Hi Anthony, I came across your posting in google/deja. I have exactly
the same problem. Your solution using intermediate cast to "void *"
works great! (I can't believe you found it!) But have you found out a
better/official way to access derived class?
I have searched the gnu.gdb.bug group for that in vain.
Thanks!
> Hi,
> (I'm a little concerned having searched the archives that nobody
> has mentioned this - perhaps it's a total misunderstanding on my part.
> If so apologies...)
>
> The behaviour of gdb looks incorrect when casting a base class
> pointer back to a derived class. The example below is trying to
> print a "Big" object via a cast of a "Small" pointer. Each time,
> different junk gets printed. An intermediate cast to "void *" seems
> to avoid the problem.
>
> This is gdb 4.18, compiled with gcc 2.95.2(pre) as shown below.
> Also see same behaviour in gdb 19990928 (current debian potato).
>
>
>
> Regards
>
> Anthony
>
>
> ~% cat tst.C
> #include <string>
>
> struct Small {
> int i;
> };
>
> struct Big : Small {
> string s;
> };
>
> int main() {
> Big *foo = new Big;
> Small *bar = foo;
> return(0);
> }
>
> ~% c++ -g -v tst.C
> Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.2/specs
> gcc version 2.95.2 19991019 (prerelease)
> GNU CPP version 2.95.2 19991019 (prerelease) (i386 Linux/ELF)
> GNU C++ version 2.95.2 19991019 (prerelease) (i386-linux) compiled by
> GNU C version 2.95.2
> 19991019 (prerelease).
> GNU assembler version 2.9.5 (i386-linux) using BFD version 2.9.5.0.16
>
> ~% gdb a.out
> GNU gdb 4.18
> Copyright 1998 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
> conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB. Type "show warranty" for
> details.
> This GDB was configured as "i686-pc-linux-gnu"...
> (gdb) break main
> Breakpoint 1 at 0x80495d7: file tst.C, line 12.
> (gdb) run
> Starting program: /export/home/ajrh/a.out
>
> Breakpoint 1, main () at tst.C:12
> 12 Big *foo = new Big;
> (gdb) n
> 13 Small *bar = foo;
> (gdb) n
> 14 return(0);
> (gdb) print *foo
> $1 = {<Small> = {i = 0}, s = {static npos = 4294967295,
> static nilRep = {len = 0, res = 0, ref = 2, selfish = false},
> dat = 0x804b838 ""}}
>
>
>
> (gdb) print *(Big*)bar
> $2 = {<Small> = {i = 0}, s = {static npos = 4294967295,
> static nilRep = {len = 0, res = 0, ref = 2, selfish = false},
> dat = 0x208 <Address 0x208 out of bounds>}}
> (gdb)
> $3 = {<Small> = {i = 0}, s = {static npos = 4294967295,
> static nilRep = {len = 0, res = 0, ref = 2, selfish = false},
> dat = 0x400034fc "U\211å\203ì\024Sè"}}
> (gdb)
> $4 = {<Small> = {i = 0}, s = {static npos = 4294967295,
> static nilRep = {len = 0, res = 0, ref = 2, selfish = false},
> dat = 0x0}}
> (gdb)
> $5 = {<Small> = {i = 0}, s = {static npos = 4294967295,
> static nilRep = {len = 0, res = 0, ref = 2, selfish = false},
> dat = 0x83512d0 <Address 0x83512d0 out of bounds>}}
>
>
>
> (gdb) print *(Big*)(void*)bar
> $6 = {<Small> = {i = 0}, s = {static npos = 4294967295,
> static nilRep = {len = 0, res = 0, ref = 2, selfish = false},
> dat = 0x804b838 ""}}
>
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