Why does the boolean type in C++ support ++ but not --?

Question

Why does the operator -- not exist for bool whereas it does for operator ++?

I tried in C++, and I do not know if my question apply to another language. I will be glad to know also.

I know, I can use the operator ++ with a bool. It makes any bool equal to true.

bool b = false;
b++;
// Now b == true.

Why can't we use the operator -- in an opposite way?

bool b = true;
b--;
// Now b == false;

It is not very useful, but I am curious.

Answer 1

In the old days of C, there was no boolean type. People used the int for storing boolean data, and it worked mostly. Zero was false and everything else was true.

This meant if you took an int flag = 0; and later did flag++ the value would be true. This would work no matter what the value of flag was _{^{(unless you did it a lot, it rolled over and you got back to zero, but lets ignore that)}} - incrementing the flag when its value was 1 would give 2, which was still true.

Some people used this for unconditionally setting a boolean value to true. I'm not sure it ever became idiomatic, but its in some code.

This never worked for --, because if the value was anything other than 1 (which it could be), the value would still not be false. And if it was already false (0) and you did a decrement operator on it, it wouldn't remain false.

When moving code from C to C++ in the early days, it was very important that C code included in C++ was still able to work. And so in the specification for C++ (section 5.2.6 (its on page 71)) it reads:

The value obtained by applying a postfix ++ is the value that the operand had before applying the operator. [Note: the value obtained is a copy of the original value ] The operand shall be a modifiable lvalue. The type of the operand shall be an arithmetic type or a pointer to a complete object type. After the result is noted, the value of the object is modified by adding 1 to it, unless the object is of type bool, in which case it is set to true. [Note: this use is deprecated, see annex D. ]

The operand of postfix -- is decremented analogously to the postfix ++ operator, except that the operand shall not be of type bool.

This is again mentioned in section 5.3.2 (for the prefix operator - 5.2.6 was on postfix)

As you can see, this is deprecated (Annex D in the document, page 709) and shouldn't be used.

But thats why. And sometimes you may see the code. But don't do it.

Answer 2

To partially cope with legacy code which used int or similar as its boolean type.

Answer 3

To understand the historical significance of this question, you must consider the case of the Therac-25. The Therac-25 was a medical device that delivered radiation to cancer patients. It was plagued with bad programming practices that contributed to its poor safety track record (with multiple deaths attributed to it).

http://courses.cs.vt.edu/professionalism/Therac_25/Therac_1.html

(go to the bottom of page 3)

Every pass through the Set-Up Test routine increments the upper collimator position check, a shared variable called Class3. If Class3 is nonzero, there is an inconsistency and treatment should not proceed. A zero value for Class3 indicates that the relevant parameters are consistent with treatment, and the beam is not inhibited.

...

During machine setup, Set-Up Test will be executed several hundred times since it reschedules itself waiting for other events to occur. In the code, the Class3 variable is incremented by one in each pass through Set-Up Test. Since the Class3 variable is 1 byte, it can only contain a maximum value of 255 decimal. Thus, on every 256th pass through the Set-Up Test code, the variable overflows and has a zero value. That means that on every 256th pass through Set-Up Test, the upper collimator will not be checked and an upper collimator fault will not be detected. The overexposure occurred when the operator hit the "set" button at the precise moment that Class3 rolled over to zero. Thus Chkcol was not executed, and F$mal was not set to indicate the upper collimator was still in field-light position. The software turned on the full 25 MeV without the target in place and without scanning. A highly concentrated electron beam resulted, which was scattered and deflected by the stainless steel mirror that was in the path.

The Therac-25 used something like the equivalent of operator++ on a bool. However, the programming language they used was not C++, and their data type was not bool. Unlike the guarantee in C++, however, a regular integer type simply keeps going up. Their data type was the equivalent of uint8_t.

C++ decided to keep the operator++ around for people used to programming like this, but instead of incrementing the value, it simply sets it to true to prevent things like this.

Note that operator++(bool) is deprecated.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf

Annex D of C++14:

D.1 Increment operator with bool operand
The use of an operand of type bool with the ++ operator is deprecated (see 5.3.2 and 5.2.6).