+
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.
+