Undoc'd Feature?

[ijabbott]

(07-12-2022 11:03 AM)RPNerd Wrote:  

(07-11-2022 07:49 PM)ijabbott Wrote:  However, for `i = i++;`, the `i++` part says that the old value of `i` plus 1 will be stored in `i`, and the `i = i` part says that the old value of `i` will be stored in `i`, so there is a conflict.

I see no conflict there.

in:

Code:

i = i++;

...as in any assignment, the rhs is evaluated so that the result can be assigned to the lhs variable, which just happens to be the same variable in this particular case.

"i++" is evaluated as the initial value of i. A by-product of that evaluation is post-incrementing i but that doesn't matter because the initial value is then assigned back to i.

EDIT: IGNORE THE FOLLOWING - IT IS WRONG

The problem is that the two stores to `i` are unsequenced relative to each other. You are under the impression that the store caused by `++` on the r.h.s. occurs before the store caused by the `=`, but there is nothing in the standard to support that.

TBH, I don't know why they bothered allowing `i = i++ + 1;` as a special case as it is a pretty pointless construct. The blanket ban in the earlier version is easier to understand, IMHO.

EDIT: CORRECTION FOLLOWS

I missed the sentence added to C++17 for the assignment operators that reads as follows

Quote:The right operand is sequenced before the left operand.

That little sentence makes all the difference!