Double increment on an integer variable, does it work as intended?

Question

I have some code with lines which increment a counter.

++ count;

Sometimes I have an if condition which means I should increment count by 2.

count += 2;

Does "double increment'ing" work in the same way?

++ ++ count;

It would be helpful to know if both C and C++ compilers interpret this the same way.

I don't believe it's required by the standard, for the same reasons as found in [this question](http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points). However, I'm not certain enough of that to do any more than leave a comment. — R_Kapp, Mar 08 '16 at 14:05
@AchmadJP: Trying something that may lead to UB is an entirely worthless endeavor. It may appear to work, while it doesn't. — R_Kapp, Mar 08 '16 at 14:06
@R_Kapp: It is well defined: http://port70.net/~nsz/c/c11/n1570.html#6.4p4 . But is horrible style. — too honest for this site, Mar 08 '16 at 14:12
I personally think `++ ++ count` is more readable, when working with only `++ count` and `++ ++ count` — FreelanceConsultant, Mar 08 '16 at 14:13
... probably a lot of people disagree with me. Perhaps `count += 1` and `count += 2` is more, more readable, if you "get me" — FreelanceConsultant, Mar 08 '16 at 14:13
@user3728501: You might change your mind if you have to fix code which includes this and similar constructs. I'd refuse such code as hard to maintain and so would some friends of mine. But feel free to use it, until I have to review your code :-) — too honest for this site, Mar 08 '16 at 14:15
To paraphrase Chris Rock; just because you can do something don't mean it's to be _done_. — Michael, Mar 08 '16 at 14:16
I find it bizarre that people don't like this. What's the point of `++` if you can't stick'em together to make your code look nice? — FreelanceConsultant, Mar 08 '16 at 14:26
Do not add tags for unrelated langauges. C and C++ are different languages. — too honest for this site, Mar 08 '16 at 14:29
@user3728501 So, in that case, what's the point of having to write `count += 10`, when we can write `++++++++++++++++++++count`? It looks _so_ nice. It would be way nicer if we rewrite `count += 20` in that form! — Algirdas Preidžius, Mar 08 '16 at 14:38
@AlgirdasPreidžius Of course that would be silly. Like I said in my question, I only ever require increment by 1 or 2. — FreelanceConsultant, Mar 08 '16 at 15:16
@user3728501: how about first trying yourself before posting? — too honest for this site, Mar 08 '16 at 15:16
@Olaf Sorry what? Well obviously I did try, I wanted to know if it was legal or not and why — FreelanceConsultant, Mar 08 '16 at 15:17
@user3728501: "Surely applies to both C and C++?" - If you had tried, you would not have stated that. — too honest for this site, Mar 08 '16 at 16:02
@olaf does it matter that much? why not choose not to answer the question of it bothers you so much — FreelanceConsultant, Mar 08 '16 at 19:25
@user3728501: Because you got it wrong. (btw. I did not answer). Re-read my comments then think again. — too honest for this site, Mar 08 '16 at 20:27

Baum mit Augen · Answer 1 · 2016-03-08T14:23:56.600

7

As this is clearly syntactically correct, the question that remains is: "Is this UB because of unsequenced writes?"

It is not (in C++11 and later) because

5) The side effect of the built-in pre-increment and pre-decrement operators is sequenced before its value computation (implicit rule due to definition as compound assignment)

(From here)

So the code is fine as of C++11.

However, the sequencing rules were different before that, and pre-C++11, the code actually has UB.

In C, that code does not even compile.

The fact that the behavior is different between C and C++ and even between different C++ standards and that this question arises in the first place is a hint that the simple count += 2; is the safer and more readable version. You should prefer it over the "cute and clever" ++ ++count;.

edited Mar 08 '16 at 14:23

answered Mar 08 '16 at 14:11

Baum mit Augen

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I think it was UB before C++11. – juanchopanza Mar 08 '16 at 14:13
See [intro.execution](http://eel.is/c++draft/intro.execution#15) for the standard reference – Barry Mar 08 '16 at 14:17
@juanchopanza Thanks, added. I learned C++ after C++11 was published, so I tend to miss such things. – Baum mit Augen Mar 08 '16 at 14:18
Lucky you! I can't even use the stuff at the moment. – juanchopanza Mar 08 '16 at 14:19
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@Olaf Well, `++++i` just doesn't compile in C, so that makes it easy right? – Barry Mar 08 '16 at 14:22

score 0 · Answer 2 · answered Mar 08 '16 at 14:20

0

There are two methods. One that obviously works, one where you have to ask a question on StackOverflow. There are comments saying "in C++ 11 or later"... How sure are you that your C++ code runs with C++ 11 and not something older? Even if you use extensions that were not part of an earlier language, your language could be C++0x with some extensions.

Clearly you shouldn't care whether the second method works or not, but use the one that obviously works.

answered Mar 08 '16 at 14:20

gnasher729

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Compatibility with an older version of the standard cannot be a reason not to use a feature. Otherwise no new features would be used. But you have other valid points. – too honest for this site Mar 08 '16 at 14:32
@Olaf The thing is, there is UB, which the compiler will most likely not warn you about. With most other features, you get either a compiler error, or well defined behaviour. – juanchopanza Mar 08 '16 at 15:58
@juanchopanza: I still wounder why this is allowed anyway. Which lvalue does the "outer" `++` increment as side-effect? – too honest for this site Mar 08 '16 at 16:01
@Olaf I guess the same as the inner one? Pre-increment yields an lvalue (at least in C++) – juanchopanza Mar 08 '16 at 16:06
@juanchopanza: Ok, so there is another difference. It does not in C. So, that intermediate lvalue is incremented then and just dropped, I suppose? Makes the construct look even more obscure to me. Anyway, thanks for the infos. – too honest for this site Mar 08 '16 at 16:08

Double increment on an integer variable, does it work as intended?

2 Answers2