Weird gdb behavior regarding conditional breakpoint(a && b not equal to b && a)

Question

I have a small C++ program:

// test.cpp
#include <vector>
#include <iostream>
using namespace std;

template class vector<int>;
int main()
{
   vector<int> v;
   for (size_t i = 0; i < 10; ++i)
   {   
      if (!v.empty())
         cout << v[0] << endl;   //<== Add a cond breakpoint here later

      v.clear();
      for (size_t j = 0; j < i; ++j)
         v.push_back(i);
   }   

   return 0;
}

This program is compiled in Ubuntu 14.04.3 LTS with gcc 4.8.4.

g++ -g -O0 -o test test.cpp

Start the program with gdb, and add a conditional breakpoint on line 13. The condition is "v.size() == 3 && v[0] == 3", as shown below. However, this condition doesn't work as expected. It stops when condition is not met.

(gdb) b 13 if v.size() == 3 && v[0] == 3
Breakpoint 1 at 0x400ca1: file test.cpp, line 13.
(gdb) info b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000000000400ca1 in main() at test.cpp:13
        stop only if v.size() == 3 && v[0] == 3

Run the program and an error message is given:

...
Error in testing breakpoint condition:
Cannot access memory at address 0x0

Breakpoint 1, main () at test.cpp:13
13           cout << v[0] << endl;

And printing v.size() shows it's 1, which is clearly not the condition that breakpoint should stop.

(gdb) p v.size()
$1 = 1

But if I replace the condition breakpoint with "v[0] == 3 && v.size() == 3" everything is fine.

(gdb) p v.size()
$2 = 3

What's the problem of the first condition?

[Update]

I upgraded gcc and gdb to 6.1.0 and 7.10 respectively on Ubuntu 14.04(both built from source), but this problem is still there.

$g++ -v

Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/6.1.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: ../gcc-6.1.0/configure --prefix=/usr --enable-languages=c,c++ --disable-multilib --disable-bootstrap --with-system-zlib Thread model: posix gcc version 6.1.0 (GCC)

$gdb -v

GNU gdb (GDB) 7.10

Answer 1

I upgraded gdb to 7.11.1(by switching to Ubuntu 16.04.1 LTE) and this problem is gone.

And for this gdb version, printing a std::vector is naturally supported, which is nice.

(gdb) print v
$3 = std::vector of length 3, capacity 4 = {3, 3, 3}