Internals for STL containers

[Nick Roberts <nickrob at snap dot net dot nz>]

Emacs 22.1 has a graphical interface to Gdb which allows the display of
watch expressions in the Speedbar.

Using variable objects, if I display a watch expression for an STL container
in a program compiled with gcc, e.g.,

  vector<int> v (3);
  v[0] = 1;
  v[1] = 11;
  v[2] = 22;

in the Speedbar, I get something like this:

v std::vector<int,std::allocator<int> >
    std::_Vector_base<int,std::allocator<int> > std::_Vector_base<int,std::allocator<int> >
      public	 
        _M_impl	std::_Vector_base<int,std::allocator<int> >::_Vector_impl
          std::allocator<int>	std::allocator<int>
            __gnu_cxx::new_allocator<int>	{...}
          public	 
            _M_start             int * 	    0x804c008
              *_M_start	                    0
            _M_finish	         int *	    0x804c014	
              *_M_finish                    135153	
            _M_end_of_storage    int *      0x804c014
              *_M_end_of_storage            135153	

which is a bit meaningless to the end user.  In this case, I know where the
values are really stored:

 v._M_impl._M_start	int *	0x804c008
 *v._M_impl._M_start		1
 *(v._M_impl._M_start+1)	11
 *(v._M_impl._M_start+2)	22

(gdb) p v._M_impl._M_finish - v._M_impl._M_start
$1 = 3

and it would be better for Emacs to display some of these.

However, _M_impl, _M_start etc are gcc internals and it might be risky for
Emacs to rely on them.  Can anyone say how likely these internals are to
change?  I'm not asking for a guarantee.  It would be helpful just to know
when they last changed as an indication of how volatile this code might be.


-- 
Nick                                           http://www.inet.net.nz/~nickrob