On Sat, Aug 3, 2019 at 4:30 PM Sam Varshavchik <mrsam@xxxxxxxxxxxxxxx> wrote:
>
> Tom Hughes writes:
>
> > But I think upstream is giving very bad advice...
> >
> > That define does not "add extra crashes" in the way that they
> > seem to think - well I mean it does literally but those crashes
> > are reports of program errors on their part.
> >
> > Specifically in this case they appear to be accessing a
> > std::vector at an index beyond the end, so they are accessing
> > memory that may not be allocated at all, and if it is does
> > not belong to the vector in question. So the program is quite
> > likely to crash there one day anyway, the extra assertion just
> > makes sure it always does.
>
> I believe that the following construct trips this assertion:
>
> # std::vector<int> foo;
> #
> # std::vector<int> bar;
> #
> # // Populate foo with something.
> #
> # std::copy(&foo[0], &foo[foo.size()], std::back_insert_iterator{bar});
>
> There's nothing wrong with this. There is no out of bounds access.
You just formed a reference past the end of an array. I doubt that is
valid according to the standard. It certainly fails the smell test.
I *think* the standard defines &foo[foo.size] as being equivalent to
&*(foo.begin() + foo.size()), which certainly appears to be invalid.
On a cursory search of the standard, I couldn't find where it says
what operator* on this type of iterator does at all, let alone whether
it's valid for one-past-the-end iterators, but I'm pretty sure that
your code is, indeed, wrong.
> I do not
> believe that this is undefined behavior. The defined semantics of
> std::vector, and its operator[], are well defined here.
>
> I ran into these new assertions with my own code as well, after updating to
> F28 (where they were enabled by default the first time, IIRC, not sure why
> this shows up only now, for that package).
>
> I ended up tweaking my code to avoid the assertions, rather than disabling
> them. For this particular situation, my original change was to try
>
> std::copy(&foo[0], &foo[0]+foo.size(), std::back_insert_iterator{bar});
>
What you want is foo.begin() and foo.end() or, if you really want to
play with pointers, foo.data().
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