Semantics
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## Semantics

A program that uses the C++ standard library correctly will maintain the same semantics under debug mode as it had with the normal (release) library. All functional and exception-handling guarantees made by the normal library also hold for the debug mode library, with one exception: performance guarantees made by the normal library may not hold in the debug mode library. For instance, erasing an element in a `std::list` is a constant-time operation in normal library, but in debug mode it is linear in the number of iterators that reference that particular list. So while your (correct) program won't change its results, it is likely to execute more slowly.

libstdc++ includes many extensions to the C++ standard library. In some cases the extensions are obvious, such as the hashed associative containers, whereas other extensions give predictable results to behavior that would otherwise be undefined, such as throwing an exception when a `std::basic_string` is constructed from a NULL character pointer. This latter category also includes implementation-defined and unspecified semantics, such as the growth rate of a vector. Use of these extensions is not considered incorrect, so code that relies on them will not be rejected by debug mode. However, use of these extensions may affect the portability of code to other implementations of the C++ standard library, and is therefore somewhat hazardous. For this reason, the libstdc++ debug mode offers a "pedantic" mode (similar to GCC's `-pedantic` compiler flag) that attempts to emulate the semantics guaranteed by the C++ standard. For instance, constructing a `std::basic_string` with a NULL character pointer would result in an exception under normal mode or non-pedantic debug mode (this is a libstdc++ extension), whereas under pedantic debug mode libstdc++ would signal an error. To enable the pedantic debug mode, compile your program with both `-D_GLIBCXX_DEBUG` and `-D_GLIBCXX_DEBUG_PEDANTIC` . (N.B. In GCC 3.4.x and 4.0.0, due to a bug, `-D_GLIBXX_DEBUG_PEDANTIC` was also needed. The problem has been fixed in GCC 4.0.1 and later versions.)

The following library components provide extra debugging capabilities in debug mode:

- `std::basic_string` (no safe iterators and see note below)

- `std::bitset`

- `std::deque`

- `std::list`

- `std::map`

- `std::multimap`

- `std::multiset`

- `std::set`

- `std::vector`

- `std::unordered_map`

- `std::unordered_multimap`

- `std::unordered_set`

- `std::unordered_multiset`

N.B. although there are precondition checks for some string operations, e.g. `operator[]`, they will not always be run when using the `char` and `wchar_t` specialisations (`std::string` and `std::wstring`). This is because libstdc++ uses GCC's `extern template` extension to provide explicit instantiations of `std::string` and `std::wstring`, and those explicit instantiations don't include the debug-mode checks. If the containing functions are inlined then the checks will run, so compiling with `-O1` might be enough to enable them. Alternatively `-D_GLIBCXX_EXTERN_TEMPLATE=0` will suppress the declarations of the explicit instantiations and cause the functions to be instantiated with the debug-mode checks included, but this is unsupported and not guaranteed to work. For full debug-mode support you can use the `__gnu_debug::basic_string` debugging container directly, which always works correctly.

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