[Bug libstdc++/81482] New: by-value lambda capture in remove_if

[fabian-gcc at kloetzl dot info]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81482

            Bug ID: 81482
           Summary: by-value lambda capture in remove_if
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fabian-gcc at kloetzl dot info
  Target Milestone: ---

Hi all,

There is a little quirks with `remove_if` and lambdas that mutably capture by
value. As a contrived example let's print all the odd numbers from a list and
also count how many elements were removed already.

----
#include <vector>
#include <algorithm>
#include <cstdio>

int main(int argc, char *argv[])
{
        auto numbers = std::vector<int>{0,1,2,3,4,5,6,7,8,9};

        auto count = 0;
        auto odd = [=] (int n) mutable {
                if (n & 1) {
                        printf("%i %i\n", n, count);
                        count++;
                        return true;
                }
                return false;
        };

        std::remove_if(std::begin(numbers), std::end(numbers), odd);
        return 0;
}
----

Note that the use of `printf` is to avoid a libc++ bug [1].

The expected output is as follows.
----
1 0
3 1
5 2
7 3
9 4
----

However, with GCC 7.1.1 and libstdc++ 3.3.6 (current arch) the output is
different.

----
1 0
3 0
5 1
7 2
9 3
----

Clang with libc++ produces the desired output. As far as I can tell, both
behaviours are correct, according to the standard. But I guess the second is
unexpected.

A possible fix would be to pass the lambda by reference to `find_if`, instead
of by-value which makes an implicit copy [2]. That is how libc++ does it [3].

Some other standard algorithms might also be affected by this, didn't check. ☺

Best,
Fabian


[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=808086
[2]:
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/stl_algo.h#L863
[3]:
https://github.com/llvm-mirror/libcxx/blob/master/include/algorithm#L2218-L2219