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<meta name="AUTHOR" content="gregod@cs.rpi.edu (Doug Gregor)" />
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<meta name="KEYWORDS" content="C++, GCC, libstdc++, g++, debug" />
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<meta name="DESCRIPTION" content="Design of the libstdc++ debug mode." />
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<meta name="GENERATOR" content="vi and eight fingers" />
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<title>Design of the libstdc++ debug mode</title>
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<body>
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<h1 class="centered"><a name="top">Design of the libstdc++ debug mode</a></h1>
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<p class="fineprint"><em>
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The latest version of this document is always available at
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<a href="http://gcc.gnu.org/onlinedocs/libstdc++/debug_mode.html">
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http://gcc.gnu.org/onlinedocs/libstdc++/debug_mode.html</a>.
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</em></p>
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<p><em>
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To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
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</em></p>
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<!-- ####################################################### -->
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<hr />
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<h1>Debug mode design</h1>
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<p> The libstdc++ debug mode replaces unsafe (but efficient) standard
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containers and iterators with semantically equivalent safe standard
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containers and iterators to aid in debugging user programs. The
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following goals directed the design of the libstdc++ debug mode:</p>
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<ul>
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<li><b>Correctness</b>: the libstdc++ debug mode must not change
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the semantics of the standard library for all cases specified in
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the ANSI/ISO C++ standard. The essence of this constraint is that
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any valid C++ program should behave in the same manner regardless
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of whether it is compiled with debug mode or release mode. In
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particular, entities that are defined in namespace std in release
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mode should remain defined in namespace std in debug mode, so that
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legal specializations of namespace std entities will remain
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valid. A program that is not valid C++ (e.g., invokes undefined
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behavior) is not required to behave similarly, although the debug
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mode will abort with a diagnostic when it detects undefined
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behavior.</li>
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<li><b>Performance</b>: the additional of the libstdc++ debug mode
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must not affect the performance of the library when it is compiled
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in release mode. Performance of the libstdc++ debug mode is
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secondary (and, in fact, will be worse than the release
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mode).</li>
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<li><b>Usability</b>: the libstdc++ debug mode should be easy to
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use. It should be easily incorporated into the user's development
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environment (e.g., by requiring only a single new compiler switch)
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and should produce reasonable diagnostics when it detects a
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problem with the user program. Usability also involves detection
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of errors when using the debug mode incorrectly, e.g., by linking
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a release-compiled object against a debug-compiled object if in
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fact the resulting program will not run correctly.</li>
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<li><b>Minimize recompilation</b>: While it is expected that
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users recompile at least part of their program to use debug
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mode, the amount of recompilation affects the
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detect-compile-debug turnaround time. This indirectly affects the
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usefulness of the debug mode, because debugging some applications
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may require rebuilding a large amount of code, which may not be
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feasible when the suspect code may be very localized. There are
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several levels of conformance to this requirement, each with its
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own usability and implementation characteristics. In general, the
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higher-numbered conformance levels are more usable (i.e., require
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less recompilation) but are more complicated to implement than
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the lower-numbered conformance levels.
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<ol>
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<li><b>Full recompilation</b>: The user must recompile his or
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her entire application and all C++ libraries it depends on,
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including the C++ standard library that ships with the
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compiler. This must be done even if only a small part of the
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program can use debugging features.</li>
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<li><b>Full user recompilation</b>: The user must recompile
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his or her entire application and all C++ libraries it depends
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on, but not the C++ standard library itself. This must be done
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even if only a small part of the program can use debugging
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features. This can be achieved given a full recompilation
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system by compiling two versions of the standard library when
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the compiler is installed and linking against the appropriate
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one, e.g., a multilibs approach.</li>
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<li><b>Partial recompilation</b>: The user must recompile the
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parts of his or her application and the C++ libraries it
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depends on that will use the debugging facilities
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directly. This means that any code that uses the debuggable
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standard containers would need to be recompiled, but code
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that does not use them (but may, for instance, use IOStreams)
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would not have to be recompiled.</li>
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<li><b>Per-use recompilation</b>: The user must recompile the
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parts of his or her application and the C++ libraries it
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depends on where debugging should occur, and any other code
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that interacts with those containers. This means that a set of
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translation units that accesses a particular standard
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container instance may either be compiled in release mode (no
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checking) or debug mode (full checking), but must all be
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compiled in the same way; a translation unit that does not see
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that standard container instance need not be recompiled. This
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also means that a translation unit <em>A</em> that contains a
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particular instantiation
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(say, <code>std::vector<int></code>) compiled in release
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mode can be linked against a translation unit <em>B</em> that
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contains the same instantiation compiled in debug mode (a
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feature not present with partial recompilation). While this
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behavior is technically a violation of the One Definition
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Rule, this ability tends to be very important in
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practice. The libstdc++ debug mode supports this level of
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recompilation. </li>
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<li><b>Per-unit recompilation</b>: The user must only
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recompile the translation units where checking should occur,
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regardless of where debuggable standard containers are
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used. This has also been dubbed "<code>-g</code> mode",
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because the <code>-g</code> compiler switch works in this way,
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emitting debugging information at a per--translation-unit
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granularity. We believe that this level of recompilation is in
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fact not possible if we intend to supply safe iterators, leave
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the program semantics unchanged, and not regress in
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performance under release mode because we cannot associate
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extra information with an iterator (to form a safe iterator)
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without either reserving that space in release mode
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(performance regression) or allocating extra memory associated
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with each iterator with <code>new</code> (changes the program
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semantics).</li>
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</ol>
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</li>
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</ul>
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<h2><a name="other">Other implementations</a></h2>
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<p> There are several existing implementations of debug modes for C++
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standard library implementations, although none of them directly
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supports debugging for programs using libstdc++. The existing
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implementations include:</p>
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<ul>
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<li><a
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href="http://www.mathcs.sjsu.edu/faculty/horstman/safestl.html">SafeSTL</a>:
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SafeSTL was the original debugging version of the Standard Template
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Library (STL), implemented by Cay S. Horstmann on top of the
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Hewlett-Packard STL. Though it inspired much work in this area, it
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has not been kept up-to-date for use with modern compilers or C++
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standard library implementations.</li>
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<li><a href="http://www.stlport.org/">STLport</a>: STLport is a free
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implementation of the C++ standard library derived from the <a
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href="http://www.sgi.com/tech/stl/">SGI implementation</a>, and
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ported to many other platforms. It includes a debug mode that uses a
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wrapper model (that in some way inspired the libstdc++ debug mode
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design), although at the time of this writing the debug mode is
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somewhat incomplete and meets only the "Full user recompilation" (2)
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recompilation guarantee by requiring the user to link against a
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different library in debug mode vs. release mode.</li>
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<li><a href="http://www.metrowerks.com/mw/default.htm">Metrowerks
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CodeWarrior</a>: The C++ standard library that ships with Metrowerks
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CodeWarrior includes a debug mode. It is a full debug-mode
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implementation (including debugging for CodeWarrior extensions) and
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is easy to use, although it meets only the "Full recompilation" (1)
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recompilation guarantee.</li>
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</ul>
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<h2><a name="design">Debug mode design methodology</a></h2>
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<p>This section provides an overall view of the design of the
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libstdc++ debug mode and details the relationship between design
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decisions and the stated design goals.</p>
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<h3><a name="wrappers">The wrapper model</a></h3>
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<p>The libstdc++ debug mode uses a wrapper model where the debugging
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versions of library components (e.g., iterators and containers) form
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a layer on top of the release versions of the library
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components. The debugging components first verify that the operation
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is correct (aborting with a diagnostic if an error is found) and
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will then forward to the underlying release-mode container that will
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perform the actual work. This design decision ensures that we cannot
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regress release-mode performance (because the release-mode
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containers are left untouched) and partially enables <a
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href="#mixing">mixing debug and release code</a> at link time,
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although that will not be discussed at this time.</p>
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<p>Two types of wrappers are used in the implementation of the debug
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mode: container wrappers and iterator wrappers. The two types of
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wrappers interact to maintain relationships between iterators and
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their associated containers, which are necessary to detect certain
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types of standard library usage errors such as dereferencing
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past-the-end iterators or inserting into a container using an
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iterator from a different container.</p>
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<h4><a name="safe_iterator">Safe iterators</a></h4>
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<p>Iterator wrappers provide a debugging layer over any iterator that
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is attached to a particular container, and will manage the
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information detailing the iterator's state (singular,
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dereferenceable, etc.) and tracking the container to which the
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iterator is attached. Because iterators have a well-defined, common
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interface the iterator wrapper is implemented with the iterator
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adaptor class template <code>__gnu_debug::_Safe_iterator</code>,
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which takes two template parameters:</p>
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<ul>
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<li><code>Iterator</code>: The underlying iterator type, which must
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be either the <code>iterator</code> or <code>const_iterator</code>
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typedef from the sequence type this iterator can reference.</li>
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<li><code>Sequence</code>: The type of sequence that this iterator
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references. This sequence must be a safe sequence (discussed below)
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whose <code>iterator</code> or <code>const_iterator</code> typedef
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is the type of the safe iterator.</li>
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</ul>
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<h4><a name="safe_sequence">Safe sequences (containers)</a></h4>
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<p>Container wrappers provide a debugging layer over a particular
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container type. Because containers vary greatly in the member
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functions they support and the semantics of those member functions
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(especially in the area of iterator invalidation), container
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wrappers are tailored to the container they reference, e.g., the
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debugging version of <code>std::list</code> duplicates the entire
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interface of <code>std::list</code>, adding additional semantic
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checks and then forwarding operations to the
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real <code>std::list</code> (a public base class of the debugging
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version) as appropriate. However, all safe containers inherit from
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the class template <code>__gnu_debug::_Safe_sequence</code>,
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instantiated with the type of the safe container itself (an instance
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of the curiously recurring template pattern).</p>
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<p>The iterators of a container wrapper will be
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<a href="#safe_iterator">safe iterators</a> that reference sequences
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of this type and wrap the iterators provided by the release-mode
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base class. The debugging container will use only the safe
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iterators within its own interface (therefore requiring the user to
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use safe iterators, although this does not change correct user
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code) and will communicate with the release-mode base class with
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only the underlying, unsafe, release-mode iterators that the base
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class exports.</p>
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<p> The debugging version of <code>std::list</code> will have the
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following basic structure:</p>
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<pre>
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template<typename _Tp, typename _Allocator = allocator<_Tp>
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class debug-list :
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public release-list<_Tp, _Allocator>,
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public __gnu_debug::_Safe_sequence<debug-list<_Tp, _Allocator> >
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{
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typedef release-list<_Tp, _Allocator> _Base;
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typedef debug-list<_Tp, _Allocator> _Self;
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public:
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typedef __gnu_debug::_Safe_iterator<typename _Base::iterator, _Self> iterator;
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typedef __gnu_debug::_Safe_iterator<typename _Base::const_iterator, _Self> const_iterator;
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// duplicate std::list interface with debugging semantics
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};
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</pre>
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<h3><a name="precondition">Precondition checking</a></h3>
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<p>The debug mode operates primarily by checking the preconditions of
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all standard library operations that it supports. Preconditions that
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are always checked (regardless of whether or not we are in debug
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mode) are checked via the <code>__check_xxx</code> macros defined
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and documented in the source
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file <code>include/debug/debug.h</code>. Preconditions that may or
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may not be checked, depending on the debug-mode
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macro <code>_GLIBCXX_DEBUG</code>, are checked via
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the <code>__requires_xxx</code> macros defined and documented in the
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same source file. Preconditions are validated using any additional
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information available at run-time, e.g., the containers that are
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associated with a particular iterator, the position of the iterator
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within those containers, the distance between two iterators that may
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form a valid range, etc. In the absence of suitable information,
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e.g., an input iterator that is not a safe iterator, these
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precondition checks will silently succeed.</p>
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<p>The majority of precondition checks use the aforementioned macros,
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which have the secondary benefit of having prewritten debug
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messages that use information about the current status of the
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objects involved (e.g., whether an iterator is singular or what
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sequence it is attached to) along with some static information
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(e.g., the names of the function parameters corresponding to the
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objects involved). When not using these macros, the debug mode uses
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either the debug-mode assertion
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macro <code>_GLIBCXX_DEBUG_ASSERT</code> , its pedantic
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cousin <code>_GLIBCXX_DEBUG_PEDASSERT</code>, or the assertion
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check macro that supports more advance formulation of error
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messages, <code>_GLIBCXX_DEBUG_VERIFY</code>. These macros are
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documented more thoroughly in the debug mode source code.</p>
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<h3><a name="coexistence">Release- and debug-mode coexistence</a></h3>
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<p>The libstdc++ debug mode is the first debug mode we know of that
|
| 302 |
|
|
is able to provide the "Per-use recompilation" (4) guarantee, that
|
| 303 |
|
|
allows release-compiled and debug-compiled code to be linked and
|
| 304 |
|
|
executed together without causing unpredictable behavior. This
|
| 305 |
|
|
guarantee minimizes the recompilation that users are required to
|
| 306 |
|
|
perform, shortening the detect-compile-debug bughunting cycle
|
| 307 |
|
|
and making the debug mode easier to incorporate into development
|
| 308 |
|
|
environments by minimizing dependencies.</p>
|
| 309 |
|
|
|
| 310 |
|
|
<p>Achieving link- and run-time coexistence is not a trivial
|
| 311 |
|
|
implementation task. To achieve this goal we required a small
|
| 312 |
|
|
extension to the GNU C++ compiler (described in the GCC Manual for
|
| 313 |
|
|
C++ Extensions, see <a
|
| 314 |
|
|
href="http://gcc.gnu.org/onlinedocs/gcc/Strong-Using.html">strong
|
| 315 |
|
|
using</a>), and a complex organization of debug- and
|
| 316 |
|
|
release-modes. The end result is that we have achieved per-use
|
| 317 |
|
|
recompilation but have had to give up some checking of the
|
| 318 |
|
|
<code>std::basic_string</code> class template (namely, safe
|
| 319 |
|
|
iterators).
|
| 320 |
|
|
</p>
|
| 321 |
|
|
|
| 322 |
|
|
<h4><a name="compile_coexistence">Compile-time coexistence of release- and
|
| 323 |
|
|
debug-mode components</a></h4>
|
| 324 |
|
|
<p>Both the release-mode components and the debug-mode
|
| 325 |
|
|
components need to exist within a single translation unit so that
|
| 326 |
|
|
the debug versions can wrap the release versions. However, only one
|
| 327 |
|
|
of these components should be user-visible at any particular
|
| 328 |
|
|
time with the standard name, e.g., <code>std::list</code>. </p>
|
| 329 |
|
|
|
| 330 |
|
|
<p>In release mode, we define only the release-mode version of the
|
| 331 |
|
|
component with its standard name and do not include the debugging
|
| 332 |
|
|
component at all. The release mode version is defined within the
|
| 333 |
|
|
namespace <code>__gnu_norm</code>, and then associated with namespace
|
| 334 |
|
|
<code>std</code> via a "strong using" directive. Minus the
|
| 335 |
|
|
namespace associations, this method leaves the behavior of release
|
| 336 |
|
|
mode completely unchanged from its behavior prior to the
|
| 337 |
|
|
introduction of the libstdc++ debug mode. Here's an example of what
|
| 338 |
|
|
this ends up looking like, in C++.</p>
|
| 339 |
|
|
|
| 340 |
|
|
<pre>
|
| 341 |
|
|
namespace __gnu_norm
|
| 342 |
|
|
{
|
| 343 |
|
|
using namespace std;
|
| 344 |
|
|
|
| 345 |
|
|
template<typename _Tp, typename _Alloc = allocator<_Tp> >
|
| 346 |
|
|
class list
|
| 347 |
|
|
{
|
| 348 |
|
|
// ...
|
| 349 |
|
|
};
|
| 350 |
|
|
} // namespace __gnu_norm
|
| 351 |
|
|
|
| 352 |
|
|
namespace std
|
| 353 |
|
|
{
|
| 354 |
|
|
using namespace __gnu_norm __attribute__ ((strong));
|
| 355 |
|
|
}
|
| 356 |
|
|
</pre>
|
| 357 |
|
|
|
| 358 |
|
|
<p>In debug mode we include the release-mode container and also the
|
| 359 |
|
|
debug-mode container. The release mode version is defined exactly as
|
| 360 |
|
|
before, and the debug-mode container is defined within the namespace
|
| 361 |
|
|
<code>__gnu_debug</code>, which is associated with namespace
|
| 362 |
|
|
<code>std</code> via a "strong using" directive. This method allows
|
| 363 |
|
|
the debug- and release-mode versions of the same component to coexist
|
| 364 |
|
|
at compile-time without causing an unreasonable maintenance burden,
|
| 365 |
|
|
while minimizing confusion. Again, this boils down to C++ code as
|
| 366 |
|
|
follows:</p>
|
| 367 |
|
|
|
| 368 |
|
|
<pre>
|
| 369 |
|
|
namespace __gnu_norm
|
| 370 |
|
|
{
|
| 371 |
|
|
using namespace std;
|
| 372 |
|
|
|
| 373 |
|
|
template<typename _Tp, typename _Alloc = allocator<_Tp> >
|
| 374 |
|
|
class list
|
| 375 |
|
|
{
|
| 376 |
|
|
// ...
|
| 377 |
|
|
};
|
| 378 |
|
|
} // namespace __gnu_norm
|
| 379 |
|
|
|
| 380 |
|
|
namespace __gnu_debug
|
| 381 |
|
|
{
|
| 382 |
|
|
using namespace std;
|
| 383 |
|
|
|
| 384 |
|
|
template<typename _Tp, typename _Alloc = allocator<_Tp> >
|
| 385 |
|
|
class list
|
| 386 |
|
|
: public __gnu_norm::list<_Tp, _Alloc>,
|
| 387 |
|
|
public __gnu_debug::_Safe_sequence<list<_Tp, _Alloc> >
|
| 388 |
|
|
{
|
| 389 |
|
|
// ...
|
| 390 |
|
|
};
|
| 391 |
|
|
} // namespace __gnu_norm
|
| 392 |
|
|
|
| 393 |
|
|
namespace std
|
| 394 |
|
|
{
|
| 395 |
|
|
using namespace __gnu_debug __attribute__ ((strong));
|
| 396 |
|
|
}
|
| 397 |
|
|
</pre>
|
| 398 |
|
|
|
| 399 |
|
|
<h4><a name="mixing">Link- and run-time coexistence of release- and
|
| 400 |
|
|
debug-mode components</a></h4>
|
| 401 |
|
|
|
| 402 |
|
|
<p>Because each component has a distinct and separate release and
|
| 403 |
|
|
debug implementation, there are are no issues with link-time
|
| 404 |
|
|
coexistence: the separate namespaces result in different mangled
|
| 405 |
|
|
names, and thus unique linkage.</p>
|
| 406 |
|
|
|
| 407 |
|
|
<p>However, components that are defined and used within the C++
|
| 408 |
|
|
standard library itself face additional constraints. For instance,
|
| 409 |
|
|
some of the member functions of <code> std::moneypunct</code> return
|
| 410 |
|
|
<code>std::basic_string</code>. Normally, this is not a problem, but
|
| 411 |
|
|
with a mixed mode standard library that could be using either
|
| 412 |
|
|
debug-mode or release-mode <code> basic_string</code> objects, things
|
| 413 |
|
|
get more complicated. As the return value of a function is not
|
| 414 |
|
|
encoded into the mangled name, there is no way to specify a
|
| 415 |
|
|
release-mode or a debug-mode string. In practice, this results in
|
| 416 |
|
|
runtime errors. A simplified example of this problem is as follows.
|
| 417 |
|
|
</p>
|
| 418 |
|
|
|
| 419 |
|
|
<p> Take this translation unit, compiled in debug-mode: </p>
|
| 420 |
|
|
<pre>
|
| 421 |
|
|
// -D_GLIBCXX_DEBUG
|
| 422 |
|
|
#include <string>
|
| 423 |
|
|
|
| 424 |
|
|
std::string test02();
|
| 425 |
|
|
|
| 426 |
|
|
std::string test01()
|
| 427 |
|
|
{
|
| 428 |
|
|
return test02();
|
| 429 |
|
|
}
|
| 430 |
|
|
|
| 431 |
|
|
int main()
|
| 432 |
|
|
{
|
| 433 |
|
|
test01();
|
| 434 |
|
|
return 0;
|
| 435 |
|
|
}
|
| 436 |
|
|
</pre>
|
| 437 |
|
|
|
| 438 |
|
|
<p> ... and linked to this translation unit, compiled in release mode:</p>
|
| 439 |
|
|
|
| 440 |
|
|
<pre>
|
| 441 |
|
|
#include <string>
|
| 442 |
|
|
|
| 443 |
|
|
std::string
|
| 444 |
|
|
test02()
|
| 445 |
|
|
{
|
| 446 |
|
|
return std::string("toast");
|
| 447 |
|
|
}
|
| 448 |
|
|
</pre>
|
| 449 |
|
|
|
| 450 |
|
|
<p> For this reason we cannot easily provide safe iterators for
|
| 451 |
|
|
the <code>std::basic_string</code> class template, as it is present
|
| 452 |
|
|
throughout the C++ standard library. For instance, locale facets
|
| 453 |
|
|
define typedefs that include <code>basic_string</code>: in a mixed
|
| 454 |
|
|
debug/release program, should that typedef be based on the
|
| 455 |
|
|
debug-mode <code>basic_string</code> or the
|
| 456 |
|
|
release-mode <code>basic_string</code>? While the answer could be
|
| 457 |
|
|
"both", and the difference hidden via renaming a la the
|
| 458 |
|
|
debug/release containers, we must note two things about locale
|
| 459 |
|
|
facets:</p>
|
| 460 |
|
|
|
| 461 |
|
|
<ol>
|
| 462 |
|
|
<li>They exist as shared state: one can create a facet in one
|
| 463 |
|
|
translation unit and access the facet via the same type name in a
|
| 464 |
|
|
different translation unit. This means that we cannot have two
|
| 465 |
|
|
different versions of locale facets, because the types would not be
|
| 466 |
|
|
the same across debug/release-mode translation unit barriers.</li>
|
| 467 |
|
|
|
| 468 |
|
|
<li>They have virtual functions returning strings: these functions
|
| 469 |
|
|
mangle in the same way regardless of the mangling of their return
|
| 470 |
|
|
types (see above), and their precise signatures can be relied upon
|
| 471 |
|
|
by users because they may be overridden in derived classes.</li>
|
| 472 |
|
|
</ol>
|
| 473 |
|
|
|
| 474 |
|
|
<p>With the design of libstdc++ debug mode, we cannot effectively hide
|
| 475 |
|
|
the differences between debug and release-mode strings from the
|
| 476 |
|
|
user. Failure to hide the differences may result in unpredictable
|
| 477 |
|
|
behavior, and for this reason we have opted to only
|
| 478 |
|
|
perform <code>basic_string</code> changes that do not require ABI
|
| 479 |
|
|
changes. The effect on users is expected to be minimal, as there are
|
| 480 |
|
|
simple alternatives (e.g., <code>__gnu_debug::basic_string</code>),
|
| 481 |
|
|
and the usability benefit we gain from the ability to mix debug- and
|
| 482 |
|
|
release-compiled translation units is enormous.</p>
|
| 483 |
|
|
|
| 484 |
|
|
<h4><a name="coexistence_alt">Alternatives for Coexistence</a></h4>
|
| 485 |
|
|
<p>The coexistence scheme above was chosen over many alternatives,
|
| 486 |
|
|
including language-only solutions and solutions that also required
|
| 487 |
|
|
extensions to the C++ front end. The following is a partial list of
|
| 488 |
|
|
solutions, with justifications for our rejection of each.</p>
|
| 489 |
|
|
|
| 490 |
|
|
<ul>
|
| 491 |
|
|
<li><em>Completely separate debug/release libraries</em>: This is by
|
| 492 |
|
|
far the simplest implementation option, where we do not allow any
|
| 493 |
|
|
coexistence of debug- and release-compiled translation units in a
|
| 494 |
|
|
program. This solution has an extreme negative affect on usability,
|
| 495 |
|
|
because it is quite likely that some libraries an application
|
| 496 |
|
|
depends on cannot be recompiled easily. This would not meet
|
| 497 |
|
|
our <b>usability</b> or <b>minimize recompilation</b> criteria
|
| 498 |
|
|
well.</li>
|
| 499 |
|
|
|
| 500 |
|
|
<li><em>Add a <code>Debug</code> boolean template parameter</em>:
|
| 501 |
|
|
Partial specialization could be used to select the debug
|
| 502 |
|
|
implementation when <code>Debug == true</code>, and the state
|
| 503 |
|
|
of <code>_GLIBCXX_DEBUG</code> could decide whether the
|
| 504 |
|
|
default <code>Debug</code> argument is <code>true</code>
|
| 505 |
|
|
or <code>false</code>. This option would break conformance with the
|
| 506 |
|
|
C++ standard in both debug <em>and</em> release modes. This would
|
| 507 |
|
|
not meet our <b>correctness</b> criteria. </li>
|
| 508 |
|
|
|
| 509 |
|
|
<li><em>Packaging a debug flag in the allocators</em>: We could
|
| 510 |
|
|
reuse the <code>Allocator</code> template parameter of containers
|
| 511 |
|
|
by adding a sentinel wrapper <code>debug<></code> that
|
| 512 |
|
|
signals the user's intention to use debugging, and pick up
|
| 513 |
|
|
the <code>debug<></code> allocator wrapper in a partial
|
| 514 |
|
|
specialization. However, this has two drawbacks: first, there is a
|
| 515 |
|
|
conformance issue because the default allocator would not be the
|
| 516 |
|
|
standard-specified <code>std::allocator<T></code>. Secondly
|
| 517 |
|
|
(and more importantly), users that specify allocators instead of
|
| 518 |
|
|
implicitly using the default allocator would not get debugging
|
| 519 |
|
|
containers. Thus this solution fails the <b>correctness</b>
|
| 520 |
|
|
criteria.</li>
|
| 521 |
|
|
|
| 522 |
|
|
<li><em>Define debug containers in another namespace, and employ
|
| 523 |
|
|
a <code>using</code> declaration (or directive)</em>: This is an
|
| 524 |
|
|
enticing option, because it would eliminate the need for
|
| 525 |
|
|
the <code>link_name</code> extension by aliasing the
|
| 526 |
|
|
templates. However, there is no true template aliasing mechanism
|
| 527 |
|
|
is C++, because both <code>using</code> directives and using
|
| 528 |
|
|
declarations disallow specialization. This method fails
|
| 529 |
|
|
the <b>correctness</b> criteria.</li>
|
| 530 |
|
|
|
| 531 |
|
|
<li><em> Use implementation-specific properties of anonymous
|
| 532 |
|
|
namespaces. </em>
|
| 533 |
|
|
See <a
|
| 534 |
|
|
href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00004.html"> this post
|
| 535 |
|
|
</a>
|
| 536 |
|
|
This method fails the <b>correctness</b> criteria.</li>
|
| 537 |
|
|
|
| 538 |
|
|
<li><em>Extension: allow reopening on namespaces</em>: This would
|
| 539 |
|
|
allow the debug mode to effectively alias the
|
| 540 |
|
|
namespace <code>std</code> to an internal namespace, such
|
| 541 |
|
|
as <code>__gnu_std_debug</code>, so that it is completely
|
| 542 |
|
|
separate from the release-mode <code>std</code> namespace. While
|
| 543 |
|
|
this will solve some renaming problems and ensure that
|
| 544 |
|
|
debug- and release-compiled code cannot be mixed unsafely, it ensures that
|
| 545 |
|
|
debug- and release-compiled code cannot be mixed at all. For
|
| 546 |
|
|
instance, the program would have two <code>std::cout</code>
|
| 547 |
|
|
objects! This solution would fails the <b>minimize
|
| 548 |
|
|
recompilation</b> requirement, because we would only be able to
|
| 549 |
|
|
support option (1) or (2).</li>
|
| 550 |
|
|
|
| 551 |
|
|
<li><em>Extension: use link name</em>: This option involves
|
| 552 |
|
|
complicated re-naming between debug-mode and release-mode
|
| 553 |
|
|
components at compile time, and then a g++ extension called <em>
|
| 554 |
|
|
link name </em> to recover the original names at link time. There
|
| 555 |
|
|
are two drawbacks to this approach. One, it's very verbose,
|
| 556 |
|
|
relying on macro renaming at compile time and several levels of
|
| 557 |
|
|
include ordering. Two, ODR issues remained with container member
|
| 558 |
|
|
functions taking no arguments in mixed-mode settings resulting in
|
| 559 |
|
|
equivalent link names, <code> vector::push_back() </code> being
|
| 560 |
|
|
one example.
|
| 561 |
|
|
See <a
|
| 562 |
|
|
href="http://gcc.gnu.org/ml/libstdc++/2003-08/msg00177.html">link
|
| 563 |
|
|
name</a> </li>
|
| 564 |
|
|
</ul>
|
| 565 |
|
|
|
| 566 |
|
|
<p>Other options may exist for implementing the debug mode, many of
|
| 567 |
|
|
which have probably been considered and others that may still be
|
| 568 |
|
|
lurking. This list may be expanded over time to include other
|
| 569 |
|
|
options that we could have implemented, but in all cases the full
|
| 570 |
|
|
ramifications of the approach (as measured against the design goals
|
| 571 |
|
|
for a libstdc++ debug mode) should be considered first. The DejaGNU
|
| 572 |
|
|
testsuite includes some testcases that check for known problems with
|
| 573 |
|
|
some solutions (e.g., the <code>using</code> declaration solution
|
| 574 |
|
|
that breaks user specialization), and additional testcases will be
|
| 575 |
|
|
added as we are able to identify other typical problem cases. These
|
| 576 |
|
|
test cases will serve as a benchmark by which we can compare debug
|
| 577 |
|
|
mode implementations.</p>
|
| 578 |
|
|
|
| 579 |
|
|
<!-- ####################################################### -->
|
| 580 |
|
|
|
| 581 |
|
|
<hr />
|
| 582 |
|
|
<p class="fineprint"><em>
|
| 583 |
|
|
See <a href="17_intro/license.html">license.html</a> for copying conditions.
|
| 584 |
|
|
Comments and suggestions are welcome, and may be sent to
|
| 585 |
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
| 586 |
|
|
</em></p>
|
| 587 |
|
|
|
| 588 |
|
|
|
| 589 |
|
|
</body>
|
| 590 |
|
|
</html>
|
