Iterator wrappers provide a debugging layer over any iterator that is attached to a particular container, and will manage the information detailing the iterator's state (singular, dereferenceable, etc.) and tracking the container to which the iterator is attached. Because iterators have a well-defined, common interface the iterator wrapper is implemented with the iterator adaptor class template __gnu_debug::_Safe_iterator, which takes two template parameters:

Container wrappers provide a debugging layer over a particular container type. Because containers vary greatly in the member functions they support and the semantics of those member functions (especially in the area of iterator invalidation), container wrappers are tailored to the container they reference, e.g., the debugging version of std::list duplicates the entire interface of std::list, adding additional semantic checks and then forwarding operations to the real std::list (a public base class of the debugging version) as appropriate. However, all safe containers inherit from the class template __gnu_debug::_Safe_sequence, instantiated with the type of the safe container itself (an instance of the curiously recurring template pattern).

The iterators of a container wrapper will be safe iterators that reference sequences of this type and wrap the iterators provided by the release-mode base class. The debugging container will use only the safe iterators within its own interface (therefore requiring the user to use safe iterators, although this does not change correct user code) and will communicate with the release-mode base class with only the underlying, unsafe, release-mode iterators that the base class exports.

The debugging version of std::list will have the following basic structure:

template<typename _Tp, typename _Allocator = allocator<_Tp>
  class debug-list :
    public release-list<_Tp, _Allocator>,
    public __gnu_debug::_Safe_sequence<debug-list<_Tp, _Allocator> >
  {
    typedef release-list<_Tp, _Allocator> _Base;
    typedef debug-list<_Tp, _Allocator>   _Self;

  public:
    typedef __gnu_debug::_Safe_iterator<typename _Base::iterator, _Self>       iterator;
    typedef __gnu_debug::_Safe_iterator<typename _Base::const_iterator, _Self> const_iterator;

    // duplicate std::list interface with debugging semantics
  };

Both the release-mode components and the debug-mode components need to exist within a single translation unit so that the debug versions can wrap the release versions. However, only one of these components should be user-visible at any particular time with the standard name, e.g., std::list.

In release mode, we define only the release-mode version of the component with its standard name and do not include the debugging component at all. The release mode version is defined within the namespace std. Minus the namespace associations, this method leaves the behavior of release mode completely unchanged from its behavior prior to the introduction of the libstdc++ debug mode. Here's an example of what this ends up looking like, in C++.

namespace std
{
  template<typename _Tp, typename _Alloc = allocator<_Tp> >
    class list
    {
      // ...
     };
} // namespace std

In debug mode we include the release-mode container (which is now defined in the namespace __cxx1998) and also the debug-mode container. The debug-mode container is defined within the namespace __debug, which is associated with namespace std via the C++11 namespace association language feature. This method allows the debug and release versions of the same component to coexist at compile-time and link-time without causing an unreasonable maintenance burden, while minimizing confusion. Again, this boils down to C++ code as follows:

namespace std
{
  namespace __cxx1998
  {
    template<typename _Tp, typename _Alloc = allocator<_Tp> >
      class list
      {
	// ...
      };
  } // namespace __gnu_norm

  namespace __debug
  {
    template<typename _Tp, typename _Alloc = allocator<_Tp> >
      class list
      : public __cxx1998::list<_Tp, _Alloc>,
	public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> >
      {
	// ...
      };
  } // namespace __cxx1998

  // namespace __debug __attribute__ ((strong));
  inline namespace __debug { }
}

Because each component has a distinct and separate release and debug implementation, there is no issue with link-time coexistence: the separate namespaces result in different mangled names, and thus unique linkage.

However, components that are defined and used within the C++ standard library itself face additional constraints. For instance, some of the member functions of std::moneypunct return std::basic_string. Normally, this is not a problem, but with a mixed mode standard library that could be using either debug-mode or release-mode basic_string objects, things get more complicated. As the return value of a function is not encoded into the mangled name, there is no way to specify a release-mode or a debug-mode string. In practice, this results in runtime errors. A simplified example of this problem is as follows.

Take this translation unit, compiled in debug-mode:

// -D_GLIBCXX_DEBUG
#include <string>

std::string test02();

std::string test01()
{
  return test02();
}

int main()
{
  test01();
  return 0;
}

... and linked to this translation unit, compiled in release mode:

#include <string>

std::string
test02()
{
  return std::string("toast");
}

For this reason we cannot easily provide safe iterators for the std::basic_string class template, as it is present throughout the C++ standard library. For instance, locale facets define typedefs that include basic_string: in a mixed debug/release program, should that typedef be based on the debug-mode basic_string or the release-mode basic_string? While the answer could be "both", and the difference hidden via renaming a la the debug/release containers, we must note two things about locale facets:

With the design of libstdc++ debug mode, we cannot effectively hide the differences between debug and release-mode strings from the user. Failure to hide the differences may result in unpredictable behavior, and for this reason we have opted to only perform basic_string changes that do not require ABI changes. The effect on users is expected to be minimal, as there are simple alternatives (e.g., __gnu_debug::basic_string), and the usability benefit we gain from the ability to mix debug- and release-compiled translation units is enormous.

The coexistence scheme above was chosen over many alternatives, including language-only solutions and solutions that also required extensions to the C++ front end. The following is a partial list of solutions, with justifications for our rejection of each.

Other options may exist for implementing the debug mode, many of which have probably been considered and others that may still be lurking. This list may be expanded over time to include other options that we could have implemented, but in all cases the full ramifications of the approach (as measured against the design goals for a libstdc++ debug mode) should be considered first. The DejaGNU testsuite includes some testcases that check for known problems with some solutions (e.g., the using declaration solution that breaks user specialization), and additional testcases will be added as we are able to identify other typical problem cases. These test cases will serve as a benchmark by which we can compare debug mode implementations.