c++ - Sequence points and partial order

Question

A few days back there was a discussion here about whether the expression

i = ++i + 1

invokes UB (Undefined Behavior) or not.

Finally the conclusion was made that it invokes UB as the value of 'i' is changing more than once between two sequence points.

I was involved in a discussion with Johannes Schaub in that same thread. According to him

i=(i,i++,i)+1 ------ (1) /* invokes UB as well */

I said (1) does not invoke UB because the side effects of the previous subexpressions are cleared by the comma operator ',' between i and i++ and between i++ and i.

Then he gave the following explanation:

"Yes the sequence point after i++ completes all side effects before it, but there is nothing that stops the assignment side effect overlapping with the side effect of i++.The underlying problem is that the side effect of an assignment is not specified to happen after or before the evaluation of both operands of the assignment, and so sequence points cannot do anything with regard to protecting this: Sequence points induce a partial order: Just because there is a sequence point after and before i++ doesn't mean all side effects are sequenced with regard to i.

Also, notice that merely a sequence point means nothing: The order of evaluations isn't dictated by the form of code. It's dictated by semantic rules. In this case, there is no semantic rule saying when the assignment side effect happens with regard to evaluating both of its operands or subexpressions of those operands".

The statement written in "bold" confused me. As far as I know:

"At certain specified points in the execution sequence called sequence points,all side effects of previous evaluations shall be complete and no side effects of subsequent evaluations shall have taken place."

Since,comma operators also specify execution order the side effect of i++ have been cancelled when we reach the last i.He(Johannes) would have been right had the order of evaluation been not specified(but in case of comma operator it is well specified).

So I just want to know whether (1) invokes UB or not?. Can someone give another valid explanation?

Thanks!

Answer 1

The C standard says this about assignment operators (C90 6.3.16 or C99 6.5.16 Assignment operators):

The side effect of updating the stored value of the left operand shall occur between the previous and the next sequence point.

It seems to me that in the statement:

i=(i,i++,i)+1;

the sequence point 'previous' to the assignment operator would be the second comma operator and the 'next' sequence point would be the end of the expression. So I'd say that the expression doesn't invoke undefined behavior.

However, this expression:

*(some_ptr + i) = (i,i++,i)+1;

would have undefined behavior because the order of evaluation of the 2 operands of the assignment operator is undefined, and in this case instead of the problem being when the assignment operator's side effect takes place, the problem is you don't know whether the value of i used in the left handle operand will be evaluated before or after the right hand side. This order of evaluation problem doesn't occur in the first example because in that expression the value of i isn't actually used in the left-hand side - all that the assignment operator is interested in is the "lvalue-ness" of i.

But I also think that all this is sketchy enough (and my understanding of the nuances involved are sketchy enough) that I wouldn't be surprised if someone can convince me otherwise (on either count).