C++ order of evaluation of operands, arguments

[Martin Sebor]

On 11/23/2015 04:01 PM, Jason Merrill wrote:
> There's a proposal working through the C++ committee to define the order
> of evaluation of subexpressions that previously had unspecified ordering:
>
> http://www.open-std.org/Jtc1/sc22/wg21/docs/papers/2015/p0145r0.pdf
>
> I agree with much of this, but was concerned about the proposal to
> define order of evaluation of function arguments as left-to-right, since
> GCC does right-to-left on PUSH_ARGS_REVERSED targets, including x86_64.
>
> Any thoughts?

I have no comments on the impact on the implementation but I do
have a couple of general concerns with the proposal.

As others have already suggested, I think a change like this needs
to be coordinated with WG14 and made either in both languages or
in neither.  If C++ were to make it alone, C++ programmers who only
occasionally write C code would be prone to introducing bugs into
their C code by relying on the C++ guarantees.  As WG14 is just
starting to talk about the goals for C2x, this seems like a good
time to bring it up for consideration.

Even in C++, though, I'd worry that specifying the evaluation order
for just a subset of expression would lead to portability bugs
creeping in as programmers inadvertently start to assume that
the same order is guaranteed for all expressions.

Martin