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+<?xml version='1.0'?>
+<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
+[ ]>
+
+<chapter id="manual.ext.debug_mode" xreflabel="Debug Mode">
+<?dbhtml filename="debug_mode.html"?>
+
+<chapterinfo>
+ <keywordset>
+ <keyword>
+ C++
+ </keyword>
+ <keyword>
+ library
+ </keyword>
+ <keyword>
+ debug
+ </keyword>
+ </keywordset>
+</chapterinfo>
+
+<title>Debug Mode</title>
+
+<sect1 id="manual.ext.debug_mode.intro" xreflabel="Intro">
+ <title>Intro</title>
+ <para>
+ 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.
+ </para>
+
+ <para>
+ 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:
+ </para>
+
+ <itemizedlist>
+ <listitem><para><emphasis>Safe iterators</emphasis>: 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.</para></listitem>
+
+ <listitem><para><emphasis>Algorithm preconditions</emphasis>: Algorithms attempt to
+ validate their input parameters to detect errors as early as
+ possible. For instance, the <code>set_intersection</code>
+ algorithm requires that its iterator
+ parameters <code>first1</code> and <code>last1</code> form a valid
+ iterator range, and that the sequence
+ [<code>first1</code>, <code>last1</code>) is sorted according to
+ the same predicate that was passed
+ to <code>set_intersection</code>; the libstdc++ debug mode will
+ detect an error if the sequence is not sorted or was sorted by a
+ different predicate.</para></listitem>
+ </itemizedlist>
+
+</sect1>
+
+<sect1 id="manual.ext.debug_mode.semantics" xreflabel="Semantics">
+ <title>Semantics</title>
+ <para>
+ </para>
+
+<para>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 <code>std::list</code> 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.</para>
+
+<para>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 <code>std::basic_string</code> 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 <code>-pedantic</code> compiler flag) that attempts to emulate
+ the semantics guaranteed by the C++ standard. For
+ instance, constructing a <code>std::basic_string</code> 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 <code>-D_GLIBCXX_DEBUG</code>
+ and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .
+ (N.B. In GCC 3.4.x and 4.0.0, due to a bug,
+ <code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
+ been fixed in GCC 4.0.1 and later versions.) </para>
+
+<para>The following library components provide extra debugging
+ capabilities in debug mode:</para>
+<itemizedlist>
+ <listitem><para><code>std::basic_string</code> (no safe iterators and see note below)</para></listitem>
+ <listitem><para><code>std::bitset</code></para></listitem>
+ <listitem><para><code>std::deque</code></para></listitem>
+ <listitem><para><code>std::list</code></para></listitem>
+ <listitem><para><code>std::map</code></para></listitem>
+ <listitem><para><code>std::multimap</code></para></listitem>
+ <listitem><para><code>std::multiset</code></para></listitem>
+ <listitem><para><code>std::set</code></para></listitem>
+ <listitem><para><code>std::vector</code></para></listitem>
+ <listitem><para><code>std::unordered_map</code></para></listitem>
+ <listitem><para><code>std::unordered_multimap</code></para></listitem>
+ <listitem><para><code>std::unordered_set</code></para></listitem>
+ <listitem><para><code>std::unordered_multiset</code></para></listitem>
+</itemizedlist>
+
+<para>N.B. although there are precondition checks for some string operations,
+e.g. <code>operator[]</code>,
+they will not always be run when using the <code>char</code> and
+<code>wchar_t</code> specialisations (<code>std::string</code> and
+<code>std::wstring</code>). This is because libstdc++ uses GCC's
+<code>extern template</code> extension to provide explicit instantiations
+of <code>std::string</code> and <code>std::wstring</code>, 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 <code>-O1</code> might be enough to enable them. Alternatively
+<code>-D_GLIBCXX_EXTERN_TEMPLATE=0</code> 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
+<code>__gnu_debug::basic_string</code> debugging container directly,
+which always works correctly.
+</para>
+
+</sect1>
+
+<sect1 id="manual.ext.debug_mode.using" xreflabel="Using">
+ <title>Using</title>
+ <para>
+ </para>
+<sect2 id="debug_mode.using.mode" xreflabel="Using Mode">
+ <title>Using the Debug Mode</title>
+
+<para>To use the libstdc++ debug mode, compile your application with the
+ compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
+ changes the sizes and behavior of standard class templates such
+ as <code>std::vector</code>, and therefore you can only link code
+ compiled with debug mode and code compiled without debug mode if no
+ instantiation of a container is passed between the two translation
+ units.</para>
+
+<para>By default, error messages are formatted to fit on lines of about
+ 78 characters. The environment variable
+ <code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a
+ different length.</para>
+
+</sect2>
+
+<sect2 id="debug_mode.using.specific" xreflabel="Using Specific">
+ <title>Using a Specific Debug Container</title>
+<para>When it is not feasible to recompile your entire application, or
+ only specific containers need checking, debugging containers are
+ available as GNU extensions. These debugging containers are
+ functionally equivalent to the standard drop-in containers used in
+ debug mode, but they are available in a separate namespace as GNU
+ extensions and may be used in programs compiled with either release
+ mode or with debug mode. The
+ following table provides the names and headers of the debugging
+ containers:
+</para>
+
+<table frame='all'>
+<title>Debugging Containers</title>
+<tgroup cols='6' align='left' colsep='1' rowsep='1'>
+<colspec colname='c1'></colspec>
+<colspec colname='c2'></colspec>
+<colspec colname='c3'></colspec>
+<colspec colname='c4'></colspec>
+
+<thead>
+ <row>
+ <entry>Container</entry>
+ <entry>Header</entry>
+ <entry>Debug container</entry>
+ <entry>Debug header</entry>
+ </row>
+</thead>
+<tbody>
+ <row>
+ <entry><classname>std::bitset</classname></entry>
+ <entry><filename class="headerfile">bitset</filename></entry>
+ <entry><classname>__gnu_debug::bitset</classname></entry>
+ <entry><filename class="headerfile">bitset</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::deque</classname></entry>
+ <entry><filename class="headerfile">deque</filename></entry>
+ <entry><classname>__gnu_debug::deque</classname></entry>
+ <entry><filename class="headerfile">deque</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::list</classname></entry>
+ <entry><filename class="headerfile">list</filename></entry>
+ <entry><classname>__gnu_debug::list</classname></entry>
+ <entry><filename class="headerfile">list</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::map</classname></entry>
+ <entry><filename class="headerfile">map</filename></entry>
+ <entry><classname>__gnu_debug::map</classname></entry>
+ <entry><filename class="headerfile">map</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::multimap</classname></entry>
+ <entry><filename class="headerfile">map</filename></entry>
+ <entry><classname>__gnu_debug::multimap</classname></entry>
+ <entry><filename class="headerfile">map</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::multiset</classname></entry>
+ <entry><filename class="headerfile">set</filename></entry>
+ <entry><classname>__gnu_debug::multiset</classname></entry>
+ <entry><filename class="headerfile">set</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::set</classname></entry>
+ <entry><filename class="headerfile">set</filename></entry>
+ <entry><classname>__gnu_debug::set</classname></entry>
+ <entry><filename class="headerfile">set</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::string</classname></entry>
+ <entry><filename class="headerfile">string</filename></entry>
+ <entry><classname>__gnu_debug::string</classname></entry>
+ <entry><filename class="headerfile">string</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::wstring</classname></entry>
+ <entry><filename class="headerfile">string</filename></entry>
+ <entry><classname>__gnu_debug::wstring</classname></entry>
+ <entry><filename class="headerfile">string</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::basic_string</classname></entry>
+ <entry><filename class="headerfile">string</filename></entry>
+ <entry><classname>__gnu_debug::basic_string</classname></entry>
+ <entry><filename class="headerfile">string</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::vector</classname></entry>
+ <entry><filename class="headerfile">vector</filename></entry>
+ <entry><classname>__gnu_debug::vector</classname></entry>
+ <entry><filename class="headerfile">vector</filename></entry>
+ </row>
+</tbody>
+</tgroup>
+</table>
+
+<para>In addition, when compiling in C++0x mode, these additional
+containers have additional debug capability.
+</para>
+
+<table frame='all'>
+<title>Debugging Containers C++0x</title>
+<tgroup cols='6' align='left' colsep='1' rowsep='1'>
+<colspec colname='c1'></colspec>
+<colspec colname='c2'></colspec>
+<colspec colname='c3'></colspec>
+<colspec colname='c4'></colspec>
+
+<thead>
+ <row>
+ <entry>Container</entry>
+ <entry>Header</entry>
+ <entry>Debug container</entry>
+ <entry>Debug header</entry>
+ </row>
+</thead>
+<tbody>
+ <row>
+ <entry><classname>std::unordered_map</classname></entry>
+ <entry><filename class="headerfile">unordered_map</filename></entry>
+ <entry><classname>__gnu_debug::unordered_map</classname></entry>
+ <entry><filename class="headerfile">unordered_map</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::unordered_multimap</classname></entry>
+ <entry><filename class="headerfile">unordered_map</filename></entry>
+ <entry><classname>__gnu_debug::unordered_multimap</classname></entry>
+ <entry><filename class="headerfile">unordered_map</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::unordered_set</classname></entry>
+ <entry><filename class="headerfile">unordered_set</filename></entry>
+ <entry><classname>__gnu_debug::unordered_set</classname></entry>
+ <entry><filename class="headerfile">unordered_set</filename></entry>
+ </row>
+ <row>
+ <entry><classname>std::unordered_multiset</classname></entry>
+ <entry><filename class="headerfile">unordered_set</filename></entry>
+ <entry><classname>__gnu_debug::unordered_multiset</classname></entry>
+ <entry><filename class="headerfile">unordered_set</filename></entry>
+ </row>
+</tbody>
+</tgroup>
+</table>
+</sect2>
+</sect1>
+
+<sect1 id="manual.ext.debug_mode.design" xreflabel="Design">
+ <title>Design</title>
+ <para>
+ </para>
+ <sect2 id="manual.ext.debug_mode.design.goals" xreflabel="Goals">
+ <title>Goals</title>
+ <para>
+ </para>
+<para> The libstdc++ debug mode replaces unsafe (but efficient) standard
+ containers and iterators with semantically equivalent safe standard
+ containers and iterators to aid in debugging user programs. The
+ following goals directed the design of the libstdc++ debug mode:</para>
+
+ <itemizedlist>
+
+ <listitem><para><emphasis>Correctness</emphasis>: the libstdc++ debug mode must not change
+ the semantics of the standard library for all cases specified in
+ the ANSI/ISO C++ standard. The essence of this constraint is that
+ any valid C++ program should behave in the same manner regardless
+ of whether it is compiled with debug mode or release mode. In
+ particular, entities that are defined in namespace std in release
+ mode should remain defined in namespace std in debug mode, so that
+ legal specializations of namespace std entities will remain
+ valid. A program that is not valid C++ (e.g., invokes undefined
+ behavior) is not required to behave similarly, although the debug
+ mode will abort with a diagnostic when it detects undefined
+ behavior.</para></listitem>
+
+ <listitem><para><emphasis>Performance</emphasis>: the additional of the libstdc++ debug mode
+ must not affect the performance of the library when it is compiled
+ in release mode. Performance of the libstdc++ debug mode is
+ secondary (and, in fact, will be worse than the release
+ mode).</para></listitem>
+
+ <listitem><para><emphasis>Usability</emphasis>: the libstdc++ debug mode should be easy to
+ use. It should be easily incorporated into the user's development
+ environment (e.g., by requiring only a single new compiler switch)
+ and should produce reasonable diagnostics when it detects a
+ problem with the user program. Usability also involves detection
+ of errors when using the debug mode incorrectly, e.g., by linking
+ a release-compiled object against a debug-compiled object if in
+ fact the resulting program will not run correctly.</para></listitem>
+
+ <listitem><para><emphasis>Minimize recompilation</emphasis>: While it is expected that
+ users recompile at least part of their program to use debug
+ mode, the amount of recompilation affects the
+ detect-compile-debug turnaround time. This indirectly affects the
+ usefulness of the debug mode, because debugging some applications
+ may require rebuilding a large amount of code, which may not be
+ feasible when the suspect code may be very localized. There are
+ several levels of conformance to this requirement, each with its
+ own usability and implementation characteristics. In general, the
+ higher-numbered conformance levels are more usable (i.e., require
+ less recompilation) but are more complicated to implement than
+ the lower-numbered conformance levels.
+ <orderedlist>
+ <listitem><para><emphasis>Full recompilation</emphasis>: The user must recompile his or
+ her entire application and all C++ libraries it depends on,
+ including the C++ standard library that ships with the
+ compiler. This must be done even if only a small part of the
+ program can use debugging features.</para></listitem>
+
+ <listitem><para><emphasis>Full user recompilation</emphasis>: The user must recompile
+ his or her entire application and all C++ libraries it depends
+ on, but not the C++ standard library itself. This must be done
+ even if only a small part of the program can use debugging
+ features. This can be achieved given a full recompilation
+ system by compiling two versions of the standard library when
+ the compiler is installed and linking against the appropriate
+ one, e.g., a multilibs approach.</para></listitem>
+
+ <listitem><para><emphasis>Partial recompilation</emphasis>: The user must recompile the
+ parts of his or her application and the C++ libraries it
+ depends on that will use the debugging facilities
+ directly. This means that any code that uses the debuggable
+ standard containers would need to be recompiled, but code
+ that does not use them (but may, for instance, use IOStreams)
+ would not have to be recompiled.</para></listitem>
+
+ <listitem><para><emphasis>Per-use recompilation</emphasis>: The user must recompile the
+ parts of his or her application and the C++ libraries it
+ depends on where debugging should occur, and any other code
+ that interacts with those containers. This means that a set of
+ translation units that accesses a particular standard
+ container instance may either be compiled in release mode (no
+ checking) or debug mode (full checking), but must all be
+ compiled in the same way; a translation unit that does not see
+ that standard container instance need not be recompiled. This
+ also means that a translation unit <emphasis>A</emphasis> that contains a
+ particular instantiation
+ (say, <code>std::vector<int></code>) compiled in release
+ mode can be linked against a translation unit <emphasis>B</emphasis> that
+ contains the same instantiation compiled in debug mode (a
+ feature not present with partial recompilation). While this
+ behavior is technically a violation of the One Definition
+ Rule, this ability tends to be very important in
+ practice. The libstdc++ debug mode supports this level of
+ recompilation. </para></listitem>
+
+ <listitem><para><emphasis>Per-unit recompilation</emphasis>: The user must only
+ recompile the translation units where checking should occur,
+ regardless of where debuggable standard containers are
+ used. This has also been dubbed "<code>-g</code> mode",
+ because the <code>-g</code> compiler switch works in this way,
+ emitting debugging information at a per--translation-unit
+ granularity. We believe that this level of recompilation is in
+ fact not possible if we intend to supply safe iterators, leave
+ the program semantics unchanged, and not regress in
+ performance under release mode because we cannot associate
+ extra information with an iterator (to form a safe iterator)
+ without either reserving that space in release mode
+ (performance regression) or allocating extra memory associated
+ with each iterator with <code>new</code> (changes the program
+ semantics).</para></listitem>
+ </orderedlist>
+ </para></listitem>
+ </itemizedlist>
+ </sect2>
+
+ <sect2 id="manual.ext.debug_mode.design.methods" xreflabel="Methods">
+ <title>Methods</title>
+ <para>
+ </para>
+<para>This section provides an overall view of the design of the
+ libstdc++ debug mode and details the relationship between design
+ decisions and the stated design goals.</para>
+
+ <sect3 id="debug_mode.design.methods.wrappers" xreflabel="Method Wrapper">
+ <title>The Wrapper Model</title>
+<para>The libstdc++ debug mode uses a wrapper model where the debugging
+ versions of library components (e.g., iterators and containers) form
+ a layer on top of the release versions of the library
+ components. The debugging components first verify that the operation
+ is correct (aborting with a diagnostic if an error is found) and
+ will then forward to the underlying release-mode container that will
+ perform the actual work. This design decision ensures that we cannot
+ regress release-mode performance (because the release-mode
+ containers are left untouched) and partially enables <ulink url="#mixing">mixing debug and release code</ulink> at link time,
+ although that will not be discussed at this time.</para>
+
+<para>Two types of wrappers are used in the implementation of the debug
+ mode: container wrappers and iterator wrappers. The two types of
+ wrappers interact to maintain relationships between iterators and
+ their associated containers, which are necessary to detect certain
+ types of standard library usage errors such as dereferencing
+ past-the-end iterators or inserting into a container using an
+ iterator from a different container.</para>
+
+ <sect4 id="debug_mode.design.methods.safe_iter" xreflabel="Method Safe Iter">
+ <title>Safe Iterators</title>
+<para>Iterator wrappers provide a debugging layer over any iterator that
+ is attached to a particular container, and will manage the
+ information detailing the iterator's state (singular,
+ dereferenceable, etc.) and tracking the container to which the
+ iterator is attached. Because iterators have a well-defined, common
+ interface the iterator wrapper is implemented with the iterator
+ adaptor class template <code>__gnu_debug::_Safe_iterator</code>,
+ which takes two template parameters:</para>
+
+<itemizedlist>
+ <listitem><para><code>Iterator</code>: The underlying iterator type, which must
+ be either the <code>iterator</code> or <code>const_iterator</code>
+ typedef from the sequence type this iterator can reference.</para></listitem>
+
+ <listitem><para><code>Sequence</code>: The type of sequence that this iterator
+ references. This sequence must be a safe sequence (discussed below)
+ whose <code>iterator</code> or <code>const_iterator</code> typedef
+ is the type of the safe iterator.</para></listitem>
+</itemizedlist>
+ </sect4>
+
+ <sect4 id="debug_mode.design.methods.safe_seq" xreflabel="Method Safe Seq">
+ <title>Safe Sequences (Containers)</title>
+
+<para>Container wrappers provide a debugging layer over a particular
+ container type. Because containers vary greatly in the member
+ functions they support and the semantics of those member functions
+ (especially in the area of iterator invalidation), container
+ wrappers are tailored to the container they reference, e.g., the
+ debugging version of <code>std::list</code> duplicates the entire
+ interface of <code>std::list</code>, adding additional semantic
+ checks and then forwarding operations to the
+ real <code>std::list</code> (a public base class of the debugging
+ version) as appropriate. However, all safe containers inherit from
+ the class template <code>__gnu_debug::_Safe_sequence</code>,
+ instantiated with the type of the safe container itself (an instance
+ of the curiously recurring template pattern).</para>
+
+<para>The iterators of a container wrapper will be
+ <ulink url="#safe_iterator">safe iterators</ulink> that reference sequences
+ of this type and wrap the iterators provided by the release-mode
+ base class. The debugging container will use only the safe
+ iterators within its own interface (therefore requiring the user to
+ use safe iterators, although this does not change correct user
+ code) and will communicate with the release-mode base class with
+ only the underlying, unsafe, release-mode iterators that the base
+ class exports.</para>
+
+<para> The debugging version of <code>std::list</code> will have the
+ following basic structure:</para>
+
+<programlisting>
+template<typename _Tp, typename _Allocator = allocator<_Tp>
+ class debug-list :
+ public release-list<_Tp, _Allocator>,
+ public __gnu_debug::_Safe_sequence<debug-list<_Tp, _Allocator> >
+ {
+ typedef release-list<_Tp, _Allocator> _Base;
+ typedef debug-list<_Tp, _Allocator> _Self;
+
+ public:
+ typedef __gnu_debug::_Safe_iterator<typename _Base::iterator, _Self> iterator;
+ typedef __gnu_debug::_Safe_iterator<typename _Base::const_iterator, _Self> const_iterator;
+
+ // duplicate std::list interface with debugging semantics
+ };
+</programlisting>
+ </sect4>
+ </sect3>
+
+ <sect3 id="debug_mode.design.methods.precond" xreflabel="Precondition check">
+ <title>Precondition Checking</title>
+<para>The debug mode operates primarily by checking the preconditions of
+ all standard library operations that it supports. Preconditions that
+ are always checked (regardless of whether or not we are in debug
+ mode) are checked via the <code>__check_xxx</code> macros defined
+ and documented in the source
+ file <code>include/debug/debug.h</code>. Preconditions that may or
+ may not be checked, depending on the debug-mode
+ macro <code>_GLIBCXX_DEBUG</code>, are checked via
+ the <code>__requires_xxx</code> macros defined and documented in the
+ same source file. Preconditions are validated using any additional
+ information available at run-time, e.g., the containers that are
+ associated with a particular iterator, the position of the iterator
+ within those containers, the distance between two iterators that may
+ form a valid range, etc. In the absence of suitable information,
+ e.g., an input iterator that is not a safe iterator, these
+ precondition checks will silently succeed.</para>
+
+<para>The majority of precondition checks use the aforementioned macros,
+ which have the secondary benefit of having prewritten debug
+ messages that use information about the current status of the
+ objects involved (e.g., whether an iterator is singular or what
+ sequence it is attached to) along with some static information
+ (e.g., the names of the function parameters corresponding to the
+ objects involved). When not using these macros, the debug mode uses
+ either the debug-mode assertion
+ macro <code>_GLIBCXX_DEBUG_ASSERT</code> , its pedantic
+ cousin <code>_GLIBCXX_DEBUG_PEDASSERT</code>, or the assertion
+ check macro that supports more advance formulation of error
+ messages, <code>_GLIBCXX_DEBUG_VERIFY</code>. These macros are
+ documented more thoroughly in the debug mode source code.</para>
+ </sect3>
+
+ <sect3 id="debug_mode.design.methods.coexistence" xreflabel="Coexistence">
+ <title>Release- and debug-mode coexistence</title>
+<para>The libstdc++ debug mode is the first debug mode we know of that
+ is able to provide the "Per-use recompilation" (4) guarantee, that
+ allows release-compiled and debug-compiled code to be linked and
+ executed together without causing unpredictable behavior. This
+ guarantee minimizes the recompilation that users are required to
+ perform, shortening the detect-compile-debug bug hunting cycle
+ and making the debug mode easier to incorporate into development
+ environments by minimizing dependencies.</para>
+
+<para>Achieving link- and run-time coexistence is not a trivial
+ implementation task. To achieve this goal we required a small
+ extension to the GNU C++ compiler (described in the GCC Manual for
+ C++ Extensions, see <ulink url="http://gcc.gnu.org/onlinedocs/gcc/Strong-Using.html">strong
+ using</ulink>), and a complex organization of debug- and
+ release-modes. The end result is that we have achieved per-use
+ recompilation but have had to give up some checking of the
+ <code>std::basic_string</code> class template (namely, safe
+ iterators).
+</para>
+
+ <sect4 id="methods.coexistence.compile" xreflabel="Compile">
+ <title>Compile-time coexistence of release- and debug-mode components</title>
+
+<para>Both the release-mode components and the debug-mode
+ components need to exist within a single translation unit so that
+ the debug versions can wrap the release versions. However, only one
+ of these components should be user-visible at any particular
+ time with the standard name, e.g., <code>std::list</code>. </para>
+
+<para>In release mode, we define only the release-mode version of the
+ component with its standard name and do not include the debugging
+ component at all. The release mode version is defined within the
+ namespace <code>std</code>. Minus the namespace associations, this
+ method leaves the behavior of release mode completely unchanged from
+ its behavior prior to the introduction of the libstdc++ debug
+ mode. Here's an example of what this ends up looking like, in
+ C++.</para>
+
+<programlisting>
+namespace std
+{
+ template<typename _Tp, typename _Alloc = allocator<_Tp> >
+ class list
+ {
+ // ...
+ };
+} // namespace std
+</programlisting>
+
+<para>In debug mode we include the release-mode container (which is now
+defined in in the namespace <code>__norm</code>) and also the
+debug-mode container. The debug-mode container is defined within the
+namespace <code>__debug</code>, which is associated with namespace
+<code>std</code> via the GNU namespace association extension. This
+method allows the debug and release versions of the same component to
+coexist at compile-time and link-time without causing an unreasonable
+maintenance burden, while minimizing confusion. Again, this boils down
+to C++ code as follows:</para>
+
+<programlisting>
+namespace std
+{
+ namespace __norm
+ {
+ template<typename _Tp, typename _Alloc = allocator<_Tp> >
+ class list
+ {
+ // ...
+ };
+ } // namespace __gnu_norm
+
+ namespace __debug
+ {
+ template<typename _Tp, typename _Alloc = allocator<_Tp> >
+ class list
+ : public __norm::list<_Tp, _Alloc>,
+ public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> >
+ {
+ // ...
+ };
+ } // namespace __norm
+
+ using namespace __debug __attribute__ ((strong));
+}
+</programlisting>
+ </sect4>
+
+ <sect4 id="methods.coexistence.link" xreflabel="Link">
+ <title>Link- and run-time coexistence of release- and
+ debug-mode components</title>
+
+<para>Because each component has a distinct and separate release and
+debug implementation, there are are no issues with link-time
+coexistence: the separate namespaces result in different mangled
+names, and thus unique linkage.</para>
+
+<para>However, components that are defined and used within the C++
+standard library itself face additional constraints. For instance,
+some of the member functions of <code> std::moneypunct</code> return
+<code>std::basic_string</code>. Normally, this is not a problem, but
+with a mixed mode standard library that could be using either
+debug-mode or release-mode <code> basic_string</code> objects, things
+get more complicated. As the return value of a function is not
+encoded into the mangled name, there is no way to specify a
+release-mode or a debug-mode string. In practice, this results in
+runtime errors. A simplified example of this problem is as follows.
+</para>
+
+<para> Take this translation unit, compiled in debug-mode: </para>
+<programlisting>
+// -D_GLIBCXX_DEBUG
+#include <string>
+
+std::string test02();
+
+std::string test01()
+{
+ return test02();
+}
+
+int main()
+{
+ test01();
+ return 0;
+}
+</programlisting>
+
+<para> ... and linked to this translation unit, compiled in release mode:</para>
+
+<programlisting>
+#include <string>
+
+std::string
+test02()
+{
+ return std::string("toast");
+}
+</programlisting>
+
+<para> For this reason we cannot easily provide safe iterators for
+ the <code>std::basic_string</code> class template, as it is present
+ throughout the C++ standard library. For instance, locale facets
+ define typedefs that include <code>basic_string</code>: in a mixed
+ debug/release program, should that typedef be based on the
+ debug-mode <code>basic_string</code> or the
+ release-mode <code>basic_string</code>? While the answer could be
+ "both", and the difference hidden via renaming a la the
+ debug/release containers, we must note two things about locale
+ facets:</para>
+
+<orderedlist>
+ <listitem><para>They exist as shared state: one can create a facet in one
+ translation unit and access the facet via the same type name in a
+ different translation unit. This means that we cannot have two
+ different versions of locale facets, because the types would not be
+ the same across debug/release-mode translation unit barriers.</para></listitem>
+
+ <listitem><para>They have virtual functions returning strings: these functions
+ mangle in the same way regardless of the mangling of their return
+ types (see above), and their precise signatures can be relied upon
+ by users because they may be overridden in derived classes.</para></listitem>
+</orderedlist>
+
+<para>With the design of libstdc++ debug mode, we cannot effectively hide
+ the differences between debug and release-mode strings from the
+ user. Failure to hide the differences may result in unpredictable
+ behavior, and for this reason we have opted to only
+ perform <code>basic_string</code> changes that do not require ABI
+ changes. The effect on users is expected to be minimal, as there are
+ simple alternatives (e.g., <code>__gnu_debug::basic_string</code>),
+ and the usability benefit we gain from the ability to mix debug- and
+ release-compiled translation units is enormous.</para>
+ </sect4>
+
+ <sect4 id="methods.coexistence.alt" xreflabel="Alternatives">
+<title>Alternatives for Coexistence</title>
+
+<para>The coexistence scheme above was chosen over many alternatives,
+ including language-only solutions and solutions that also required
+ extensions to the C++ front end. The following is a partial list of
+ solutions, with justifications for our rejection of each.</para>
+
+<itemizedlist>
+ <listitem><para><emphasis>Completely separate debug/release libraries</emphasis>: This is by
+ far the simplest implementation option, where we do not allow any
+ coexistence of debug- and release-compiled translation units in a
+ program. This solution has an extreme negative affect on usability,
+ because it is quite likely that some libraries an application
+ depends on cannot be recompiled easily. This would not meet
+ our <emphasis>usability</emphasis> or <emphasis>minimize recompilation</emphasis> criteria
+ well.</para></listitem>
+
+ <listitem><para><emphasis>Add a <code>Debug</code> boolean template parameter</emphasis>:
+ Partial specialization could be used to select the debug
+ implementation when <code>Debug == true</code>, and the state
+ of <code>_GLIBCXX_DEBUG</code> could decide whether the
+ default <code>Debug</code> argument is <code>true</code>
+ or <code>false</code>. This option would break conformance with the
+ C++ standard in both debug <emphasis>and</emphasis> release modes. This would
+ not meet our <emphasis>correctness</emphasis> criteria. </para></listitem>
+
+ <listitem><para><emphasis>Packaging a debug flag in the allocators</emphasis>: We could
+ reuse the <code>Allocator</code> template parameter of containers
+ by adding a sentinel wrapper <code>debug<></code> that
+ signals the user's intention to use debugging, and pick up
+ the <code>debug<></code> allocator wrapper in a partial
+ specialization. However, this has two drawbacks: first, there is a
+ conformance issue because the default allocator would not be the
+ standard-specified <code>std::allocator<T></code>. Secondly
+ (and more importantly), users that specify allocators instead of
+ implicitly using the default allocator would not get debugging
+ containers. Thus this solution fails the <emphasis>correctness</emphasis>
+ criteria.</para></listitem>
+
+ <listitem><para><emphasis>Define debug containers in another namespace, and employ
+ a <code>using</code> declaration (or directive)</emphasis>: This is an
+ enticing option, because it would eliminate the need for
+ the <code>link_name</code> extension by aliasing the
+ templates. However, there is no true template aliasing mechanism
+ is C++, because both <code>using</code> directives and using
+ declarations disallow specialization. This method fails
+ the <emphasis>correctness</emphasis> criteria.</para></listitem>
+
+ <listitem><para><emphasis> Use implementation-specific properties of anonymous
+ namespaces. </emphasis>
+ See <ulink url="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00004.html"> this post
+ </ulink>
+ This method fails the <emphasis>correctness</emphasis> criteria.</para></listitem>
+
+ <listitem><para><emphasis>Extension: allow reopening on namespaces</emphasis>: This would
+ allow the debug mode to effectively alias the
+ namespace <code>std</code> to an internal namespace, such
+ as <code>__gnu_std_debug</code>, so that it is completely
+ separate from the release-mode <code>std</code> namespace. While
+ this will solve some renaming problems and ensure that
+ debug- and release-compiled code cannot be mixed unsafely, it ensures that
+ debug- and release-compiled code cannot be mixed at all. For
+ instance, the program would have two <code>std::cout</code>
+ objects! This solution would fails the <emphasis>minimize
+ recompilation</emphasis> requirement, because we would only be able to
+ support option (1) or (2).</para></listitem>
+
+ <listitem><para><emphasis>Extension: use link name</emphasis>: This option involves
+ complicated re-naming between debug-mode and release-mode
+ components at compile time, and then a g++ extension called <emphasis>
+ link name </emphasis> to recover the original names at link time. There
+ are two drawbacks to this approach. One, it's very verbose,
+ relying on macro renaming at compile time and several levels of
+ include ordering. Two, ODR issues remained with container member
+ functions taking no arguments in mixed-mode settings resulting in
+ equivalent link names, <code> vector::push_back() </code> being
+ one example.
+ See <ulink url="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00177.html">link
+ name</ulink> </para></listitem>
+</itemizedlist>
+
+<para>Other options may exist for implementing the debug mode, many of
+ which have probably been considered and others that may still be
+ lurking. This list may be expanded over time to include other
+ options that we could have implemented, but in all cases the full
+ ramifications of the approach (as measured against the design goals
+ for a libstdc++ debug mode) should be considered first. The DejaGNU
+ testsuite includes some testcases that check for known problems with
+ some solutions (e.g., the <code>using</code> declaration solution
+ that breaks user specialization), and additional testcases will be
+ added as we are able to identify other typical problem cases. These
+ test cases will serve as a benchmark by which we can compare debug
+ mode implementations.</para>
+ </sect4>
+ </sect3>
+ </sect2>
+
+ <sect2 id="manual.ext.debug_mode.design.other" xreflabel="Other">
+ <title>Other Implementations</title>
+ <para>
+ </para>
+<para> There are several existing implementations of debug modes for C++
+ standard library implementations, although none of them directly
+ supports debugging for programs using libstdc++. The existing
+ implementations include:</para>
+<itemizedlist>
+ <listitem><para><ulink url="http://www.mathcs.sjsu.edu/faculty/horstman/safestl.html">SafeSTL</ulink>:
+ SafeSTL was the original debugging version of the Standard Template
+ Library (STL), implemented by Cay S. Horstmann on top of the
+ Hewlett-Packard STL. Though it inspired much work in this area, it
+ has not been kept up-to-date for use with modern compilers or C++
+ standard library implementations.</para></listitem>
+
+ <listitem><para><ulink url="http://www.stlport.org/">STLport</ulink>: STLport is a free
+ implementation of the C++ standard library derived from the <ulink url="http://www.sgi.com/tech/stl/">SGI implementation</ulink>, and
+ ported to many other platforms. It includes a debug mode that uses a
+ wrapper model (that in some way inspired the libstdc++ debug mode
+ design), although at the time of this writing the debug mode is
+ somewhat incomplete and meets only the "Full user recompilation" (2)
+ recompilation guarantee by requiring the user to link against a
+ different library in debug mode vs. release mode.</para></listitem>
+
+ <listitem><para><ulink url="http://www.metrowerks.com/mw/default.htm">Metrowerks
+ CodeWarrior</ulink>: The C++ standard library that ships with Metrowerks
+ CodeWarrior includes a debug mode. It is a full debug-mode
+ implementation (including debugging for CodeWarrior extensions) and
+ is easy to use, although it meets only the "Full recompilation" (1)
+ recompilation guarantee.</para></listitem>
+</itemizedlist>
+
+ </sect2>
+</sect1>
+
+</chapter>
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