Debug Mode

Using Debug Mode

Libc++ provides a debug mode that enables assertions meant to detect incorrect usage of the standard library. By default these assertions are disabled but they can be enabled using the _LIBCPP_DEBUG macro.



This macro is used to enable assertions and iterator debugging checks within libc++. By default it is undefined.

Values: 0, 1

Defining _LIBCPP_DEBUG to 0 or greater enables most of libc++’s assertions. Defining _LIBCPP_DEBUG to 1 enables “iterator debugging” which provides additional assertions about the validity of iterators used by the program.

Note that this option has no effect on libc++’s ABI

When this macro is defined _LIBCPP_ASSERT failures throw __libcpp_debug_exception instead of aborting. Additionally this macro disables exception specifications on functions containing _LIBCPP_ASSERT checks. This allows assertion failures to correctly throw through these functions.

Handling Assertion Failures

When a debug assertion fails the assertion handler is called via the std::__libcpp_debug_function function pointer. It is possible to override this function pointer using a different handler function. Libc++ provides two different assertion handlers, the default handler std::__libcpp_abort_debug_handler which aborts the program, and std::__libcpp_throw_debug_handler which throws an instance of std::__libcpp_debug_exception. Libc++ can be changed to use the throwing assertion handler as follows:

#define _LIBCPP_DEBUG 1
#include <string>
int main() {
  std::__libcpp_debug_function = std::__libcpp_throw_debug_function;
  try {
    std::string::iterator bad_it;
    std::string str("hello world");
    str.insert(bad_it, '!'); // causes debug assertion
  } catch (std::__libcpp_debug_exception const&) {
    return EXIT_SUCCESS;
  return EXIT_FAILURE;

Debug Mode Checks

Libc++’s debug mode offers two levels of checking. The first enables various precondition checks throughout libc++. The second additionally enables “iterator debugging” which checks the validity of iterators used by the program.

Basic Checks

These checks are enabled when _LIBCPP_DEBUG is defined to either 0 or 1.

The following checks are enabled by _LIBCPP_DEBUG:

  • FIXME: Update this list

Iterator Debugging Checks

These checks are enabled when _LIBCPP_DEBUG is defined to 1.

The following containers and STL classes support iterator debugging:

  • std::string
  • std::vector<T> (T != bool)
  • std::list
  • std::unordered_map
  • std::unordered_multimap
  • std::unordered_set
  • std::unordered_multiset

The remaining containers do not currently support iterator debugging. Patches welcome.