Menu
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.
508
To get the size of a C++ Vector, you can use size() function on the vector. size() function returns the number of elements in the vector.
The C++ function std::vector::resize() changes the size of vector. If n is smaller than current size then extra elements are destroyed. If n is greater than current container size then new elements are inserted at the end of vector. If val is specified then new elements are initialed with val.
Use the vector::size() method: i < v. size() . The sizeof operator returns the size in bytes of the object or expression at compile time, which is constant for a std::vector .
Important concepts to learn: The vector size and its capacity and what the difference between them is. The capacity is the number of elements the vector have allocated memory for. The size is the current number of elements in the vector.
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... If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
