Does *&++i cause undefined behaviour in C++03?

Question

In another answer it was stated that prior to C++11, where i is an int, then use of the expression:

*&++i

caused undefined behaviour. Is this true?

On the other answer there was a little discussion in comments but it seems unconvincing.

Answer 1

It makes little sense to ask whether *&++i in itself has UB. The deferencing doesn't necessarily access the stored value (prior or new) of i, as you can see by using this as an initializer expression for a reference. Only if an rvalue conversion is involved (usage in such context) is there any question to discuss at all. And then, since we can use the value of ++i, we can use the value of *&++i with exactly the same caveats as for ++i.

The original question concerned essentially i = ++i, which is the same as i = *&++i. That was undefined behavior in C++03, due to i being modified twice between sequence points, and is well-defined in C++11, due to the side-effects of the assignment operator being sequenced after the value computations of the left and right hand sides.

It is perhaps relevant to note that the non-normative examples in the C++98 and C++03 standards, were incorrect, describing some cases of formally Undefined Behavior as merely unspecified behavior. Thus, the intent has not been entirely clear, all the way back. A good rule of thumb is to simply not rely on such obscure corner cases of the language, to avoid them: one should not need to be a language lawyer in order to make sense of the code…