548 lines
28 KiB
HTML
548 lines
28 KiB
HTML
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<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta name="AUTHOR" content="dgregor@apple.com (Doug Gregor)" />
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<meta name="KEYWORDS" content="libstdc++, libstdc++-v3, GCC, 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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<link rel="StyleSheet" href="lib3styles.css" />
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</head>
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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 = std::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
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is able to provide the "Per-use recompilation" (4) guarantee, that
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allows release-compiled and debug-compiled code to be linked and
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executed together without causing unpredictable behavior. This
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guarantee minimizes the recompilation that users are required to
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perform, shortening the detect-compile-debug bughunting cycle
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and making the debug mode easier to incorporate into development
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environments by minimizing dependencies.</p>
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<p>Achieving link- and run-time coexistence is not a trivial
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implementation task. To achieve this goal we required a small
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extension to the GNU C++ compiler (described in the section on
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<a href="#mixing">link- and run-time coexistence</a>) and complex
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organization of debug- and release-modes. The end result is that we
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have achieved per-use recompilation but have had to give up some
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checking of the <code>std::basic_string</code> class template
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(namely, safe iterators).
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<h4><a name="compile_coexistence">Compile-time coexistence of release- and
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debug-mode components</a></h4>
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<p>Both the release-mode components and the debug-mode
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components need to exist within a single translation unit so that
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|
|
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>. 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 (except, perhaps, in <code>__gnu_debug</code>, if
|
||
|
|
requested via the separate debugging headers). This method leaves the
|
||
|
|
behavior of release mode completely unchanged from its behavior
|
||
|
|
prior to the introduction of the libstdc++ debug mode.</p>
|
||
|
|
|
||
|
|
<p>In debug mode we include the release-mode container into its
|
||
|
|
natural namespace but perform renaming to an implementation-defined
|
||
|
|
name using preprocessor macros. Thus the
|
||
|
|
release-mode <code>std::list</code> will be renamed
|
||
|
|
to <code>std::_Release_list</code> during debug mode, and we will
|
||
|
|
automatically include the debugging version with the
|
||
|
|
name <code>std::list</code> for users to reference. This method
|
||
|
|
allows the debug- and release-mode versions of the same component to
|
||
|
|
coexist at compile-time without causing an unreasonable maintenance
|
||
|
|
burden.</p>
|
||
|
|
|
||
|
|
<h4><a name="mixing">Link- and run-time coexistence of release- and
|
||
|
|
debug-mode components</a></h4>
|
||
|
|
<p>There is a problem with the simple compile-time coexistence
|
||
|
|
mechanism: if a user compiles some modules with release mode and
|
||
|
|
some modules with debug mode, the debuggable components will differ
|
||
|
|
in different translation units, violating the C++ One Definition
|
||
|
|
Rule (ODR). This violation will likely be detected at link time,
|
||
|
|
because the sizes of debug-mode containers will differ from the
|
||
|
|
sizes of release-mode containers, although in some cases (such as
|
||
|
|
dynamic linking) the error may be detected much later (or not at
|
||
|
|
all!).</p>
|
||
|
|
|
||
|
|
<p>Unfortunately, it is not possible to avoid violating the ODR with
|
||
|
|
most debug mode designs (see the section on <a
|
||
|
|
href="#coexistence_alt">alternatives for coexistence</a>), so the
|
||
|
|
philosophy of the libstdc++ debug mode is to acknowledge that there
|
||
|
|
is an unavoidable ODR violation in this case but to ensure that the
|
||
|
|
ODR violation does not affect execution. To accomplish this, the
|
||
|
|
libstdc++ debug mode uses the aforementioned preprocessor renaming
|
||
|
|
scheme but includes an additional renaming scheme that happens at
|
||
|
|
compile-time that essentially reverses the preprocessor
|
||
|
|
renaming <em>from the linker's point of view</em>. Thus, in debug
|
||
|
|
mode, the release-mode <code>list</code> container is
|
||
|
|
named <code>std::_Release_list</code> but will be mangled with the
|
||
|
|
name <code>std::list</code> (as it was in release mode). Similarly,
|
||
|
|
the debug-mode <code>list</code> is named <code>std::list</code>
|
||
|
|
(in debug mode) but will be mangled
|
||
|
|
as <code>std::_Debug_list</code>. Thus the
|
||
|
|
release-mode <code>list</code> always compiles down to code that
|
||
|
|
uses the name <code>std::list</code>, and the
|
||
|
|
debug-mode <code>list</code> always compiles down to code that uses
|
||
|
|
the name <code>std::_Debug_list</code>, independent of the use of
|
||
|
|
debug mode. This has several positive effects:</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>No linking conflicts between debug/release objects: because the
|
||
|
|
names of the debug- and release-mode containers are different in the
|
||
|
|
compiled object files, there are no link-time conflicts between the
|
||
|
|
two.</li>
|
||
|
|
|
||
|
|
<li>Release-mode code is shared: the release-mode code can be shared
|
||
|
|
within a program, even with it is compiled partly in release-mode
|
||
|
|
and partly in debug-mode, because the release-mode code is unchanged
|
||
|
|
in name and function. This can decrease the size of mixed
|
||
|
|
debug/release binaries.</li>
|
||
|
|
|
||
|
|
<li>Able to catch <em>most</em> invalid debug/release combinations:
|
||
|
|
because the names of debug- and release-mode containers are
|
||
|
|
different in the compiled object files, if a debug/release
|
||
|
|
interaction cannot occur (e.g., because a container a translation
|
||
|
|
unit compiled in debug mode is passed to a routine in a translation
|
||
|
|
unit compiled in release mode) the result will be an undefined
|
||
|
|
symbol at link time. The undefined symbol occurs because the mangled
|
||
|
|
name of the definition will contain the release-mode container type
|
||
|
|
and the mangled name of the reference will contain the debug-mode
|
||
|
|
container type. However, we cannot detect these collisions if the
|
||
|
|
only use of the container is in the return type, because the return
|
||
|
|
type is not part of the mangled name of a function.</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>The new <code>link_name</code> class attribute facilities
|
||
|
|
renaming. It may be attached to any class type (or any class
|
||
|
|
template) to override the name of the class used for name
|
||
|
|
mangling. For instance, a class named <code>bar</code> would
|
||
|
|
generally mangle as <code>3bar</code>; if the class has
|
||
|
|
a <code>link_name</code> attribute that specifies the string
|
||
|
|
"wibble", then it would mangle as <code>6wibble</code>.</p>
|
||
|
|
|
||
|
|
<p>Note that although we have hidden the ODR violation, it still
|
||
|
|
exists. 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:</p>
|
||
|
|
|
||
|
|
<ol>
|
||
|
|
<li>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.</li>
|
||
|
|
|
||
|
|
<li>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.
|
||
|
|
</ol>
|
||
|
|
|
||
|
|
<p>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.</p>
|
||
|
|
|
||
|
|
<h4><a name="coexistence_alt">Alternatives for Coexistence</a></h4>
|
||
|
|
<p>The coexistence scheme 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.</p>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li><em>Completely separate debug/release libraries</em>: 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 <b>usability</b> or <b>minimize recompilation</b> criteria
|
||
|
|
well.</li>
|
||
|
|
|
||
|
|
<li><em>Add a <code>Debug</code> boolean template parameter</em>:
|
||
|
|
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 <em>and</em> release modes. This would
|
||
|
|
not meet our <b>correctness</b> criteria. </li>
|
||
|
|
|
||
|
|
<li><em>Packaging a debug flag in the allocators</em>: 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&lr;></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 <b>correctness</b>
|
||
|
|
criteria.</li>
|
||
|
|
|
||
|
|
<li><em>Define debug containers in another namespace, and employ
|
||
|
|
a <code>using</code> declaration (or directive)</em>: 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 <b>correctness</b> criteria.</li>
|
||
|
|
|
||
|
|
<li><em>Extension: allow template aliasing/renaming</em>: This is
|
||
|
|
the runner-up to the <code>link_name</code> solution, eliminated
|
||
|
|
only because it requires more extensive compiler changes
|
||
|
|
than <code>link_name</code>. In this model, we would define the
|
||
|
|
debug containers in a different namespace
|
||
|
|
(e.g., <code>__gnu_debug</code>) and then import them (e.g., with
|
||
|
|
an extended <code>using</code> declaration that aliases templates,
|
||
|
|
such as that of <a
|
||
|
|
href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1449.pdf">template
|
||
|
|
aliases</a> proposal). This solution is workable, and in fact
|
||
|
|
would be desirable in the long run, but requires a sizeable change
|
||
|
|
to the C++ compiler front-end that is not within the scope of
|
||
|
|
this project.</li>
|
||
|
|
|
||
|
|
<li><em>Extension: allow reopening on namespaces</em>: 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 <b>minimize
|
||
|
|
recompilation</b> requirement, because we would only be able to
|
||
|
|
support option (1) or (2).</li>
|
||
|
|
</li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>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.</p>
|
||
|
|
|
||
|
|
<!-- ####################################################### -->
|
||
|
|
|
||
|
|
<hr />
|
||
|
|
<p class="fineprint"><em>
|
||
|
|
See <a href="17_intro/license.html">license.html</a> for copying conditions.
|
||
|
|
Comments and suggestions are welcome, and may be sent to
|
||
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
||
|
|
</em></p>
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|