Bug or undefined behaviour?

[Daniel Junglas]

Hi all,

consider the following program self-assign.cpp
        #include <string>
        #include <iostream>

        int main (int argc, char** argv)
        {
          std::string const s =
            (argc == 2) ? std::string(argv[1]) : s;

          std::cout << s.c_str() << std::endl;

          return 0;
        }
I can compile it without warnings using 'g++ -W -Wall'.
When I run the program with one argument everything is fine. However, when
I run the program without any arguments I get a segmentation fault. The
problem seems to be the assignment of 's' to itself in the ternary
expression. I know that I should not write code like above but I
accidently ran into that problem. Is it a bug in g++ or is it just
undefiend behaviour (I do not have a C++ standard here to check that).
In my opinion the compiler should at least warn about this stuff.

My system is debian:
  g++ --version -> 3.0.4
  uname -a      -> Linux fb04383 2.4.24-2-686 #1 Wed Feb 25 21:46:26
                   EST 2004 i686 unknown
The backtrace of gdb is
  GNU gdb 2002-04-01-cvs
  Copyright 2002 Free Software Foundation, Inc.
  GDB is free software, covered by the GNU General Public License, and you
  are
  welcome to change it and/or distribute copies of it under certain
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  Type "show copying" to see the conditions.
  There is absolutely no warranty for GDB.  Type "show warranty" for
  details.
  This GDB was configured as "i386-linux"...
  (gdb) r
  Starting program: /home/junglas/bugs/self-assign

  Program received signal SIGSEGV, Segmentation fault.
  0x0804c793 in std::char_traits<char>::assign(char&, char const&) (
      address@hidden, address@hidden)
      at /usr/include/g++-v3/bits/char_traits.h:142
  142           { __c1 = __c2; }
  (gdb) bt
  #0  0x0804c793 in std::char_traits<char>::assign(char&, char const&) (
      address@hidden, address@hidden)
      at /usr/include/g++-v3/bits/char_traits.h:142
  #1  0x0804a843 in std::string::c_str() const (this=0xbffff794)
      at /usr/include/g++-v3/bits/basic_string.h:717
  #2  0x0804a4c8 in main (argc=1, argv=0xbffff814) at self-assign.cpp:9
  (gdb)

Thanks for any comments

        Daniel