[Bug libstdc++/56166] New: std::string::clear() can allocate memory despite being marked noexcept

[zackw at panix dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56166

             Bug #: 56166
           Summary: std::string::clear() can allocate memory despite being
                    marked noexcept
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zackw@panix.com


The attached test case (which is regrettably large, but I haven't found a
shorter way to trigger the problem) will call terminate() when compiled with
-std=c++11, despite there being no point at which an exception should escape
main().  The problem is deep inside std::basic_string:

/usr/include/c++/4.7/bits/basic_string.h
    796       /**
    797        *  Erases the string, making it empty.
    798        */
    799       void
    800       clear() _GLIBCXX_NOEXCEPT
    801       { _M_mutate(0, this->size(), 0); }

_M_mutate can allocate memory even when it's being asked to erase the string
(presumably due to internal reference-counting -- if you take 's2' out of the
test case the crash goes away), and thus can throw bad_alloc.  In C++11 mode,
clear() is marked 'noexcept', so we wind up in terminate().