debug/vector anomalous behaviour

[Stefano Soffia <soffia at cs dot unipr dot it>]

I was playing with the -D_GLIBCXX_DEBUG flag when I stumbled upon the
following example.

  #include <vector>
  #include <algorithm>
  #include <iterator>

  typedef std::vector<unsigned> array_t;
  typedef std::back_insert_iterator<array_t> bii_t;

  array_t a;

  int main() {
    // Push 5 elements.
    a.push_back(0);
    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    a.push_back(4);
    // Ensure that there is enough space for other two elements.
    // (2 + 5 = 7)
    if (a.capacity() < 7)
      a.reserve(7);
    // Add two new elements.
    std::copy(a.begin(),
              a.begin() + 2,
              bii_t(a));
  }

  $ g++ -o test -D_GLIBCXX_DEBUG test.cc
  $ ./test
  /usr/lib/gcc/i686-pc-linux-gnu/4.2.3/include/g++-v4/debug/safe_iterator.h:209:
      error: attempt to increment a singular iterator.

      Objects involved in the operation:
      iterator "this" @ 0x0xbffa87cc {
        type =
          N11__gnu_debug14_Safe_iteratorIN9__gnu_cxx17__normal_iteratorIPjNSt6__nor
  m6vectorIjSaIjEEEEENSt7__debug6vectorIjS6_EEEE
          (mutable iterator);
          state = singular;
            references sequence with type `NSt7__debug6vectorIjSaIjEEE' @
              0x0xbffa87cc
      }
  Aborted
  $

Digging in the include files I found the following:

  /4.2.3/include/g++-v4/debug/vector

  180       using _Base::capacity;
  ...
  362       bool
  363       _M_requires_reallocation(size_type __elements)
  364       {
  365 #ifdef _GLIBCXX_DEBUG_PEDANTIC
  366         return __elements > this->capacity();
  367 #else
  368         return __elements > _M_guaranteed_capacity;
  369 #endif
  370       }

which seems quite strange to me. Indeed, compiled in pedantic mode, it gives
no errors.

  $ g++ -o test -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC test.cc
  $ ./test
  $

Am I missing something?

Stefano Soffia
Applied Formal Methods Laboratory
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/afmlab/