ae8a08ff59
The previous URL to an entry in the wayback machine now redirects to a page saying "SGI.com Tech Archive Resources now retired" so use an older entry from the archive. * doc/xml/faq.xml: Use working link for SGI STL FAQ. * doc/html/*: Regenerate.
1318 lines
49 KiB
XML
1318 lines
49 KiB
XML
<book xmlns="http://docbook.org/ns/docbook" version="5.0">
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<article xml:id="faq" xreflabel="Frequently Asked Questions">
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<?dbhtml filename="faq.html"?>
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<info><title>Frequently Asked Questions</title>
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<copyright>
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<year>
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2008-2018
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</year>
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<holder>
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://www.fsf.org">FSF</link>
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</holder>
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</copyright>
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</info>
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<!-- FAQ starts here -->
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<qandaset xml:id="faq.faq">
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<!-- General Information -->
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<qandadiv xml:id="faq.info" xreflabel="General Information">
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<qandaentry xml:id="faq.what">
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<question xml:id="faq.what.q">
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<para>
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What is libstdc++?
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</para>
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</question>
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<answer xml:id="faq.what.a">
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<para>
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The GNU Standard C++ Library v3 is an ongoing project to
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implement the ISO 14882 C++ Standard Library as described in
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clauses 20 through 33 and annex D (prior to the 2017 standard
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the library clauses started with 17). For those who want to see
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exactly how far the project has come, or just want the latest
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bleeding-edge code, the up-to-date source can be cloned via
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/git.html">Git</link>.
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</para>
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<para>
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N.B. The library is called libstdc++ <emphasis>not</emphasis> stdlibc++.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.why">
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<question xml:id="q-why">
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<para>
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Why should I use libstdc++?
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</para>
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</question>
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<answer xml:id="a-why">
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<para>
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The completion of the initial ISO C++ standardization effort gave the C++
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community a powerful set of reuseable tools in the form of the C++
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Standard Library. However, for several years C++ implementations were
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(as the Draft Standard used to say) <quote>incomplet and
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incorrekt</quote>, and many suffered from limitations of the compilers
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that used them.
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</para>
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<para>
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The GNU compiler collection
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(<command>gcc</command>, <command>g++</command>, etc) is widely
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considered to be one of the leading compilers in the world. Its
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development is overseen by the
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/">GCC team</link>. All of
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the rapid development and near-legendary
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/buildstat.html">portability</link>
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that are the hallmarks of an open-source project are applied to libstdc++.
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</para>
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<para>
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All of the standard classes and functions from C++98/C++03, C++11 and C++14
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(such as <classname>string</classname>,
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<classname>vector<></classname>, iostreams, algorithms etc.)
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are freely available and attempt to be fully compliant.
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Work is ongoing to complete support for the current revision of the
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ISO C++ Standard.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.who">
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<question xml:id="q-who">
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<para>
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Who's in charge of it?
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</para>
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</question>
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<answer xml:id="a-who">
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<para>
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The libstdc++ project is contributed to by several developers
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all over the world, in the same way as GCC or the Linux kernel.
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The current maintainers are listed in the
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/viewcvs/gcc/trunk/MAINTAINERS?view=co"><filename>MAINTAINERS</filename></link>
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file (look for "c++ runtime libs").
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</para>
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<para>
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Development and discussion is held on the libstdc++ mailing
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list. Subscribing to the list, or searching the list
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archives, is open to everyone. You can read instructions for
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doing so on the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/lists.html">GCC mailing lists</link> page.
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If you have questions, ideas, code, or are just curious, sign up!
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.when">
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<question xml:id="q-when">
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<para>
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When is libstdc++ going to be finished?
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</para>
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</question>
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<answer xml:id="a-when">
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<para>
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Nathan Myers gave the best of all possible answers, responding to
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a Usenet article asking this question: <emphasis>Sooner, if you
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help.</emphasis>
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.how">
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<question xml:id="q-how">
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<para>
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How do I contribute to the effort?
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</para>
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</question>
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<answer xml:id="a-how">
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<para>
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See the <link linkend="appendix.contrib">Contributing</link> section in
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the manual. Subscribing to the mailing list (see above, or
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the homepage) is a very good idea if you have something to
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contribute, or if you have spare time and want to
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help. Contributions don't have to be in the form of source code;
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anybody who is willing to help write documentation, for example,
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or has found a bug in code that we all thought was working and is
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willing to provide details, is more than welcome!
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.whereis_old">
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<question xml:id="q-whereis_old">
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<para>
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What happened to the older libg++? I need that!
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</para>
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</question>
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<answer xml:id="a-whereis_old">
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<para>
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The last libg++ README states
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<quote>This package is considered obsolete and is no longer
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being developed.</quote>
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It should not be used for new projects, and won't even compile with
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recent releases of GCC (or most other C++ compilers).
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</para>
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<para>
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More information can be found in the
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<link linkend="manual.appendix.porting.backwards">Backwards
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Compatibility</link> section of the libstdc++ manual.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.more_questions">
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<question xml:id="q-more_questions">
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<para>
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What if I have more questions?
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</para>
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</question>
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<answer xml:id="a-more_questions">
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<para>
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If you have read the documentation, and your question remains
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unanswered, then just ask the mailing list. At present, you do not
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need to be subscribed to the list to send a message to it. More
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information is available on the homepage (including how to browse
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the list archives); to send a message to the list,
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use <email>libstdc++@gcc.gnu.org</email>.
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</para>
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<para>
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If you have a question that you think should be included
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here, or if you have a question <emphasis>about</emphasis> a question/answer
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here, please send email to the libstdc++ mailing list, as above.
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</para>
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</answer>
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</qandaentry>
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</qandadiv>
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<!-- License -->
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<qandadiv xml:id="faq.license" xreflabel="License QA">
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<qandaentry xml:id="faq.license.what">
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<question xml:id="q-license.what">
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<para>
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What are the license terms for libstdc++?
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</para>
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</question>
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<answer xml:id="a-license.what">
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<para>
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See <link linkend="manual.intro.status.license">our license description</link>
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for these and related questions.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.license.any_program">
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<question xml:id="q-license.any_program">
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<para>
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So any program which uses libstdc++ falls under the GPL?
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</para>
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</question>
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<answer xml:id="a-license.any_program">
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<para>
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No. The special exception permits use of the library in
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proprietary applications.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.license.lgpl">
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<question xml:id="q-license.lgpl">
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<para>
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How is that different from the GNU {Lesser,Library} GPL?
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</para>
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</question>
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<answer xml:id="a-license.lgpl">
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<para>
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The LGPL requires that users be able to replace the LGPL code with a
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modified version; this is trivial if the library in question is a C
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shared library. But there's no way to make that work with C++, where
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much of the library consists of inline functions and templates, which
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are expanded inside the code that uses the library. So to allow people
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to replace the library code, someone using the library would have to
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distribute their own source, rendering the LGPL equivalent to the GPL.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.license.what_restrictions">
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<question xml:id="q-license.what_restrictions">
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<para>
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I see. So, what restrictions are there on programs that use the library?
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</para>
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</question>
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<answer xml:id="a-license.what_restrictions">
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<para>
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None. We encourage such programs to be released as free software,
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but we won't punish you or sue you if you choose otherwise.
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</para>
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</answer>
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</qandaentry>
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</qandadiv>
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<!-- Installation -->
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<qandadiv xml:id="faq.installation" xreflabel="Installation">
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<qandaentry xml:id="faq.how_to_install">
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<question xml:id="q-how_to_install">
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<para>How do I install libstdc++?
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</para>
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</question>
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<answer xml:id="a-how_to_install">
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<para>
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Often libstdc++ comes pre-installed as an integral part of many
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existing GNU/Linux and Unix systems, as well as many embedded
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development tools. It may be necessary to install extra
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development packages to get the headers, or the documentation, or
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the source: please consult your vendor for details.
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</para>
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<para>
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To build and install from the GNU GCC sources, please consult the
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<link linkend="manual.intro.setup">setup
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documentation</link> for detailed
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instructions. You may wish to browse those files ahead
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of time to get a feel for what's required.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.how_to_get_sources">
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<question xml:id="q-how_to_get_sources">
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<para>How does one get current libstdc++ sources?
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</para>
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</question>
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<answer xml:id="a-how_to_get_sources">
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<para>
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Libstdc++ sources for all official releases can be obtained as
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part of the GCC sources, available from various sites and
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mirrors. A full <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/mirrors.html">list of
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download sites</link> is provided on the main GCC site.
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</para>
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<para>
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Current libstdc++ sources can always be found in the main GCC source
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repository, available using the appropriate version control tool.
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At this time, that tool is <application>Git</application>.
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For more details see the documentation on
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/git.html">using the Git repository</link>.
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.how_to_test">
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<question xml:id="q-how_to_test">
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<para>How do I know if it works?
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</para>
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</question>
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<answer xml:id="a-how_to_test">
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<para>
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Libstdc++ comes with its own validation testsuite, which includes
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conformance testing, regression testing, ABI testing, and
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performance testing. Please consult the
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/install/test.html">testing
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documentation</link> for GCC and
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<link linkend="manual.intro.setup.test">Testing</link> in the libstdc++
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manual for more details.
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</para>
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<para>
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If you find bugs in the testsuite programs themselves, or if you
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think of a new test program that should be added to the suite,
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<emphasis>please</emphasis> write up your idea and send it to the list!
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</para>
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</answer>
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</qandaentry>
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<qandaentry xml:id="faq.how_to_set_paths">
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<question xml:id="q-how_to_set_paths">
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<para>How do I insure that the dynamically linked library will be found?
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</para>
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</question>
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<answer xml:id="a-how_to_set_paths">
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<para>
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Depending on your platform and library version, the error message might
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be similar to one of the following:
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</para>
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<screen>
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./a.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory
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/usr/libexec/ld-elf.so.1: Shared object "libstdc++.so.6" not found
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</screen>
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<para>
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This doesn't mean that the shared library isn't installed, only
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that the dynamic linker can't find it. When a dynamically-linked
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executable is run the linker finds and loads the required shared
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libraries by searching a pre-configured list of directories. If
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the directory where you've installed libstdc++ is not in this list
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then the libraries won't be found.
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</para>
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<para>
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If you already have an older version of libstdc++ installed then the
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error might look like one of the following instead:
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</para>
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<screen>
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./a.out: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.20' not found
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./a.out: /usr/lib/libstdc++.so.6: version `CXXABI_1.3.8' not found
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</screen>
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<para>
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This means the linker found <filename>/usr/lib/libstdc++.so.6</filename>
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but that library belongs to an older version of GCC than was used to
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compile and link the program <filename>a.out</filename> (or some part
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of it). The program depends on code defined in the newer libstdc++
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that belongs to the newer version of GCC, so the linker must be told
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how to find the newer libstdc++ shared library.
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</para>
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<para>
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The simplest way to fix this is
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to use the <envar>LD_LIBRARY_PATH</envar> environment variable,
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which is a colon-separated list of directories in which the linker
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will search for shared libraries:
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</para>
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<screen><command>
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export LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH
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</command></screen>
|
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|
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<para>
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Here the shell variable <varname>${prefix}</varname> is assumed to contain
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the directory prefix where GCC was installed to. The directory containing
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the library might depend on whether you want the 32-bit or 64-bit copy
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of the library, so for example would be
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<filename class="directory">${prefix}/lib64</filename> on some systems.
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The exact environment variable to use will depend on your
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platform, e.g. <envar>DYLD_LIBRARY_PATH</envar> for Darwin,
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<envar>LD_LIBRARY_PATH_32</envar>/<envar>LD_LIBRARY_PATH_64</envar>
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for Solaris 32-/64-bit,
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and <envar>SHLIB_PATH</envar> for HP-UX.
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</para>
|
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<para>
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See the man pages for <command>ld</command>, <command>ldd</command>
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and <command>ldconfig</command> for more information. The dynamic
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linker has different names on different platforms but the man page
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is usually called something such as <filename>ld.so</filename>,
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<filename>rtld</filename> or <filename>dld.so</filename>.
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</para>
|
|
<para>
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Using <envar>LD_LIBRARY_PATH</envar> is not always the best solution,
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<link linkend="manual.intro.using.linkage.dynamic">Finding Dynamic or Shared
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Libraries</link> in the manual gives some alternatives.
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</para>
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|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.what_is_libsupcxx">
|
|
<question xml:id="q-what_is_libsupcxx">
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<para>
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|
What's libsupc++?
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</para>
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</question>
|
|
<answer xml:id="a-what_is_libsupcxx">
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<para>
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If the only functions from <filename class="libraryfile">libstdc++.a</filename>
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which you need are language support functions (those listed in
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<link linkend="std.support">clause 18</link> of the
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|
standard, e.g., <function>new</function> and
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<function>delete</function>), then try linking against
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<filename class="libraryfile">libsupc++.a</filename>, which is a subset of
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<filename class="libraryfile">libstdc++.a</filename>. (Using <command>gcc</command>
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|
instead of <command>g++</command> and explicitly linking in
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<filename class="libraryfile">libsupc++.a</filename> via <option>-lsupc++</option>
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for the final link step will do it). This library contains only
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|
those support routines, one per object file. But if you are
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|
using anything from the rest of the library, such as IOStreams
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|
or vectors, then you'll still need pieces from
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|
<filename class="libraryfile">libstdc++.a</filename>.
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|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.size">
|
|
<question xml:id="q-size">
|
|
<para>
|
|
This library is HUGE!
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|
</para>
|
|
</question>
|
|
<answer xml:id="a-size">
|
|
<para>
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|
Usually the size of libraries on disk isn't noticeable. When a
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link editor (or simply <quote>linker</quote>) pulls things from a
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|
static archive library, only the necessary object files are copied
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|
into your executable, not the entire library. Unfortunately, even
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|
if you only need a single function or variable from an object file,
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|
the entire object file is extracted. (There's nothing unique to C++
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|
or libstdc++ about this; it's just common behavior, given here
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|
for background reasons.)
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|
</para>
|
|
<para>
|
|
Some of the object files which make up
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<filename class="libraryfile">libstdc++.a</filename> are rather large.
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|
If you create a statically-linked executable with
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|
<option>-static</option>, those large object files are suddenly part
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|
of your executable. Historically the best way around this was to
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|
only place a very few functions (often only a single one) in each
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|
source/object file; then extracting a single function is the same
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|
as extracting a single <filename>.o</filename> file. For libstdc++ this
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|
is only possible to a certain extent; the object files in question contain
|
|
template classes and template functions, pre-instantiated, and
|
|
splitting those up causes severe maintenance headaches.
|
|
</para>
|
|
<para>
|
|
On supported platforms, libstdc++ takes advantage of garbage
|
|
collection in the GNU linker to get a result similar to separating
|
|
each symbol into a separate source and object files. On these platforms,
|
|
GNU ld can place each function and variable into its own
|
|
section in a <filename>.o</filename> file. The GNU linker can then perform
|
|
garbage collection on unused sections; this reduces the situation to only
|
|
copying needed functions into the executable, as before, but all
|
|
happens automatically.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
</qandadiv>
|
|
|
|
|
|
<!-- Platform-Specific Issues -->
|
|
<qandadiv xml:id="faq.platform-specific" xreflabel="Platform-Specific Issues">
|
|
|
|
|
|
<qandaentry xml:id="faq.other_compilers">
|
|
<question xml:id="q-other_compilers">
|
|
<para>
|
|
Can libstdc++ be used with non-GNU compilers?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-other_compilers">
|
|
<para>
|
|
Perhaps.
|
|
</para>
|
|
<para>
|
|
Since the goal of ISO Standardization is for all C++
|
|
implementations to be able to share code, libstdc++ should be
|
|
usable under any ISO-compliant compiler, at least in theory.
|
|
</para>
|
|
<para>
|
|
However, the reality is that libstdc++ is targeted and optimized
|
|
for GCC/G++. This means that often libstdc++ uses specific,
|
|
non-standard features of G++ that are not present in older
|
|
versions of proprietary compilers. It may take as much as a year or two
|
|
after an official release of GCC that contains these features for
|
|
proprietary tools to support these constructs.
|
|
</para>
|
|
<para>
|
|
Recent versions of libstdc++ are known to work with the Clang compiler.
|
|
In the near past, specific released versions of libstdc++ have
|
|
been known to work with versions of the EDG C++ compiler, and
|
|
vendor-specific proprietary C++ compilers such as the Intel ICC
|
|
C++ compiler.
|
|
</para>
|
|
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.solaris_long_long">
|
|
<question xml:id="q-solaris_long_long">
|
|
<para>
|
|
No '<type>long long</type>' type on Solaris?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-solaris_long_long">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
By default we try to support the C99 <type>long long</type> type.
|
|
This requires that certain functions from your C library be present.
|
|
</para>
|
|
<para>
|
|
Up through release 3.0.2 the platform-specific tests performed by
|
|
libstdc++ were too general, resulting in a conservative approach
|
|
to enabling the <type>long long</type> code paths. The most
|
|
commonly reported platform affected was Solaris.
|
|
</para>
|
|
<para>
|
|
This has been fixed for libstdc++ releases greater than 3.0.3.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.predefined">
|
|
<question xml:id="q-predefined">
|
|
<para>
|
|
<constant>_XOPEN_SOURCE</constant> and <constant>_GNU_SOURCE</constant> are always defined?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-predefined">
|
|
<para>On Solaris, <command>g++</command> (but not <command>gcc</command>)
|
|
always defines the preprocessor macro
|
|
<constant>_XOPEN_SOURCE</constant>. On GNU/Linux, the same happens
|
|
with <constant>_GNU_SOURCE</constant>. (This is not an exhaustive list;
|
|
other macros and other platforms are also affected.)
|
|
</para>
|
|
<para>These macros are typically used in C library headers, guarding new
|
|
versions of functions from their older versions. The C++98 standard
|
|
library includes the C standard library, but it requires the C90
|
|
version, which for backwards-compatibility reasons is often not the
|
|
default for many vendors.
|
|
</para>
|
|
<para>More to the point, the C++ standard requires behavior which is only
|
|
available on certain platforms after certain symbols are defined.
|
|
Usually the issue involves I/O-related typedefs. In order to
|
|
ensure correctness, the compiler simply predefines those symbols.
|
|
</para>
|
|
<para>Note that it's not enough to <literal>#define</literal> them only when the library is
|
|
being built (during installation). Since we don't have an 'export'
|
|
keyword, much of the library exists as headers, which means that
|
|
the symbols must also be defined as your programs are parsed and
|
|
compiled.
|
|
</para>
|
|
<para>To see which symbols are defined, look for
|
|
<varname>CPLUSPLUS_CPP_SPEC</varname> in
|
|
the gcc config headers for your target (and try changing them to
|
|
see what happens when building complicated code). You can also run
|
|
<command>g++ -E -dM - < /dev/null"</command> to display
|
|
a list of predefined macros for any particular installation.
|
|
</para>
|
|
<para>This has been discussed on the mailing lists
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/cgi-bin/htsearch?method=and&format=builtin-long&sort=score&words=_XOPEN_SOURCE+Solaris">quite a bit</link>.
|
|
</para>
|
|
<para>This method is something of a wart. We'd like to find a cleaner
|
|
solution, but nobody yet has contributed the time.
|
|
</para>
|
|
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.darwin_ctype">
|
|
<question xml:id="q-darwin_ctype">
|
|
<para>
|
|
Mac OS X <filename class="headerfile">ctype.h</filename> is broken! How can I fix it?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-darwin_ctype">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
This was a long-standing bug in the OS X support. Fortunately, the
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html">patch</link>
|
|
was quite simple, and well-known.
|
|
</para>
|
|
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.threads_i386">
|
|
<question xml:id="q-threads_i386">
|
|
<para>
|
|
Threading is broken on i386?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-threads_i386">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>Support for atomic integer operations was broken on i386
|
|
platforms. The assembly code accidentally used opcodes that are
|
|
only available on the i486 and later. So if you configured GCC
|
|
to target, for example, i386-linux, but actually used the programs
|
|
on an i686, then you would encounter no problems. Only when
|
|
actually running the code on a i386 will the problem appear.
|
|
</para>
|
|
<para>This is fixed in 3.2.2.
|
|
</para>
|
|
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.atomic_mips">
|
|
<question xml:id="q-atomic_mips">
|
|
<para>
|
|
MIPS atomic operations
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-atomic_mips">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
The atomic locking routines for MIPS targets requires MIPS II
|
|
and later. A patch went in just after the 3.3 release to
|
|
make mips* use the generic implementation instead. You can also
|
|
configure for mipsel-elf as a workaround.
|
|
</para>
|
|
<para>
|
|
The mips*-*-linux* port continues to use the MIPS II routines, and more
|
|
work in this area is expected.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.linux_glibc">
|
|
<question xml:id="q-linux_glibc">
|
|
<para>
|
|
Recent GNU/Linux glibc required?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-linux_glibc">
|
|
<para>When running on GNU/Linux, libstdc++ 3.2.1 (shared library version
|
|
5.0.1) and later uses localization and formatting code from the system
|
|
C library (glibc) version 2.2.5 which contains necessary bugfixes.
|
|
All GNU/Linux distros make more recent versions available now.
|
|
libstdc++ 4.6.0 and later require glibc 2.3 or later for this
|
|
localization and formatting code.
|
|
</para>
|
|
<para>The guideline is simple: the more recent the C++ library, the
|
|
more recent the C library. (This is also documented in the main
|
|
GCC installation instructions.)
|
|
</para>
|
|
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.freebsd_wchar">
|
|
<question xml:id="q-freebsd_wchar">
|
|
<para>
|
|
Can't use <type>wchar_t</type>/<classname>wstring</classname> on FreeBSD
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-freebsd_wchar">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
Older versions of FreeBSD's C library do not have sufficient
|
|
support for wide character functions, and as a result the
|
|
libstdc++ configury decides that <type>wchar_t</type> support should be
|
|
disabled. In addition, the libstdc++ platform checks that
|
|
enabled <type>wchar_t</type> were quite strict, and not granular
|
|
enough to detect when the minimal support to
|
|
enable <type>wchar_t</type> and C++ library structures
|
|
like <classname>wstring</classname> were present. This impacted Solaris,
|
|
Darwin, and BSD variants, and is fixed in libstdc++ versions post 4.1.0.
|
|
</para>
|
|
<para>
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
</qandadiv>
|
|
|
|
|
|
<!-- Known Bugs -->
|
|
<qandadiv xml:id="faq.known_bugs" xreflabel="Known Bugs">
|
|
|
|
|
|
<qandaentry xml:id="faq.what_works">
|
|
<question xml:id="q-what_works">
|
|
<para>
|
|
What works already?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-what_works">
|
|
<para>
|
|
Short answer: Pretty much everything <emphasis>works</emphasis>
|
|
except for some corner cases. Support for localization
|
|
in <classname>locale</classname> may be incomplete on some non-GNU
|
|
platforms. Also dependent on the underlying platform is support
|
|
for <type>wchar_t</type> and <type>long long</type> specializations,
|
|
and details of thread support.
|
|
</para>
|
|
<para>
|
|
Long answer: See the implementation status pages for
|
|
<link linkend="status.iso.1998">C++98</link>,
|
|
<link linkend="status.iso.tr1">TR1</link>,
|
|
<link linkend="status.iso.2011">C++11</link>,
|
|
<link linkend="status.iso.2014">C++14</link>, and
|
|
<link linkend="status.iso.2017">C++17</link>.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.standard_bugs">
|
|
<question xml:id="q-standard_bugs">
|
|
<para>
|
|
Bugs in the ISO C++ language or library specification
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-standard_bugs">
|
|
<para>
|
|
Unfortunately, there are some.
|
|
</para>
|
|
<para>
|
|
For those people who are not part of the ISO Library Group
|
|
(i.e., nearly all of us needing to read this page in the first
|
|
place), a public list of the library defects is occasionally
|
|
published on <link xmlns:xlink="http://www.w3.org/1999/xlink"
|
|
xlink:href="http://www.open-std.org/jtc1/sc22/wg21/">the WG21
|
|
website</link>.
|
|
Many of these issues have resulted in
|
|
<link linkend="manual.intro.status.bugs.iso">code changes in libstdc++</link>.
|
|
</para>
|
|
<para>
|
|
If you think you've discovered a new bug that is not listed,
|
|
please post a message describing your problem to the author of
|
|
the library issues list.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.compiler_bugs">
|
|
<question xml:id="q-compiler_bugs">
|
|
<para>
|
|
Bugs in the compiler (gcc/g++) and not libstdc++
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-compiler_bugs">
|
|
<para>
|
|
On occasion, the compiler is wrong. Please be advised that this
|
|
happens much less often than one would think, and avoid jumping to
|
|
conclusions.
|
|
</para>
|
|
<para>
|
|
First, examine the ISO C++ standard. Second, try another compiler
|
|
or an older version of the GNU compilers. Third, you can find more
|
|
information on the libstdc++ and the GCC mailing lists: search
|
|
these lists with terms describing your issue.
|
|
</para>
|
|
<para>
|
|
Before reporting a bug, please examine the
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/bugs/">bugs database</link>, with the
|
|
component set to <quote>c++</quote>.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
</qandadiv>
|
|
|
|
<!-- Known Non-Bugs -->
|
|
<qandadiv xml:id="faq.known_non-bugs" xreflabel="Known Non-Bugs">
|
|
|
|
|
|
<qandaentry xml:id="faq.stream_reopening_fails">
|
|
<question xml:id="q-stream_reopening_fails">
|
|
<para>
|
|
Reopening a stream fails
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-stream_reopening_fails">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
Prior to GCC 4.0 this was one of the most-reported non-bug reports.
|
|
Executing a sequence like this would fail:
|
|
</para>
|
|
|
|
<programlisting>
|
|
#include <fstream>
|
|
...
|
|
std::fstream fs("a_file");
|
|
// .
|
|
// . do things with fs...
|
|
// .
|
|
fs.close();
|
|
fs.open("a_new_file");
|
|
</programlisting>
|
|
|
|
<para>
|
|
All operations on the re-opened <varname>fs</varname> would fail, or at
|
|
least act very strangely, especially if <varname>fs</varname> reached the
|
|
EOF state on the previous file.
|
|
The original C++98 standard did not specify behavior in this case, and
|
|
the <link linkend="manual.bugs.dr22">resolution of DR #22</link> was to
|
|
leave the state flags unchanged on a successful call to
|
|
<function>open()</function>.
|
|
You had to insert a call to <function>fs.clear()</function> between the
|
|
calls to <function>close()</function> and <function>open()</function>,
|
|
and then everything will work as expected.
|
|
<emphasis>Update:</emphasis> For GCC 4.0 we implemented the resolution
|
|
of <link linkend="manual.bugs.dr409">DR #409</link> and
|
|
<function>open()</function>
|
|
now calls <function>clear()</function> on success.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.wefcxx_verbose">
|
|
<question xml:id="q-wefcxx_verbose">
|
|
<para>
|
|
-Weffc++ complains too much
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-wefcxx_verbose">
|
|
<para>
|
|
Many warnings are emitted when <option>-Weffc++</option> is used. Making
|
|
libstdc++ <option>-Weffc++</option>-clean is not a goal of the project,
|
|
for a few reasons. Mainly, that option tries to enforce
|
|
object-oriented programming, while the Standard Library isn't
|
|
necessarily trying to be OO. The option also enforces outdated guidelines
|
|
from old editions of the books, and the advice isn't all relevant to
|
|
modern C++ (especially C++11 and later).
|
|
</para>
|
|
<para>
|
|
We do, however, try to have libstdc++ sources as clean as possible. If
|
|
you see some simple changes that pacify <option>-Weffc++</option>
|
|
without other drawbacks, send us a patch.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.ambiguous_overloads">
|
|
<question xml:id="q-ambiguous_overloads">
|
|
<para>
|
|
Ambiguous overloads after including an old-style header
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-ambiguous_overloads">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
Another problem is the <literal>rel_ops</literal> namespace and the template
|
|
comparison operator functions contained therein. If they become
|
|
visible in the same namespace as other comparison functions
|
|
(e.g., <quote>using</quote> them and the
|
|
<filename class="headerfile"><iterator></filename> header),
|
|
then you will suddenly be faced with huge numbers of ambiguity
|
|
errors. This was discussed on the mailing list; Nathan Myers
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html">sums
|
|
things up here</link>. The collisions with vector/string iterator
|
|
types have been fixed for 3.1.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.v2_headers">
|
|
<question xml:id="q-v2_headers">
|
|
<para>
|
|
The g++-3 headers are <emphasis>not ours</emphasis>
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-v2_headers">
|
|
<note>
|
|
<para>This answer is old and probably no longer be relevant.</para>
|
|
</note>
|
|
<para>
|
|
If you are using headers in
|
|
<filename class="directory">${prefix}/include/g++-3</filename>, or if
|
|
the installed library's name looks like
|
|
<filename class="libraryfile">libstdc++-2.10.a</filename> or
|
|
<filename class="libraryfile">libstdc++-libc6-2.10.so</filename>, then
|
|
you are using the old libstdc++-v2 library, which is non-standard and
|
|
unmaintained. Do not report problems with -v2 to the -v3
|
|
mailing list.
|
|
</para>
|
|
<para>
|
|
For GCC versions 3.0 and 3.1 the libstdc++ header files are installed in
|
|
<filename class="directory">${prefix}/include/g++-v3</filename>
|
|
(see the 'v'?). Starting with version 3.2 the headers are installed in
|
|
<filename class="directory">${prefix}/include/c++/${version}</filename>
|
|
as this prevents headers from previous versions being found by mistake.
|
|
</para>
|
|
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.boost_concept_checks">
|
|
<question xml:id="q-boost_concept_checks">
|
|
<para>
|
|
Errors about <emphasis>*Concept</emphasis> and
|
|
<emphasis>constraints</emphasis> in the STL
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-boost_concept_checks">
|
|
<para>
|
|
If you see compilation errors containing messages about
|
|
<errortext>foo Concept</errortext> and something to do with a
|
|
<errortext>constraints</errortext> member function, then most
|
|
likely you have violated one of the requirements for types used
|
|
during instantiation of template containers and functions. For
|
|
example, EqualityComparableConcept appears if your types must be
|
|
comparable with == and you have not provided this capability (a
|
|
typo, or wrong visibility, or you just plain forgot, etc).
|
|
</para>
|
|
<para>
|
|
More information, including how to optionally enable/disable the
|
|
checks, is available in the
|
|
<link linkend="std.diagnostics.concept_checking">Diagnostics</link>.
|
|
chapter of the manual.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.dlopen_crash">
|
|
<question xml:id="q-dlopen_crash">
|
|
<para>
|
|
Program crashes when using library code in a
|
|
dynamically-loaded library
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-dlopen_crash">
|
|
<para>
|
|
If you are using the C++ library across dynamically-loaded
|
|
objects, make certain that you are passing the correct options
|
|
when compiling and linking:
|
|
</para>
|
|
|
|
<literallayout class="normal">
|
|
Compile your library components:
|
|
<command>g++ -fPIC -c a.cc</command>
|
|
<command>g++ -fPIC -c b.cc</command>
|
|
...
|
|
<command>g++ -fPIC -c z.cc</command>
|
|
|
|
Create your library:
|
|
<command>g++ -fPIC -shared -rdynamic -o libfoo.so a.o b.o ... z.o</command>
|
|
|
|
Link the executable:
|
|
<command>g++ -fPIC -rdynamic -o foo ... -L. -lfoo -ldl</command>
|
|
</literallayout>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.memory_leaks">
|
|
<question xml:id="q-memory_leaks">
|
|
<para>
|
|
<quote>Memory leaks</quote> in libstdc++
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-memory_leaks">
|
|
<para>
|
|
Since GCC 5.1.0, libstdc++ automatically allocates a pool
|
|
of a few dozen kilobytes on startup. This pool is used to ensure it's
|
|
possible to throw exceptions (such as <classname>bad_alloc</classname>)
|
|
even when <code>malloc</code> is unable to allocate any more memory.
|
|
With some versions of <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://valgrind.org/"><command>valgrind</command></link>
|
|
this pool will be shown as "still reachable" when the process exits, e.g.
|
|
<code>still reachable: 72,704 bytes in 1 blocks</code>.
|
|
This memory is not a leak, because it's still in use by libstdc++,
|
|
and the memory will be returned to the OS when the process exits.
|
|
Later versions of <command>valgrind</command> know how to free this
|
|
pool as the process exits, and so won't show any "still reachable" memory.
|
|
</para>
|
|
<para>
|
|
In the past, a few people reported that the standard containers appear
|
|
to leak memory when tested with memory checkers such as
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://valgrind.org/"><command>valgrind</command></link>.
|
|
Under some (non-default) configurations the library's allocators keep
|
|
free memory in a
|
|
pool for later reuse, rather than deallocating it with <code>delete</code>
|
|
Although this memory is always reachable by the library and is never
|
|
lost, memory debugging tools can report it as a leak. If you
|
|
want to test the library for memory leaks please read
|
|
<link linkend="debug.memory">Tips for memory leak hunting</link>
|
|
first.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.list_size_on">
|
|
<question xml:id="q-list_size_on">
|
|
<para>
|
|
<code>list::size()</code> is O(n)!
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-list_size_on">
|
|
<para>
|
|
See
|
|
the <link linkend="std.containers">Containers</link>
|
|
chapter.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.easy_to_fix">
|
|
<question xml:id="q-easy_to_fix">
|
|
<para>
|
|
Aw, that's easy to fix!
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-easy_to_fix">
|
|
<para>
|
|
If you have found a bug in the library and you think you have
|
|
a working fix, then send it in! The main GCC site has a page
|
|
on <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/contribute.html">submitting
|
|
patches</link> that covers the procedure, but for libstdc++ you
|
|
should also send the patch to our mailing list in addition to
|
|
the GCC patches mailing list. The libstdc++
|
|
<link linkend="appendix.contrib">contributors' page</link>
|
|
also talks about how to submit patches.
|
|
</para>
|
|
<para>
|
|
In addition to the description, the patch, and the ChangeLog
|
|
entry, it is a Good Thing if you can additionally create a small
|
|
test program to test for the presence of the bug that your patch
|
|
fixes. Bugs have a way of being reintroduced; if an old bug
|
|
creeps back in, it will be caught immediately by the testsuite -
|
|
but only if such a test exists.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
</qandadiv>
|
|
|
|
|
|
<!-- Miscellaneous -->
|
|
<qandadiv xml:id="faq.misc" xreflabel="Miscellaneous">
|
|
|
|
|
|
<qandaentry xml:id="faq.iterator_as_pod">
|
|
<question xml:id="faq.iterator_as_pod_q">
|
|
<para>
|
|
<classname>string::iterator</classname> is not <code>char*</code>;
|
|
<classname>vector<T>::iterator</classname> is not <code>T*</code>
|
|
</para>
|
|
</question>
|
|
<answer xml:id="faq.iterator_as_pod_a">
|
|
<para>
|
|
If you have code that depends on container<T> iterators
|
|
being implemented as pointer-to-T, your code is broken. It's
|
|
considered a feature, not a bug, that libstdc++ points this out.
|
|
</para>
|
|
<para>
|
|
While there are arguments for iterators to be implemented in
|
|
that manner, A) they aren't very good ones in the long term,
|
|
and B) they were never guaranteed by the Standard anyway. The
|
|
type-safety achieved by making iterators a real class rather
|
|
than a typedef for <type>T*</type> outweighs nearly all opposing
|
|
arguments.
|
|
</para>
|
|
<para>
|
|
Code which does assume that a vector/string iterator <varname>i</varname>
|
|
is a pointer can often be fixed by changing <varname>i</varname> in
|
|
certain expressions to <varname>&*i</varname>.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.what_is_next">
|
|
<question xml:id="q-what_is_next">
|
|
<para>
|
|
What's next after libstdc++?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-what_is_next">
|
|
<para>
|
|
The goal of libstdc++ is to produce a
|
|
fully-compliant, fully-portable Standard Library.
|
|
While the C++ Standard continues to evolve the libstdc++ will
|
|
continue to track it.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.sgi_stl">
|
|
<question xml:id="q-sgi_stl">
|
|
<para>
|
|
What about the STL from SGI?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-sgi_stl">
|
|
<para>
|
|
The STL (Standard Template Library) was the inspiration for large chunks
|
|
of the C++ Standard Library, but the terms are not interchangeable and
|
|
they don't mean the same thing. The C++ Standard Library includes lots of
|
|
things that didn't come from the STL, and some of them aren't even
|
|
templates, such as <classname>std::locale</classname> and
|
|
<classname>std::thread</classname>.
|
|
</para>
|
|
<para>
|
|
Libstdc++-v3 incorporates a lot of code from
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/">the SGI STL</link>
|
|
(the final merge was from
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/whats_new.html">release 3.3</link>).
|
|
The code in libstdc++ contains many fixes and changes compared to the
|
|
original SGI code.
|
|
</para>
|
|
<para>
|
|
In particular, <classname>string</classname> is not from SGI and makes no
|
|
use of their "rope" class (although that is included as an optional
|
|
extension), neither is <classname>valarray</classname> nor some others.
|
|
Classes like <classname>vector<></classname> were from SGI, but have
|
|
been extensively modified.
|
|
</para>
|
|
<para>
|
|
More information on the evolution of libstdc++ can be found at the
|
|
<link linkend="appendix.porting.api">API
|
|
evolution</link>
|
|
and <link linkend="manual.appendix.porting.backwards">backwards
|
|
compatibility</link> documentation.
|
|
</para>
|
|
<para>
|
|
The <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171104092813/http://www.sgi.com/tech/stl/FAQ.html">FAQ</link>
|
|
for SGI's STL is still recommended reading.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.extensions_and_backwards_compat">
|
|
<question xml:id="q-extensions_and_backwards_compat">
|
|
<para>
|
|
Extensions and Backward Compatibility
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-extensions_and_backwards_compat">
|
|
<para>
|
|
See the <link linkend="manual.appendix.porting.backwards">link</link> on backwards compatibility and <link linkend="appendix.porting.api">link</link> on evolution.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.tr1_support">
|
|
<question xml:id="q-tr1_support">
|
|
<para>
|
|
Does libstdc++ support TR1?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-tr1_support">
|
|
<para>
|
|
Yes.
|
|
</para>
|
|
<para>
|
|
The C++ Standard Library
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
|
|
Technical Report 1</link> added many new features to the library.
|
|
</para>
|
|
<para>
|
|
The implementation status of TR1 in libstdc++ can be tracked
|
|
<link linkend="status.iso.tr1">on the TR1 status page</link>.
|
|
</para>
|
|
<para>
|
|
New code should probably not use TR1, because almost everything in it has
|
|
been added to the main C++ Standard Library (usually with significant
|
|
improvements).
|
|
The TR1 implementation in libstdc++ is no longer actively maintained.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.get_iso_cxx">
|
|
<question xml:id="q-get_iso_cxx">
|
|
<para>How do I get a copy of the ISO C++ Standard?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-get_iso_cxx">
|
|
<para>
|
|
Please refer to the <link linkend="appendix.contrib">Contributing</link>
|
|
section in our manual.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.what_is_abi">
|
|
<question xml:id="q-what_is_abi">
|
|
<para>
|
|
What's an ABI and why is it so messy?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-what_is_abi">
|
|
<para>
|
|
<acronym>ABI</acronym> stands for <quote>Application Binary
|
|
Interface</quote>. Conventionally, it refers to a great
|
|
mass of details about how arguments are arranged on the call
|
|
stack and/or in registers, and how various types are arranged
|
|
and padded in structs. A single CPU design may suffer
|
|
multiple ABIs designed by different development tool vendors
|
|
who made different choices, or even by the same vendor for
|
|
different target applications or compiler versions. In ideal
|
|
circumstances the CPU designer presents one ABI and all the
|
|
OSes and compilers use it. In practice every ABI omits
|
|
details that compiler implementers (consciously or
|
|
accidentally) must choose for themselves.
|
|
</para>
|
|
<para>
|
|
That ABI definition suffices for compilers to generate code so a
|
|
program can interact safely with an OS and its lowest-level libraries.
|
|
Users usually want an ABI to encompass more detail, allowing libraries
|
|
built with different compilers (or different releases of the same
|
|
compiler!) to be linked together. For C++, this includes many more
|
|
details than for C, and most CPU designers (for good reasons elaborated
|
|
below) have not stepped up to publish C++ ABIs. Such an ABI has been
|
|
defined for the Itanium architecture (see
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://itanium-cxx-abi.github.io/cxx-abi/">C++
|
|
ABI for Itanium</link>) and that is used by G++ and other compilers
|
|
as the de facto standard ABI on many common architectures (including x86).
|
|
G++ can also use the ARM architecture's EABI, for embedded
|
|
systems relying only on a <quote>free-standing implementation</quote> that
|
|
doesn't include (much of) the standard library, and the GNU EABI for
|
|
hosted implementations on ARM. Those ABIs cover low-level details
|
|
such as virtual function implementation, struct inheritance layout,
|
|
name mangling, and exception handling.
|
|
</para>
|
|
<para>
|
|
A useful C++ ABI must also incorporate many details of the standard
|
|
library implementation. For a C ABI, the layouts of a few structs
|
|
(such as <type>FILE</type>, <type>stat</type>, <type>jmpbuf</type>,
|
|
and the like) and a few macros suffice.
|
|
For C++, the details include the complete set of names of functions
|
|
and types used, the offsets of class members and virtual functions,
|
|
and the actual definitions of all inlines. C++ exposes many more
|
|
library details to the caller than C does. It makes defining
|
|
a complete ABI a much bigger undertaking, and requires not just
|
|
documenting library implementation details, but carefully designing
|
|
those details so that future bug fixes and optimizations don't
|
|
force breaking the ABI.
|
|
</para>
|
|
<para>
|
|
There are ways to help isolate library implementation details from the
|
|
ABI, but they trade off against speed. Library details used in inner
|
|
loops (e.g., <function>getchar</function>) must be exposed and frozen for
|
|
all time, but many others may reasonably be kept hidden from user code,
|
|
so they may later be changed. Deciding which, and implementing
|
|
the decisions, must happen before you can reasonably document a
|
|
candidate C++ ABI that encompasses the standard library.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
<qandaentry xml:id="faq.size_equals_capacity">
|
|
<question xml:id="q-size_equals_capacity">
|
|
<para>
|
|
How do I make <code>std::vector<T>::capacity() == std::vector<T>::size</code>?
|
|
</para>
|
|
</question>
|
|
<answer xml:id="a-size_equals_capacity">
|
|
<para>
|
|
Since C++11 just call the <function>shrink_to_fit()</function> member
|
|
function.
|
|
</para>
|
|
<para>
|
|
Before C++11, the standard idiom for deallocating a
|
|
<classname>vector<T></classname>'s
|
|
unused memory was to create a temporary copy of the vector and swap their
|
|
contents, e.g. for <classname>vector<T> v</classname>
|
|
</para>
|
|
<literallayout class="normal">
|
|
std::vector<T>(v).swap(v);
|
|
</literallayout>
|
|
<para>
|
|
The copy will take O(n) time and the swap is constant time.
|
|
</para>
|
|
<para>
|
|
See <link linkend="strings.string.shrink">Shrink-to-fit
|
|
strings</link> for a similar solution for strings.
|
|
</para>
|
|
</answer>
|
|
</qandaentry>
|
|
|
|
</qandadiv>
|
|
|
|
|
|
<!-- FAQ ends here -->
|
|
</qandaset>
|
|
|
|
</article>
|
|
|
|
</book>
|