answer 3

Due to StackOverflow discussion that booleans could overflow, I explored the documentation discrepancy saying ++ just sets booleans true.
Related copy+paste code for below c++17 compiler:
https://code.sololearn.com/c4Nyt8o40DgP/?ref=app
Output at http://cpp.sh and c++14:
Max value if bool were incrementing: 255
boolean: false (0) (initial value)
Boolean data: 0 (union before increment)
Boolean data: 1 (increment 100)
Boolean data: 1 (increment 200)
loop stopped: 256 (should overflow to 0)
boolean: true (1) (still set to 1)

Answer 4

Hatsy Rei
true is 1, so I think...only non-zeroes if converted to the bool type first.
Hopefully making sense, I mean in comparisons:
(42 == true) // 0 by equality
(42 ? "true" : "false") // "true" by condition, converted to bool
https://softwareengineering.stackexchange.com/a/136933
"C: any scalar can be used in the boolean context..."
"In C and C++, equality comparisons to true and false are dangerous..."

Answer 5

Kirk Schafer Perhaps it concerns the idea that any non-zero integer is evaluated to true? Somehow, I forgotten to consider that part.

Answer 6

Thanks Guys For Helping Me Out.
Always Smile😊#The-champ-921

Answer 7

thank you agent