#include <sstream> #include <iostream> int main() { std::istringstream s("++"); s.ignore(2, '-'); std::cout << "EOF? " << s.eof() << std::endl; } This shows that we set eofbit when the end of the stream is reached, even though we have already discarded as many characters as requested. Libc++ and MSVC do not set eofbit here.
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>: https://gcc.gnu.org/g:ba8fe4b4832e30277f2e4a73b5d35b2e55074d07 commit r11-2054-gba8fe4b4832e30277f2e4a73b5d35b2e55074d07 Author: Jonathan Wakely <jwakely@redhat.com> Date: Mon Jul 13 10:26:39 2020 +0100 libstdc++: Fix istream::ignore exit conditions (PR 94749, PR 96161) My previous fix for PR 94749 did fix the reported case, so that the next character is not discarded if it happens to equal the delimiter when __n characters have already been read. But it introduced a new bug, which is that the delimiter character would *not* be discarded if the number of characters discarded is numeric_limits<streamsize>::max() or more before reaching the delimiter. The new bug happens because I changed the code to check _M_gcount < __n. But when __n == numeric_limits<streamsize>::max() that is false, and so we don't discard the delimiter. It's not sufficient to check for the delimiter when the __large_ignore condition is true, because there's an edge case where the delimiter is reached when _M_gcount == __n and so we break out of the loop without setting __large_ignore. PR 96161 is a similar bug to the original PR 94749 report, where eofbit is set after discarding __n characters if there happen to be no more characters in the stream. This patch fixes both cases (and the regression) by checking different conditions for the __n == max case and the __n < max case. For the former case, we know that we must have either reached the delimiter or EOF, and the value of _M_gcount doesn't matter (except to avoid integer overflow). For the latter case we need to check _M_gcount first and only set eofbit or discard the delimiter if it didn't reach __n. For the latter case overflow can't happen because _M_gcount <= __n < max. libstdc++-v3/ChangeLog: PR libstdc++/94749 PR libstdc++/96161 * include/bits/istream.tcc (basic_istream::ignore(streamsize)) [n == max]: Check overflow conditions on _M_gcount. Rely on the fact that either EOF or the delimiter was reached. [n < max]: Check _M_gcount < n before checking for EOF or delimiter. (basic_istream::ignore(streamsize, char_type): Likewise. * src/c++98/compatibility.cc (istream::ignore(streamsize)) (wistream::ignore(streamsize)): Likewise. * src/c++98/istream.cc (istream::ignore(streamsize, char_type)) (wistream::ignore(streamsize, char_type)): Likewise. * testsuite/27_io/basic_istream/ignore/char/94749.cc: Check that delimiter is discarded if the number of characters ignored doesn't fit in streamsize. * testsuite/27_io/basic_istream/ignore/wchar_t/94749.cc: Likewise. * testsuite/27_io/basic_istream/ignore/char/96161.cc: New test. * testsuite/27_io/basic_istream/ignore/wchar_t/96161.cc: New test.
Fixed on trunk so far.
See also g:4e39f563c0cd25401f689c2093cb8c13692156ef