How to "watch" the size of a C++ std::vector in gdb?

Question

I have a std::vector as part of a class, that contains a custom type. Its contents seems to be mysteriously changed from somewhere in the program. I am having trouble trying to figure out where this is happening.

Is there a way to "watch" the contents (or size) of a std::vector from gdb?

Thanks.

Answer 1

Is there a way to "watch" the contents (or size) of a std::vector from gdb?

Assuming you are using GCC, set watchpoints on theVector->_M_impl._M_start and _M_finish. If you are using some other std::vector implementation, adjust accordingly.

Example:

#include <vector>

int main()
{
  std::vector<int> v;

  v.push_back(1);
  v.push_back(2);
}

g++ -g t.cc
gdb -q ./a.out

Reading symbols from /tmp/a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x40090f: file t.cc, line 5.

Temporary breakpoint 1, main () at t.cc:5
5     std::vector<int> v;
(gdb) n
7     v.push_back(1);
(gdb) p v._M_impl._M_start
$1 = (int *) 0x0
(gdb) p v._M_impl._M_finish 
$2 = (int *) 0x0
(gdb) p &v._M_impl._M_finish
$3 = (int **) 0x7fffffffd878
(gdb) watch *$3
Hardware watchpoint 2: *$3
(gdb) p &v._M_impl._M_start
$4 = (int **) 0x7fffffffd870
(gdb) watch *$4
Hardware watchpoint 3: *$4
(gdb) c
Hardware watchpoint 3: *$4

Old value = (int *) 0x0
New value = (int *) 0x604010
std::vector<int, std::allocator<int> >::_M_insert_aux (this=0x7fffffffd870, __position=0x0) at /usr/include/c++/4.4/bits/vector.tcc:365
365       this->_M_impl._M_finish = __new_finish;
(gdb) bt
#0  std::vector<int, std::allocator<int> >::_M_insert_aux (this=0x7fffffffd870, __position=0x0) at /usr/include/c++/4.4/bits/vector.tcc:365
#1  0x0000000000400a98 in std::vector<int, std::allocator<int> >::push_back (this=0x7fffffffd870, __x=@0x7fffffffd88c) at /usr/include/c++/4.4/bits/stl_vector.h:741
#2  0x0000000000400935 in main () at t.cc:7
(gdb) c
Hardware watchpoint 2: *$3

Old value = (int *) 0x0
New value = (int *) 0x604014
std::vector<int, std::allocator<int> >::_M_insert_aux (this=0x7fffffffd870, __position=0x0) at /usr/include/c++/4.4/bits/vector.tcc:366
366       this->_M_impl._M_end_of_storage = __new_start + __len;
(gdb) bt
#0  std::vector<int, std::allocator<int> >::_M_insert_aux (this=0x7fffffffd870, __position=0x0) at /usr/include/c++/4.4/bits/vector.tcc:366
#1  0x0000000000400a98 in std::vector<int, std::allocator<int> >::push_back (this=0x7fffffffd870, __x=@0x7fffffffd88c) at /usr/include/c++/4.4/bits/stl_vector.h:741
#2  0x0000000000400935 in main () at t.cc:7

... etc...