1 <chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
2 xml:id="manual.ext.debug_mode" xreflabel="Debug Mode">
3 <?dbhtml filename="debug_mode.html"?>
5 <info><title>Debug Mode</title>
21 <section xml:id="manual.ext.debug_mode.intro" xreflabel="Intro"><info><title>Intro</title></info>
24 By default, libstdc++ is built with efficiency in mind, and
25 therefore performs little or no error checking that is not
26 required by the C++ standard. This means that programs that
27 incorrectly use the C++ standard library will exhibit behavior
28 that is not portable and may not even be predictable, because they
29 tread into implementation-specific or undefined behavior. To
30 detect some of these errors before they can become problematic,
31 libstdc++ offers a debug mode that provides additional checking of
32 library facilities, and will report errors in the use of libstdc++
33 as soon as they can be detected by emitting a description of the
34 problem to standard error and aborting the program. This debug
35 mode is available with GCC 3.4.0 and later versions.
39 The libstdc++ debug mode performs checking for many areas of the
40 C++ standard, but the focus is on checking interactions among
41 standard iterators, containers, and algorithms, including:
45 <listitem><para><emphasis>Safe iterators</emphasis>: Iterators keep track of the
46 container whose elements they reference, so errors such as
47 incrementing a past-the-end iterator or dereferencing an iterator
48 that points to a container that has been destructed are diagnosed
49 immediately.</para></listitem>
51 <listitem><para><emphasis>Algorithm preconditions</emphasis>: Algorithms attempt to
52 validate their input parameters to detect errors as early as
53 possible. For instance, the <code>set_intersection</code>
54 algorithm requires that its iterator
55 parameters <code>first1</code> and <code>last1</code> form a valid
56 iterator range, and that the sequence
57 [<code>first1</code>, <code>last1</code>) is sorted according to
58 the same predicate that was passed
59 to <code>set_intersection</code>; the libstdc++ debug mode will
60 detect an error if the sequence is not sorted or was sorted by a
61 different predicate.</para></listitem>
66 <section xml:id="manual.ext.debug_mode.semantics" xreflabel="Semantics"><info><title>Semantics</title></info>
71 <para>A program that uses the C++ standard library correctly
72 will maintain the same semantics under debug mode as it had with
73 the normal (release) library. All functional and exception-handling
74 guarantees made by the normal library also hold for the debug mode
75 library, with one exception: performance guarantees made by the
76 normal library may not hold in the debug mode library. For
77 instance, erasing an element in a <code>std::list</code> is a
78 constant-time operation in normal library, but in debug mode it is
79 linear in the number of iterators that reference that particular
80 list. So while your (correct) program won't change its results, it
81 is likely to execute more slowly.</para>
83 <para>libstdc++ includes many extensions to the C++ standard library. In
84 some cases the extensions are obvious, such as the hashed
85 associative containers, whereas other extensions give predictable
86 results to behavior that would otherwise be undefined, such as
87 throwing an exception when a <code>std::basic_string</code> is
88 constructed from a NULL character pointer. This latter category also
89 includes implementation-defined and unspecified semantics, such as
90 the growth rate of a vector. Use of these extensions is not
91 considered incorrect, so code that relies on them will not be
92 rejected by debug mode. However, use of these extensions may affect
93 the portability of code to other implementations of the C++ standard
94 library, and is therefore somewhat hazardous. For this reason, the
95 libstdc++ debug mode offers a "pedantic" mode (similar to
96 GCC's <code>-pedantic</code> compiler flag) that attempts to emulate
97 the semantics guaranteed by the C++ standard. For
98 instance, constructing a <code>std::basic_string</code> with a NULL
99 character pointer would result in an exception under normal mode or
100 non-pedantic debug mode (this is a libstdc++ extension), whereas
101 under pedantic debug mode libstdc++ would signal an error. To enable
102 the pedantic debug mode, compile your program with
103 both <code>-D_GLIBCXX_DEBUG</code>
104 and <code>-D_GLIBCXX_DEBUG_PEDANTIC</code> .
105 (N.B. In GCC 3.4.x and 4.0.0, due to a bug,
106 <code>-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
107 been fixed in GCC 4.0.1 and later versions.) </para>
109 <para>The following library components provide extra debugging
110 capabilities in debug mode:</para>
112 <listitem><para><code>std::basic_string</code> (no safe iterators and see note below)</para></listitem>
113 <listitem><para><code>std::bitset</code></para></listitem>
114 <listitem><para><code>std::deque</code></para></listitem>
115 <listitem><para><code>std::list</code></para></listitem>
116 <listitem><para><code>std::map</code></para></listitem>
117 <listitem><para><code>std::multimap</code></para></listitem>
118 <listitem><para><code>std::multiset</code></para></listitem>
119 <listitem><para><code>std::set</code></para></listitem>
120 <listitem><para><code>std::vector</code></para></listitem>
121 <listitem><para><code>std::unordered_map</code></para></listitem>
122 <listitem><para><code>std::unordered_multimap</code></para></listitem>
123 <listitem><para><code>std::unordered_set</code></para></listitem>
124 <listitem><para><code>std::unordered_multiset</code></para></listitem>
127 <para>N.B. although there are precondition checks for some string operations,
128 e.g. <code>operator[]</code>,
129 they will not always be run when using the <code>char</code> and
130 <code>wchar_t</code> specialisations (<code>std::string</code> and
131 <code>std::wstring</code>). This is because libstdc++ uses GCC's
132 <code>extern template</code> extension to provide explicit instantiations
133 of <code>std::string</code> and <code>std::wstring</code>, and those
134 explicit instantiations don't include the debug-mode checks. If the
135 containing functions are inlined then the checks will run, so compiling
136 with <code>-O1</code> might be enough to enable them. Alternatively
137 <code>-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
138 of the explicit instantiations and cause the functions to be instantiated
139 with the debug-mode checks included, but this is unsupported and not
140 guaranteed to work. For full debug-mode support you can use the
141 <code>__gnu_debug::basic_string</code> debugging container directly,
142 which always works correctly.
147 <section xml:id="manual.ext.debug_mode.using" xreflabel="Using"><info><title>Using</title></info>
151 <section xml:id="debug_mode.using.mode" xreflabel="Using Mode"><info><title>Using the Debug Mode</title></info>
154 <para>To use the libstdc++ debug mode, compile your application with the
155 compiler flag <code>-D_GLIBCXX_DEBUG</code>. Note that this flag
156 changes the sizes and behavior of standard class templates such
157 as <code>std::vector</code>, and therefore you can only link code
158 compiled with debug mode and code compiled without debug mode if no
159 instantiation of a container is passed between the two translation
162 <para>By default, error messages are formatted to fit on lines of about
163 78 characters. The environment variable
164 <code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a
165 different length.</para>
169 <section xml:id="debug_mode.using.specific" xreflabel="Using Specific"><info><title>Using a Specific Debug Container</title></info>
171 <para>When it is not feasible to recompile your entire application, or
172 only specific containers need checking, debugging containers are
173 available as GNU extensions. These debugging containers are
174 functionally equivalent to the standard drop-in containers used in
175 debug mode, but they are available in a separate namespace as GNU
176 extensions and may be used in programs compiled with either release
177 mode or with debug mode. The
178 following table provides the names and headers of the debugging
183 <title>Debugging Containers</title>
185 <tgroup cols="4" align="left" colsep="1" rowsep="1">
186 <colspec colname="c1"/>
187 <colspec colname="c2"/>
188 <colspec colname="c3"/>
189 <colspec colname="c4"/>
193 <entry>Container</entry>
194 <entry>Header</entry>
195 <entry>Debug container</entry>
196 <entry>Debug header</entry>
201 <entry><classname>std::bitset</classname></entry>
202 <entry><filename class="headerfile">bitset</filename></entry>
203 <entry><classname>__gnu_debug::bitset</classname></entry>
204 <entry><filename class="headerfile"><debug/bitset></filename></entry>
207 <entry><classname>std::deque</classname></entry>
208 <entry><filename class="headerfile">deque</filename></entry>
209 <entry><classname>__gnu_debug::deque</classname></entry>
210 <entry><filename class="headerfile"><debug/deque></filename></entry>
213 <entry><classname>std::list</classname></entry>
214 <entry><filename class="headerfile">list</filename></entry>
215 <entry><classname>__gnu_debug::list</classname></entry>
216 <entry><filename class="headerfile"><debug/list></filename></entry>
219 <entry><classname>std::map</classname></entry>
220 <entry><filename class="headerfile">map</filename></entry>
221 <entry><classname>__gnu_debug::map</classname></entry>
222 <entry><filename class="headerfile"><debug/map></filename></entry>
225 <entry><classname>std::multimap</classname></entry>
226 <entry><filename class="headerfile">map</filename></entry>
227 <entry><classname>__gnu_debug::multimap</classname></entry>
228 <entry><filename class="headerfile"><debug/map></filename></entry>
231 <entry><classname>std::multiset</classname></entry>
232 <entry><filename class="headerfile">set</filename></entry>
233 <entry><classname>__gnu_debug::multiset</classname></entry>
234 <entry><filename class="headerfile"><debug/set></filename></entry>
237 <entry><classname>std::set</classname></entry>
238 <entry><filename class="headerfile">set</filename></entry>
239 <entry><classname>__gnu_debug::set</classname></entry>
240 <entry><filename class="headerfile"><debug/set></filename></entry>
243 <entry><classname>std::string</classname></entry>
244 <entry><filename class="headerfile">string</filename></entry>
245 <entry><classname>__gnu_debug::string</classname></entry>
246 <entry><filename class="headerfile"><debug/string></filename></entry>
249 <entry><classname>std::wstring</classname></entry>
250 <entry><filename class="headerfile">string</filename></entry>
251 <entry><classname>__gnu_debug::wstring</classname></entry>
252 <entry><filename class="headerfile"><debug/string></filename></entry>
255 <entry><classname>std::basic_string</classname></entry>
256 <entry><filename class="headerfile">string</filename></entry>
257 <entry><classname>__gnu_debug::basic_string</classname></entry>
258 <entry><filename class="headerfile"><debug/string></filename></entry>
261 <entry><classname>std::vector</classname></entry>
262 <entry><filename class="headerfile">vector</filename></entry>
263 <entry><classname>__gnu_debug::vector</classname></entry>
264 <entry><filename class="headerfile"><debug/vector></filename></entry>
270 <para>In addition, when compiling in C++11 mode, these additional
271 containers have additional debug capability.
275 <title>Debugging Containers C++11</title>
277 <tgroup cols="4" align="left" colsep="1" rowsep="1">
278 <colspec colname="c1"/>
279 <colspec colname="c2"/>
280 <colspec colname="c3"/>
281 <colspec colname="c4"/>
285 <entry>Container</entry>
286 <entry>Header</entry>
287 <entry>Debug container</entry>
288 <entry>Debug header</entry>
293 <entry><classname>std::unordered_map</classname></entry>
294 <entry><filename class="headerfile">unordered_map</filename></entry>
295 <entry><classname>__gnu_debug::unordered_map</classname></entry>
296 <entry><filename class="headerfile"><debug/unordered_map></filename></entry>
299 <entry><classname>std::unordered_multimap</classname></entry>
300 <entry><filename class="headerfile">unordered_map</filename></entry>
301 <entry><classname>__gnu_debug::unordered_multimap</classname></entry>
302 <entry><filename class="headerfile"><debug/unordered_map></filename></entry>
305 <entry><classname>std::unordered_set</classname></entry>
306 <entry><filename class="headerfile">unordered_set</filename></entry>
307 <entry><classname>__gnu_debug::unordered_set</classname></entry>
308 <entry><filename class="headerfile"><debug/unordered_set></filename></entry>
311 <entry><classname>std::unordered_multiset</classname></entry>
312 <entry><filename class="headerfile">unordered_set</filename></entry>
313 <entry><classname>__gnu_debug::unordered_multiset</classname></entry>
314 <entry><filename class="headerfile"><debug/unordered_set></filename></entry>
322 <section xml:id="manual.ext.debug_mode.design" xreflabel="Design"><info><title>Design</title></info>
326 <section xml:id="debug_mode.design.goals" xreflabel="Goals"><info><title>Goals</title></info>
330 <para> The libstdc++ debug mode replaces unsafe (but efficient) standard
331 containers and iterators with semantically equivalent safe standard
332 containers and iterators to aid in debugging user programs. The
333 following goals directed the design of the libstdc++ debug mode:</para>
337 <listitem><para><emphasis>Correctness</emphasis>: the libstdc++ debug mode must not change
338 the semantics of the standard library for all cases specified in
339 the ANSI/ISO C++ standard. The essence of this constraint is that
340 any valid C++ program should behave in the same manner regardless
341 of whether it is compiled with debug mode or release mode. In
342 particular, entities that are defined in namespace std in release
343 mode should remain defined in namespace std in debug mode, so that
344 legal specializations of namespace std entities will remain
345 valid. A program that is not valid C++ (e.g., invokes undefined
346 behavior) is not required to behave similarly, although the debug
347 mode will abort with a diagnostic when it detects undefined
348 behavior.</para></listitem>
350 <listitem><para><emphasis>Performance</emphasis>: the additional of the libstdc++ debug mode
351 must not affect the performance of the library when it is compiled
352 in release mode. Performance of the libstdc++ debug mode is
353 secondary (and, in fact, will be worse than the release
354 mode).</para></listitem>
356 <listitem><para><emphasis>Usability</emphasis>: the libstdc++ debug mode should be easy to
357 use. It should be easily incorporated into the user's development
358 environment (e.g., by requiring only a single new compiler switch)
359 and should produce reasonable diagnostics when it detects a
360 problem with the user program. Usability also involves detection
361 of errors when using the debug mode incorrectly, e.g., by linking
362 a release-compiled object against a debug-compiled object if in
363 fact the resulting program will not run correctly.</para></listitem>
365 <listitem><para><emphasis>Minimize recompilation</emphasis>: While it is expected that
366 users recompile at least part of their program to use debug
367 mode, the amount of recompilation affects the
368 detect-compile-debug turnaround time. This indirectly affects the
369 usefulness of the debug mode, because debugging some applications
370 may require rebuilding a large amount of code, which may not be
371 feasible when the suspect code may be very localized. There are
372 several levels of conformance to this requirement, each with its
373 own usability and implementation characteristics. In general, the
374 higher-numbered conformance levels are more usable (i.e., require
375 less recompilation) but are more complicated to implement than
376 the lower-numbered conformance levels.
377 <orderedlist inheritnum="ignore" continuation="restarts">
378 <listitem><para><emphasis>Full recompilation</emphasis>: The user must recompile his or
379 her entire application and all C++ libraries it depends on,
380 including the C++ standard library that ships with the
381 compiler. This must be done even if only a small part of the
382 program can use debugging features.</para></listitem>
384 <listitem><para><emphasis>Full user recompilation</emphasis>: The user must recompile
385 his or her entire application and all C++ libraries it depends
386 on, but not the C++ standard library itself. This must be done
387 even if only a small part of the program can use debugging
388 features. This can be achieved given a full recompilation
389 system by compiling two versions of the standard library when
390 the compiler is installed and linking against the appropriate
391 one, e.g., a multilibs approach.</para></listitem>
393 <listitem><para><emphasis>Partial recompilation</emphasis>: The user must recompile the
394 parts of his or her application and the C++ libraries it
395 depends on that will use the debugging facilities
396 directly. This means that any code that uses the debuggable
397 standard containers would need to be recompiled, but code
398 that does not use them (but may, for instance, use IOStreams)
399 would not have to be recompiled.</para></listitem>
401 <listitem><para><emphasis>Per-use recompilation</emphasis>: The user must recompile the
402 parts of his or her application and the C++ libraries it
403 depends on where debugging should occur, and any other code
404 that interacts with those containers. This means that a set of
405 translation units that accesses a particular standard
406 container instance may either be compiled in release mode (no
407 checking) or debug mode (full checking), but must all be
408 compiled in the same way; a translation unit that does not see
409 that standard container instance need not be recompiled. This
410 also means that a translation unit <emphasis>A</emphasis> that contains a
411 particular instantiation
412 (say, <code>std::vector<int></code>) compiled in release
413 mode can be linked against a translation unit <emphasis>B</emphasis> that
414 contains the same instantiation compiled in debug mode (a
415 feature not present with partial recompilation). While this
416 behavior is technically a violation of the One Definition
417 Rule, this ability tends to be very important in
418 practice. The libstdc++ debug mode supports this level of
419 recompilation. </para></listitem>
421 <listitem><para><emphasis>Per-unit recompilation</emphasis>: The user must only
422 recompile the translation units where checking should occur,
423 regardless of where debuggable standard containers are
424 used. This has also been dubbed "<code>-g</code> mode",
425 because the <code>-g</code> compiler switch works in this way,
426 emitting debugging information at a per--translation-unit
427 granularity. We believe that this level of recompilation is in
428 fact not possible if we intend to supply safe iterators, leave
429 the program semantics unchanged, and not regress in
430 performance under release mode because we cannot associate
431 extra information with an iterator (to form a safe iterator)
432 without either reserving that space in release mode
433 (performance regression) or allocating extra memory associated
434 with each iterator with <code>new</code> (changes the program
435 semantics).</para></listitem>
441 <section xml:id="debug_mode.design.methods" xreflabel="Methods"><info><title>Methods</title></info>
445 <para>This section provides an overall view of the design of the
446 libstdc++ debug mode and details the relationship between design
447 decisions and the stated design goals.</para>
449 <section xml:id="debug_mode.design.methods.wrappers" xreflabel="Method Wrapper"><info><title>The Wrapper Model</title></info>
451 <para>The libstdc++ debug mode uses a wrapper model where the
452 debugging versions of library components (e.g., iterators and
453 containers) form a layer on top of the release versions of the
454 library components. The debugging components first verify that the
455 operation is correct (aborting with a diagnostic if an error is
456 found) and will then forward to the underlying release-mode
457 container that will perform the actual work. This design decision
458 ensures that we cannot regress release-mode performance (because the
459 release-mode containers are left untouched) and partially
460 enables <link linkend="methods.coexistence.link">mixing debug and
461 release code</link> at link time, although that will not be
462 discussed at this time.</para>
464 <para>Two types of wrappers are used in the implementation of the debug
465 mode: container wrappers and iterator wrappers. The two types of
466 wrappers interact to maintain relationships between iterators and
467 their associated containers, which are necessary to detect certain
468 types of standard library usage errors such as dereferencing
469 past-the-end iterators or inserting into a container using an
470 iterator from a different container.</para>
472 <section xml:id="debug_mode.design.methods.safe_iter" xreflabel="Method Safe Iter"><info><title>Safe Iterators</title></info>
474 <para>Iterator wrappers provide a debugging layer over any iterator that
475 is attached to a particular container, and will manage the
476 information detailing the iterator's state (singular,
477 dereferenceable, etc.) and tracking the container to which the
478 iterator is attached. Because iterators have a well-defined, common
479 interface the iterator wrapper is implemented with the iterator
480 adaptor class template <code>__gnu_debug::_Safe_iterator</code>,
481 which takes two template parameters:</para>
484 <listitem><para><code>Iterator</code>: The underlying iterator type, which must
485 be either the <code>iterator</code> or <code>const_iterator</code>
486 typedef from the sequence type this iterator can reference.</para></listitem>
488 <listitem><para><code>Sequence</code>: The type of sequence that this iterator
489 references. This sequence must be a safe sequence (discussed below)
490 whose <code>iterator</code> or <code>const_iterator</code> typedef
491 is the type of the safe iterator.</para></listitem>
495 <section xml:id="debug_mode.design.methods.safe_seq" xreflabel="Method Safe Seq"><info><title>Safe Sequences (Containers)</title></info>
498 <para>Container wrappers provide a debugging layer over a particular
499 container type. Because containers vary greatly in the member
500 functions they support and the semantics of those member functions
501 (especially in the area of iterator invalidation), container
502 wrappers are tailored to the container they reference, e.g., the
503 debugging version of <code>std::list</code> duplicates the entire
504 interface of <code>std::list</code>, adding additional semantic
505 checks and then forwarding operations to the
506 real <code>std::list</code> (a public base class of the debugging
507 version) as appropriate. However, all safe containers inherit from
508 the class template <code>__gnu_debug::_Safe_sequence</code>,
509 instantiated with the type of the safe container itself (an instance
510 of the curiously recurring template pattern).</para>
512 <para>The iterators of a container wrapper will be
513 <link linkend="debug_mode.design.methods.safe_iter">safe
514 iterators</link> that reference sequences of this type and wrap the
515 iterators provided by the release-mode base class. The debugging
516 container will use only the safe iterators within its own interface
517 (therefore requiring the user to use safe iterators, although this
518 does not change correct user code) and will communicate with the
519 release-mode base class with only the underlying, unsafe,
520 release-mode iterators that the base class exports.</para>
522 <para> The debugging version of <code>std::list</code> will have the
523 following basic structure:</para>
526 template<typename _Tp, typename _Allocator = allocator<_Tp>
528 public release-list<_Tp, _Allocator>,
529 public __gnu_debug::_Safe_sequence<debug-list<_Tp, _Allocator> >
531 typedef release-list<_Tp, _Allocator> _Base;
532 typedef debug-list<_Tp, _Allocator> _Self;
535 typedef __gnu_debug::_Safe_iterator<typename _Base::iterator, _Self> iterator;
536 typedef __gnu_debug::_Safe_iterator<typename _Base::const_iterator, _Self> const_iterator;
538 // duplicate std::list interface with debugging semantics
544 <section xml:id="debug_mode.design.methods.precond" xreflabel="Precondition check"><info><title>Precondition Checking</title></info>
546 <para>The debug mode operates primarily by checking the preconditions of
547 all standard library operations that it supports. Preconditions that
548 are always checked (regardless of whether or not we are in debug
549 mode) are checked via the <code>__check_xxx</code> macros defined
550 and documented in the source
551 file <code>include/debug/debug.h</code>. Preconditions that may or
552 may not be checked, depending on the debug-mode
553 macro <code>_GLIBCXX_DEBUG</code>, are checked via
554 the <code>__requires_xxx</code> macros defined and documented in the
555 same source file. Preconditions are validated using any additional
556 information available at run-time, e.g., the containers that are
557 associated with a particular iterator, the position of the iterator
558 within those containers, the distance between two iterators that may
559 form a valid range, etc. In the absence of suitable information,
560 e.g., an input iterator that is not a safe iterator, these
561 precondition checks will silently succeed.</para>
563 <para>The majority of precondition checks use the aforementioned macros,
564 which have the secondary benefit of having prewritten debug
565 messages that use information about the current status of the
566 objects involved (e.g., whether an iterator is singular or what
567 sequence it is attached to) along with some static information
568 (e.g., the names of the function parameters corresponding to the
569 objects involved). When not using these macros, the debug mode uses
570 either the debug-mode assertion
571 macro <code>_GLIBCXX_DEBUG_ASSERT</code> , its pedantic
572 cousin <code>_GLIBCXX_DEBUG_PEDASSERT</code>, or the assertion
573 check macro that supports more advance formulation of error
574 messages, <code>_GLIBCXX_DEBUG_VERIFY</code>. These macros are
575 documented more thoroughly in the debug mode source code.</para>
578 <section xml:id="debug_mode.design.methods.coexistence" xreflabel="Coexistence"><info><title>Release- and debug-mode coexistence</title></info>
580 <para>The libstdc++ debug mode is the first debug mode we know of that
581 is able to provide the "Per-use recompilation" (4) guarantee, that
582 allows release-compiled and debug-compiled code to be linked and
583 executed together without causing unpredictable behavior. This
584 guarantee minimizes the recompilation that users are required to
585 perform, shortening the detect-compile-debug bug hunting cycle
586 and making the debug mode easier to incorporate into development
587 environments by minimizing dependencies.</para>
589 <para>Achieving link- and run-time coexistence is not a trivial
590 implementation task. To achieve this goal we required a small
591 extension to the GNU C++ compiler (since incorporated into the C++11 language specification, described in the GCC Manual for the C++ language as
592 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/gcc/Namespace-Association.html#Namespace-Association">namespace
593 association</link>), and a complex organization of debug- and
594 release-modes. The end result is that we have achieved per-use
595 recompilation but have had to give up some checking of the
596 <code>std::basic_string</code> class template (namely, safe
600 <section xml:id="methods.coexistence.compile" xreflabel="Compile"><info><title>Compile-time coexistence of release- and debug-mode components</title></info>
603 <para>Both the release-mode components and the debug-mode
604 components need to exist within a single translation unit so that
605 the debug versions can wrap the release versions. However, only one
606 of these components should be user-visible at any particular
607 time with the standard name, e.g., <code>std::list</code>. </para>
609 <para>In release mode, we define only the release-mode version of the
610 component with its standard name and do not include the debugging
611 component at all. The release mode version is defined within the
612 namespace <code>std</code>. Minus the namespace associations, this
613 method leaves the behavior of release mode completely unchanged from
614 its behavior prior to the introduction of the libstdc++ debug
615 mode. Here's an example of what this ends up looking like, in
621 template<typename _Tp, typename _Alloc = allocator<_Tp> >
629 <para>In debug mode we include the release-mode container (which is now
630 defined in the namespace <code>__cxx1998</code>) and also the
631 debug-mode container. The debug-mode container is defined within the
632 namespace <code>__debug</code>, which is associated with namespace
633 <code>std</code> via the C++11 namespace association language feature. This
634 method allows the debug and release versions of the same component to
635 coexist at compile-time and link-time without causing an unreasonable
636 maintenance burden, while minimizing confusion. Again, this boils down
637 to C++ code as follows:</para>
644 template<typename _Tp, typename _Alloc = allocator<_Tp> >
649 } // namespace __gnu_norm
653 template<typename _Tp, typename _Alloc = allocator<_Tp> >
655 : public __cxx1998::list<_Tp, _Alloc>,
656 public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> >
660 } // namespace __cxx1998
662 // namespace __debug __attribute__ ((strong));
663 inline namespace __debug { }
668 <section xml:id="methods.coexistence.link" xreflabel="Link"><info><title>Link- and run-time coexistence of release- and
669 debug-mode components</title></info>
672 <para>Because each component has a distinct and separate release and
673 debug implementation, there is no issue with link-time
674 coexistence: the separate namespaces result in different mangled
675 names, and thus unique linkage.</para>
677 <para>However, components that are defined and used within the C++
678 standard library itself face additional constraints. For instance,
679 some of the member functions of <code> std::moneypunct</code> return
680 <code>std::basic_string</code>. Normally, this is not a problem, but
681 with a mixed mode standard library that could be using either
682 debug-mode or release-mode <code> basic_string</code> objects, things
683 get more complicated. As the return value of a function is not
684 encoded into the mangled name, there is no way to specify a
685 release-mode or a debug-mode string. In practice, this results in
686 runtime errors. A simplified example of this problem is as follows.
689 <para> Take this translation unit, compiled in debug-mode: </para>
692 #include <string>
694 std::string test02();
708 <para> ... and linked to this translation unit, compiled in release mode:</para>
711 #include <string>
716 return std::string("toast");
720 <para> For this reason we cannot easily provide safe iterators for
721 the <code>std::basic_string</code> class template, as it is present
722 throughout the C++ standard library. For instance, locale facets
723 define typedefs that include <code>basic_string</code>: in a mixed
724 debug/release program, should that typedef be based on the
725 debug-mode <code>basic_string</code> or the
726 release-mode <code>basic_string</code>? While the answer could be
727 "both", and the difference hidden via renaming a la the
728 debug/release containers, we must note two things about locale
731 <orderedlist inheritnum="ignore" continuation="restarts">
732 <listitem><para>They exist as shared state: one can create a facet in one
733 translation unit and access the facet via the same type name in a
734 different translation unit. This means that we cannot have two
735 different versions of locale facets, because the types would not be
736 the same across debug/release-mode translation unit barriers.</para></listitem>
738 <listitem><para>They have virtual functions returning strings: these functions
739 mangle in the same way regardless of the mangling of their return
740 types (see above), and their precise signatures can be relied upon
741 by users because they may be overridden in derived classes.</para></listitem>
744 <para>With the design of libstdc++ debug mode, we cannot effectively hide
745 the differences between debug and release-mode strings from the
746 user. Failure to hide the differences may result in unpredictable
747 behavior, and for this reason we have opted to only
748 perform <code>basic_string</code> changes that do not require ABI
749 changes. The effect on users is expected to be minimal, as there are
750 simple alternatives (e.g., <code>__gnu_debug::basic_string</code>),
751 and the usability benefit we gain from the ability to mix debug- and
752 release-compiled translation units is enormous.</para>
755 <section xml:id="methods.coexistence.alt" xreflabel="Alternatives"><info><title>Alternatives for Coexistence</title></info>
758 <para>The coexistence scheme above was chosen over many alternatives,
759 including language-only solutions and solutions that also required
760 extensions to the C++ front end. The following is a partial list of
761 solutions, with justifications for our rejection of each.</para>
764 <listitem><para><emphasis>Completely separate debug/release libraries</emphasis>: This is by
765 far the simplest implementation option, where we do not allow any
766 coexistence of debug- and release-compiled translation units in a
767 program. This solution has an extreme negative affect on usability,
768 because it is quite likely that some libraries an application
769 depends on cannot be recompiled easily. This would not meet
770 our <emphasis>usability</emphasis> or <emphasis>minimize recompilation</emphasis> criteria
771 well.</para></listitem>
773 <listitem><para><emphasis>Add a <code>Debug</code> boolean template parameter</emphasis>:
774 Partial specialization could be used to select the debug
775 implementation when <code>Debug == true</code>, and the state
776 of <code>_GLIBCXX_DEBUG</code> could decide whether the
777 default <code>Debug</code> argument is <code>true</code>
778 or <code>false</code>. This option would break conformance with the
779 C++ standard in both debug <emphasis>and</emphasis> release modes. This would
780 not meet our <emphasis>correctness</emphasis> criteria. </para></listitem>
782 <listitem><para><emphasis>Packaging a debug flag in the allocators</emphasis>: We could
783 reuse the <code>Allocator</code> template parameter of containers
784 by adding a sentinel wrapper <code>debug<></code> that
785 signals the user's intention to use debugging, and pick up
786 the <code>debug<></code> allocator wrapper in a partial
787 specialization. However, this has two drawbacks: first, there is a
788 conformance issue because the default allocator would not be the
789 standard-specified <code>std::allocator<T></code>. Secondly
790 (and more importantly), users that specify allocators instead of
791 implicitly using the default allocator would not get debugging
792 containers. Thus this solution fails the <emphasis>correctness</emphasis>
793 criteria.</para></listitem>
795 <listitem><para><emphasis>Define debug containers in another namespace, and employ
796 a <code>using</code> declaration (or directive)</emphasis>: This is an
797 enticing option, because it would eliminate the need for
798 the <code>link_name</code> extension by aliasing the
799 templates. However, there is no true template aliasing mechanism
800 in C++, because both <code>using</code> directives and using
801 declarations disallow specialization. This method fails
802 the <emphasis>correctness</emphasis> criteria.</para></listitem>
804 <listitem><para><emphasis> Use implementation-specific properties of anonymous
805 namespaces. </emphasis>
806 See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00004.html"> this post
808 This method fails the <emphasis>correctness</emphasis> criteria.</para></listitem>
810 <listitem><para><emphasis>Extension: allow reopening on namespaces</emphasis>: This would
811 allow the debug mode to effectively alias the
812 namespace <code>std</code> to an internal namespace, such
813 as <code>__gnu_std_debug</code>, so that it is completely
814 separate from the release-mode <code>std</code> namespace. While
815 this will solve some renaming problems and ensure that
816 debug- and release-compiled code cannot be mixed unsafely, it ensures that
817 debug- and release-compiled code cannot be mixed at all. For
818 instance, the program would have two <code>std::cout</code>
819 objects! This solution would fails the <emphasis>minimize
820 recompilation</emphasis> requirement, because we would only be able to
821 support option (1) or (2).</para></listitem>
823 <listitem><para><emphasis>Extension: use link name</emphasis>: This option involves
824 complicated re-naming between debug-mode and release-mode
825 components at compile time, and then a g++ extension called <emphasis>
826 link name </emphasis> to recover the original names at link time. There
827 are two drawbacks to this approach. One, it's very verbose,
828 relying on macro renaming at compile time and several levels of
829 include ordering. Two, ODR issues remained with container member
830 functions taking no arguments in mixed-mode settings resulting in
831 equivalent link names, <code> vector::push_back() </code> being
833 See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00177.html">link
834 name</link> </para></listitem>
837 <para>Other options may exist for implementing the debug mode, many of
838 which have probably been considered and others that may still be
839 lurking. This list may be expanded over time to include other
840 options that we could have implemented, but in all cases the full
841 ramifications of the approach (as measured against the design goals
842 for a libstdc++ debug mode) should be considered first. The DejaGNU
843 testsuite includes some testcases that check for known problems with
844 some solutions (e.g., the <code>using</code> declaration solution
845 that breaks user specialization), and additional testcases will be
846 added as we are able to identify other typical problem cases. These
847 test cases will serve as a benchmark by which we can compare debug
848 mode implementations.</para>
853 <section xml:id="debug_mode.design.other" xreflabel="Other"><info><title>Other Implementations</title></info>
857 <para> There are several existing implementations of debug modes for C++
858 standard library implementations, although none of them directly
859 supports debugging for programs using libstdc++. The existing
860 implementations include:</para>
862 <listitem><para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.cs.sjsu.edu/faculty/horstman/safestl.html">SafeSTL</link>:
863 SafeSTL was the original debugging version of the Standard Template
864 Library (STL), implemented by Cay S. Horstmann on top of the
865 Hewlett-Packard STL. Though it inspired much work in this area, it
866 has not been kept up-to-date for use with modern compilers or C++
867 standard library implementations.</para></listitem>
869 <listitem><para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.stlport.org/">STLport</link>: STLport is a free
870 implementation of the C++ standard library derived from the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.sgi.com/tech/stl/">SGI implementation</link>, and
871 ported to many other platforms. It includes a debug mode that uses a
872 wrapper model (that in some ways inspired the libstdc++ debug mode
873 design), although at the time of this writing the debug mode is
874 somewhat incomplete and meets only the "Full user recompilation" (2)
875 recompilation guarantee by requiring the user to link against a
876 different library in debug mode vs. release mode.</para></listitem>
878 <listitem><para>Metrowerks CodeWarrior: The C++ standard library
879 that ships with Metrowerks CodeWarrior includes a debug mode. It is
880 a full debug-mode implementation (including debugging for
881 CodeWarrior extensions) and is easy to use, although it meets only
882 the "Full recompilation" (1) recompilation
883 guarantee.</para></listitem>