libstdc++-v3/doc/html/manual/debug_mode_semantics.html

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   3   </p><p>A program that uses the C++ standard library correctly
   4   will maintain the same semantics under debug mode as it had with
   5   the normal (release) library. All functional and exception-handling
   6   guarantees made by the normal library also hold for the debug mode
   7   library, with one exception: performance guarantees made by the
   8   normal library may not hold in the debug mode library. For
   9   instance, erasing an element in a <code class="code">std::list</code> is a
  10   constant-time operation in normal library, but in debug mode it is
  11   linear in the number of iterators that reference that particular
  12   list. So while your (correct) program won't change its results, it
  13   is likely to execute more slowly.</p><p>libstdc++ includes many extensions to the C++ standard library. In
  14   some cases the extensions are obvious, such as the hashed
  15   associative containers, whereas other extensions give predictable
  16   results to behavior that would otherwise be undefined, such as
  17   throwing an exception when a <code class="code">std::basic_string</code> is
  18   constructed from a NULL character pointer. This latter category also
  19   includes implementation-defined and unspecified semantics, such as
  20   the growth rate of a vector. Use of these extensions is not
  21   considered incorrect, so code that relies on them will not be
  22   rejected by debug mode. However, use of these extensions may affect
  23   the portability of code to other implementations of the C++ standard
  24   library, and is therefore somewhat hazardous. For this reason, the
  25   libstdc++ debug mode offers a "pedantic" mode (similar to
  26   GCC's <code class="code">-pedantic</code> compiler flag) that attempts to emulate
  27   the semantics guaranteed by the C++ standard. For
  28   instance, constructing a <code class="code">std::basic_string</code> with a NULL
  29   character pointer would result in an exception under normal mode or
  30   non-pedantic debug mode (this is a libstdc++ extension), whereas
  31   under pedantic debug mode libstdc++ would signal an error. To enable
  32   the pedantic debug mode, compile your program with
  33   both <code class="code">-D_GLIBCXX_DEBUG</code>
  34   and <code class="code">-D_GLIBCXX_DEBUG_PEDANTIC</code> .
  35   (N.B. In GCC 3.4.x and 4.0.0, due to a bug,
  36   <code class="code">-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
  37   been fixed in GCC 4.0.1 and later versions.) </p><p>The following library components provide extra debugging
  38   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,
  39 e.g.  <code class="code">operator[]</code>,
  40 they will not always be run when using the <code class="code">char</code> and
  41 <code class="code">wchar_t</code> specialisations (<code class="code">std::string</code> and
  42 <code class="code">std::wstring</code>).  This is because libstdc++ uses GCC's
  43 <code class="code">extern template</code> extension to provide explicit instantiations
  44 of <code class="code">std::string</code> and <code class="code">std::wstring</code>, and those
  45 explicit instantiations don't include the debug-mode checks.  If the
  46 containing functions are inlined then the checks will run, so compiling
  47 with <code class="code">-O1</code> might be enough to enable them.  Alternatively
  48 <code class="code">-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
  49 of the explicit instantiations and cause the functions to be instantiated
  50 with the debug-mode checks included, but this is unsupported and not
  51 guaranteed to work.  For full debug-mode support you can use the
  52 <code class="code">__gnu_debug::basic_string</code> debugging container directly,
  53 which always works correctly.
  54 </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>