[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: unassig...@gcc.gnu.org

        ReportedBy: za...@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().