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Debug Mode
.. contents::
.. _using-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
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:
.. code-block:: cpp
#define _LIBCPP_DEBUG 1
#include <string>
int main(int, char**) {
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&) {
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.