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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Semantics</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="C++, library, debug" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="debug_mode.html" title="Chapter 17. Debug Mode" /><link rel="prev" href="debug_mode.html" title="Chapter 17. Debug Mode" /><link rel="next" href="debug_mode_using.html" title="Using" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Semantics</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="debug_mode.html">Prev</a> </td><th width="60%" align="center">Chapter 17. Debug Mode</th><td width="20%" align="right"> <a accesskey="n" href="debug_mode_using.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.ext.debug_mode.semantics"></a>Semantics</h2></div></div></div><p>
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</p><p>A program that uses the C++ standard library correctly
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will maintain the same semantics under debug mode as it had with
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the normal (release) library. All functional and exception-handling
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guarantees made by the normal library also hold for the debug mode
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library, with one exception: performance guarantees made by the
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normal library may not hold in the debug mode library. For
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instance, erasing an element in a <code class="code">std::list</code> is a
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constant-time operation in normal library, but in debug mode it is
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linear in the number of iterators that reference that particular
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list. So while your (correct) program won't change its results, it
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is likely to execute more slowly.</p><p>libstdc++ includes many extensions to the C++ standard library. In
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some cases the extensions are obvious, such as the hashed
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associative containers, whereas other extensions give predictable
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results to behavior that would otherwise be undefined, such as
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throwing an exception when a <code class="code">std::basic_string</code> is
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constructed from a NULL character pointer. This latter category also
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includes implementation-defined and unspecified semantics, such as
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the growth rate of a vector. Use of these extensions is not
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considered incorrect, so code that relies on them will not be
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rejected by debug mode. However, use of these extensions may affect
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the portability of code to other implementations of the C++ standard
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library, and is therefore somewhat hazardous. For this reason, the
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libstdc++ debug mode offers a "pedantic" mode (similar to
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GCC's <code class="code">-pedantic</code> compiler flag) that attempts to emulate
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the semantics guaranteed by the C++ standard. For
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instance, constructing a <code class="code">std::basic_string</code> with a NULL
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character pointer would result in an exception under normal mode or
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non-pedantic debug mode (this is a libstdc++ extension), whereas
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under pedantic debug mode libstdc++ would signal an error. To enable
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the pedantic debug mode, compile your program with
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both <code class="code">-D_GLIBCXX_DEBUG</code>
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and <code class="code">-D_GLIBCXX_DEBUG_PEDANTIC</code> .
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(N.B. In GCC 3.4.x and 4.0.0, due to a bug,
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<code class="code">-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
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been fixed in GCC 4.0.1 and later versions.) </p><p>The following library components provide extra debugging
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capabilities in debug mode:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><code class="code">std::basic_string</code> (no safe iterators and see note below)</p></li><li class="listitem"><p><code class="code">std::bitset</code></p></li><li class="listitem"><p><code class="code">std::deque</code></p></li><li class="listitem"><p><code class="code">std::list</code></p></li><li class="listitem"><p><code class="code">std::map</code></p></li><li class="listitem"><p><code class="code">std::multimap</code></p></li><li class="listitem"><p><code class="code">std::multiset</code></p></li><li class="listitem"><p><code class="code">std::set</code></p></li><li class="listitem"><p><code class="code">std::vector</code></p></li><li class="listitem"><p><code class="code">std::unordered_map</code></p></li><li class="listitem"><p><code class="code">std::unordered_multimap</code></p></li><li class="listitem"><p><code class="code">std::unordered_set</code></p></li><li class="listitem"><p><code class="code">std::unordered_multiset</code></p></li></ul></div><p>N.B. although there are precondition checks for some string operations,
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e.g. <code class="code">operator[]</code>,
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they will not always be run when using the <code class="code">char</code> and
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<code class="code">wchar_t</code> specialisations (<code class="code">std::string</code> and
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<code class="code">std::wstring</code>). This is because libstdc++ uses GCC's
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<code class="code">extern template</code> extension to provide explicit instantiations
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of <code class="code">std::string</code> and <code class="code">std::wstring</code>, and those
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explicit instantiations don't include the debug-mode checks. If the
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containing functions are inlined then the checks will run, so compiling
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with <code class="code">-O1</code> might be enough to enable them. Alternatively
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<code class="code">-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
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of the explicit instantiations and cause the functions to be instantiated
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with the debug-mode checks included, but this is unsupported and not
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guaranteed to work. For full debug-mode support you can use the
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<code class="code">__gnu_debug::basic_string</code> debugging container directly,
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which always works correctly.
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</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="debug_mode.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="debug_mode.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug_mode_using.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 17. Debug Mode </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Using</td></tr></table></div></body></html> |