On 03/08/19 19:29 -0400, Sam Varshavchik 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. I
do not believe that this is undefined behavior. The defined semantics
of std::vector, and its operator[], are well defined here.
The semantics are defined only when the index is < foo.size(), meaning
foo[foo.size()] is undefined.
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});
But that still tripped the assertion when the foo vector was empty, so
I had wrap this inside an if().
Or:
std::copy(foo.begin(), foo.end(), ...);
Or:
std::copy(foo.data(), foo.data()+foo.size(), ...);
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