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; but it does have broad ODR implications. Users should compile their whole program at the same debugging level.

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 a the default handler, std::__libcpp_abort_debug_handler, which aborts the program. The handler may not return. Libc++ can be changed to use a custom assertion handler as follows.

#define _LIBCPP_DEBUG 1
#include <string>
void my_handler(std::__libcpp_debug_info const&);
int main(int, char**) {
  std::__libcpp_debug_function = &my_handler;

  std::string::iterator bad_it;
  std::string str("hello world");
  str.insert(bad_it, '!'); // causes debug assertion
  // control flow doesn't return

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:

  • Many algorithms, such as binary_search, merge, next_permutation, and sort, wrap the user-provided comparator to assert that !comp(y, x) whenever comp(x, y). This can cause the user-provided comparator to be evaluated up to twice as many times as it would be without _LIBCPP_DEBUG, and causes the library to violate some of the Standard’s complexity clauses.
  • 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.