About sequence point and UB

Question

Here is a question I can not explain clearly.

Which option is wrong and why?

(A) a += (a++);   
(B) a += (++a);
(C) (a++) += a;
(D) (++a) += (a++);

What is the difference between A and B?

My understand: A is a UB but B is OK because the side effect of ++a will be completed before the assignment. Is that right?

Update: What is the difference between ++a and a++ within the sequence point? The side effect of pre-increment(decrement) may be completed any time before the next seq-point just like post-increment(decrement)?

The parentheses are unnecessary in all the statements — but do no harm. — Jonathan Leffler, Aug 22 '13 at 14:29
@JonathanLeffler (Once again, operator precedence is not the same as order of evaluation.) — , Aug 22 '13 at 14:30
@PHI; It is not recommended to see `a += ++a` as `a = a + ++a`. — haccks, Aug 22 '13 at 14:43
@H2CO3; I never said `a += ++a` is recommended (it invokes UB). I mean to say `a += b` is not equivalent to `a = a+b`. — haccks, Aug 22 '13 at 15:11
@haccks I asked OP to see this way and find why was it not appropriate and how does it look to him ? second I didn't want to go that far and include readability issues and all ....... and as far as addition is concerned its perfectly alright to do it like that... its only subtraction where confusion arises . — 0decimal0, Aug 22 '13 at 15:53

score 6 · Accepted Answer · answered Aug 22 '13 at 14:12

6

Which option is wrong and why?

All of them are wrong, and that's because the first two invoke undefined behavior, and the last two don't compile. (And if they did, they would invoke UB as well.)

answered Aug 22 '13 at 14:12

Why the second one is UB? `a+=(++a)` is equal to `++a; a+=a;` – Donglei Aug 22 '13 at 14:15
4

@Donglei Oh huh? [Assignment is not a sequence point.](http://stackoverflow.com/questions/4362501/any-good-reason-why-assignment-operator-isnt-a-sequence-point) – Aug 22 '13 at 14:17
Yes, I know assignment is not a sequence point. Can you explain what is the difference between `++a` and `a++` within the sequence point? My understand: Pre-increment will `++` before return so after `++a` the side-effect has already happend. – Donglei Aug 22 '13 at 14:35
2

@Donglei And it doesn't matter when the side effects take place, since either way, **`a` is modified twice between sequence points.** – Aug 22 '13 at 14:36
Yes, exactly. But what about the difference between `++a` and `a++` within the sequence point? I think this is what I confused actually. – Donglei Aug 22 '13 at 14:44
@Donglei Nothing, since the code invokes UB, so there's no point in reasoning about what it does. If you write code that has defined behavior, though, then what you already explained applies (i. e. `a++` yields `a` and `++a` yields `a + 1`). – Aug 22 '13 at 14:47
@Donglei: pre-increment doesn't work the way you think it does; the expression `++a` will evaluate to the current value of `a` + 1, but the *side effect* of updating `a` doesn't have to happen immediately; in that respect it behaves exactly like the postfix version. – John Bode Aug 22 '13 at 16:54

score 2 · Answer 2 · edited Aug 22 '13 at 14:27

2

The first and second is UB because C does not define what is supposed to be evaluated first.

The third and fourth do not compile - the reason: lvalue required as left operand of assignment

Let me be more specific on the first two:

a += (a++); - equal to a = a + (a++);

if the a++ is evaluated first then it's a = a + 1 + a;
if the a++ is evaluated at the end then it's a = a + a;

but C does not define what should happen first, so it depends on the implementation, and it's UB.

The same goes for the second.

edited Aug 22 '13 at 14:27

Jonathan Leffler

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answered Aug 22 '13 at 14:19

No Idea For Name

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@JonathanLeffler can it make demons fly out of your nose? XD – No Idea For Name Aug 22 '13 at 14:30
It depends on the skill of the compiler writer! I removed my comment; I'm not happy enough with it. Let's remove these two too. – Jonathan Leffler Aug 22 '13 at 14:31

About sequence point and UB

2 Answers2