explore memory layout of g++-4.4.5/libstdc++'s std::string

gdb_session

rednaxelafx@fx-laptop:/tmp
$ gedit xx.cpp
rednaxelafx@fx-laptop:/tmp
$ g++ -g xx.cpp 
rednaxelafx@fx-laptop:/tmp
$ g++ -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.4.4-14ubuntu5.1' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5.1) 
rednaxelafx@fx-laptop:/tmp
$ gdb ./a.out 
GNU gdb (GDB) 7.2-ubuntu
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /tmp/a.out...done.
(gdb) b main
Breakpoint 1 at 0x400adf: file xx.cpp, line 6.
(gdb) run
Starting program: /tmp/a.out 

Breakpoint 1, main () at xx.cpp:6
6	  string foo("foo");   // (1)
(gdb) n
7	  string goo = foo;    // (2)
(gdb) p foo
$14 = {static npos = <optimized out>, 
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x603028 "foo"}}
(gdb) p/x &foo
$15 = 0x7fffffffd750
(gdb) x/g &foo
0x7fffffffd750:	0x0000000000603028
(gdb) p *(((std::string::_Rep*)foo._M_dataplus._M_p)-1)
$16 = {<std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Rep_base> = {_M_length = 3, _M_capacity = 3, _M_refcount = 0}, 
  static _S_max_size = <optimized out>, static _S_terminal = <optimized out>, 
  static _S_empty_rep_storage = {0, 0, 0, 0}}
(gdb) x/4g (((std::string::_Rep*)foo._M_dataplus._M_p)-1)
0x603010:	0x0000000000000003	0x0000000000000003
0x603020:	0x0000000000000000	0x00000000006f6f66
(gdb) n
8	  goo[0] = 'g';        // (3)
(gdb) p foo
$17 = {static npos = <optimized out>, 
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x603028 "foo"}}
(gdb) p/x &foo
$18 = 0x7fffffffd750
(gdb) x/g &foo
0x7fffffffd750:	0x0000000000603028
(gdb) p *(((std::string::_Rep*)foo._M_dataplus._M_p)-1)
$19 = {<std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Rep_base> = {_M_length = 3, _M_capacity = 3, _M_refcount = 1}, 
  static _S_max_size = <optimized out>, static _S_terminal = <optimized out>, 
  static _S_empty_rep_storage = {0, 0, 0, 0}}
(gdb) x/4g (((std::string::_Rep*)foo._M_dataplus._M_p)-1)
0x603010:	0x0000000000000003	0x0000000000000003
0x603020:	0x0000000000000001	0x00000000006f6f66
(gdb) p goo
$20 = {static npos = <optimized out>, 
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x603028 "foo"}}
(gdb) p/x &goo
$21 = 0x7fffffffd740
(gdb) x/g &goo
0x7fffffffd740:	0x0000000000603028
(gdb) n
9	  cout << foo << endl;
(gdb) p foo
$22 = {static npos = <optimized out>, 
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x603028 "foo"}}
(gdb) p/x &foo
$23 = 0x7fffffffd750
(gdb) x/g &foo
0x7fffffffd750:	0x0000000000603028
(gdb) p *(((std::string::_Rep*)foo._M_dataplus._M_p)-1)
$24 = {<std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Rep_base> = {_M_length = 3, _M_capacity = 3, _M_refcount = 0}, 
  static _S_max_size = <optimized out>, static _S_terminal = <optimized out>, 
  static _S_empty_rep_storage = {0, 0, 0, 0}}
(gdb) x/4g (((std::string::_Rep*)foo._M_dataplus._M_p)-1)
0x603010:	0x0000000000000003	0x0000000000000003
0x603020:	0x0000000000000000	0x00000000006f6f66
(gdb) p goo
$25 = {static npos = <optimized out>, 
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x603058 "goo"}}
(gdb) p/x &goo
$26 = 0x7fffffffd740
(gdb) x/g &goo
0x7fffffffd740:	0x0000000000603058
(gdb) p *(((std::string::_Rep*)goo._M_dataplus._M_p)-1)
$27 = {<std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Rep_base> = {_M_length = 3, _M_capacity = 3, _M_refcount = -1}, 
  static _S_max_size = <optimized out>, static _S_terminal = <optimized out>, 
  static _S_empty_rep_storage = {0, 0, 0, 0}}
(gdb) x/4g (((std::string::_Rep*)goo._M_dataplus._M_p)-1)
0x603040:	0x0000000000000003	0x0000000000000003
0x603050:	0x00000000ffffffff	0x00000000006f6f67
(gdb) cont
Continuing.
foo
goo

Program exited normally.
(gdb) quit
rednaxelafx@fx-laptop:/tmp
$ 

xx.cpp

#include <string>
#include <iostream>
using namespace std;

int main() {
  string foo("foo");   // (1)
  string goo = foo;    // (2)
  goo[0] = 'g';        // (3)
  cout << foo << endl;
  cout << goo << endl;
  return 0;
}