Keywords: C++, library, debug

Chapter 17. Debug Mode
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## Chapter 17. Debug Mode

## Intro

By default, libstdc++ is built with efficiency in mind, and therefore performs little or no error checking that is not required by the C++ standard. This means that programs that incorrectly use the C++ standard library will exhibit behavior that is not portable and may not even be predictable, because they tread into implementation-specific or undefined behavior. To detect some of these errors before they can become problematic, libstdc++ offers a debug mode that provides additional checking of library facilities, and will report errors in the use of libstdc++ as soon as they can be detected by emitting a description of the problem to standard error and aborting the program. This debug mode is available with GCC 3.4.0 and later versions.

The libstdc++ debug mode performs checking for many areas of the C++ standard, but the focus is on checking interactions among standard iterators, containers, and algorithms, including:

- Safe iterators: Iterators keep track of the container whose elements they reference, so errors such as incrementing a past-the-end iterator or dereferencing an iterator that points to a container that has been destructed are diagnosed immediately.

- Algorithm preconditions: Algorithms attempt to validate their input parameters to detect errors as early as possible. For instance, the `set_intersection` algorithm requires that its iterator parameters `first1` and `last1` form a valid iterator range, and that the sequence [`first1`, `last1`) is sorted according to the same predicate that was passed to `set_intersection`; the libstdc++ debug mode will detect an error if the sequence is not sorted or was sorted by a different predicate.

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