[Bug libstdc++/65470] New: regex_search corrupts matches when haystack is destroyed

["aral at gmx dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65470

            Bug ID: 65470
           Summary: regex_search corrupts matches when haystack is
                    destroyed
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aral at gmx dot de

Created attachment 35063
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35063&action=edit
minimal bug example - compile with g++ -std=c++11 regexbug.cpp -o regexbug

Tested on g++ (Debian 4.9.2-10) 4.9.2.

regex_search with matches apparently depends on the haystack (in both the const
basic_string and const *charT  versions) to remain intact. The matches object
returned seems to point to locations in the original haystack. When the
haystack is destroyed, the matches are corrupted.

This leads to very unpleasant results when using regex_search with the
temporary strings returned by .string() or .c_str() methods of many objects
(e.g. boost::filesystem::path.filename() ), as those strings are destroyed at
the end of the line containing the regex_search.

Fix recommendation:
1) if this behavior is intended for performance, add a REALLY BIG FLASHING RED
WARNING MESSAGE to the function documentation on cplusplus.com (and anywhere)
that the haystack MUST NOT be a temporary string, and MUST be kept until after
the matches have been evaluated.

2) to fix, modify the (sub)matches class so that it creates a copy of each
match and manages that copy destruction itself


To reproduce (also submitted as attachment):
-----
#include <regex>
#include <iostream>
using namespace std;

int main( void )
{
    cmatch match;                                // store the matches here,
this object seems to depend on haystack after search

    char *haystack = strdup ("BUG DEMO");
    regex_search( haystack, match, regex(".*") );        // perform regex
search in the haystack, always matches, not checking match.size() for brevity

    cout << "correct match: "<< match[0] << endl;        // document the
correct match
     delete haystack;        // destroy the haystack
    cout << "corrupt match: "<< match[0] << endl;        // document the
correct match

    return 0;
}
-----
(compile with g++ -std=c++11 regexbug.cpp -o regexbug)