mirror of git://gcc.gnu.org/git/gcc.git
765 lines
26 KiB
HTML
765 lines
26 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
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<html>
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<head>
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<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type">
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<title>Libstdc++-porting-howto</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.48">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="article">
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<div class="titlepage">
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<div><h1 class="title">
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<a name="libstdc++-porting-howto"></a>Libstdc++-porting-howto</h1></div>
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<div><h3 class="author">Felix Natter</h3></div>
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<div><div class="legalnotice">
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<p class="legalnotice-title"><b>Legal Notice</b></p>
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<p>
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This document can be distributed under the FDL
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(<a href="http://www.gnu.org" target="_top">www.gnu.org</a>)
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</p>
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</div></div>
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<div><p class="pubdate">Tue Jun 5 20:07:49 2001</p></div>
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<div><div class="revhistory"><table border="1" width="100%" summary="Revision history">
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<tr><th align="left" valign="top" colspan="3"><b>Revision History</b></th></tr>
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<tr>
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<td align="left">Revision 0.5</td>
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<td align="left">Thu Jun 1 13:06:50 2000</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">First docbook-version.</td></tr>
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<tr>
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<td align="left">Revision 0.8</td>
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<td align="left">Sun Jul 30 20:28:40 2000</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">First released version using docbook-xml
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+ second upload to libstdc++-page.
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</td></tr>
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<tr>
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<td align="left">Revision 0.9</td>
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<td align="left">Wed Sep 6 02:59:32 2000</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">5 new sections.</td></tr>
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<tr>
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<td align="left">Revision 0.9.1</td>
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<td align="left">Sat Sep 23 14:20:15 2000</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">added information about why file-descriptors are not in the
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standard</td></tr>
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<tr>
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<td align="left">Revision 0.9.2</td>
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<td align="left">Tue Jun 5 20:07:49 2001</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">
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a fix, added hint on increased portability of C-shadow-headers,
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added autoconf-test HAVE_CONTAINER_AT
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</td></tr>
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<tr>
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<td align="left">Revision 0.9.3</td>
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<td align="left">Fri Jun 29 16:15:56 2001</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">
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changed signature of nonstandard filebuf-constructor and
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update the section on filebuf::attach to point to ../ext/howto.html,
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added link to ../21/strings/howto.html
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in sec-stringstream, changed <link>-tags to have content
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(so that these links work),
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replace "user-space" by "global namespace"
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add note about gcc 3.0 and shadow-headers
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add section about ostream::form and istream::scan
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sec-vector-at: remove hint to modify headers
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fix spelling error in sec-stringstream
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</td></tr>
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<tr>
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<td align="left">Revision 0.9.4</td>
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<td align="left">Mon Nov 5 17:01:04 2001</td>
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<td align="left">fnatter</td>
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</tr>
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<tr><td align="left" colspan="3">
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rewrite section 1.1.3 because of gnu.gcc.help-post by
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Juergen Heinzl
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</td></tr>
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</table></div></div>
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<div><div class="abstract">
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<p><b>Abstract</b></p>
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<p>
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Some notes on porting applications from libstdc++-2.90 (or earlier
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versions) to libstdc++-v3. Not speaking in terms of the GNU libstdc++
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implementations, this means porting from earlier versions of the
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C++-Standard to ISO 14882.
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</p>
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</div></div>
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<hr>
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</div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl>
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<dt>1. <a href="#sec-nsstd">Namespace std::</a>
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</dt>
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<dd><dl>
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<dt>1.1.1. <a href="#sec-gtkmm-hack">Using namespace
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composition if the project uses a separate
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namespace</a>
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</dt>
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<dt>1.1.2. <a href="#sec-emptyns">Defining an empty namespace std</a>
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</dt>
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<dt>1.1.3. <a href="#sec-avoidfqn">Avoid to use fully qualified names
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(i.e. std::string)</a>
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</dt>
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<dt>1.1.4. <a href="#sec-osprojects">How some open-source-projects deal
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with this</a>
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</dt>
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</dl></dd>
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<dt>2. <a href="#sec-nocreate">there is no ios::nocreate/ios::noreplace
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in ISO 14882</a>
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</dt>
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<dt>3. <a href="#sec-stream::attach">stream::attach(int
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fd) is not in the standard any more</a>
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</dt>
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<dt>4. <a href="#sec-headers">The new headers</a>
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</dt>
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<dd><dl>
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<dt>4.4.1. <a href="#sec-cheaders">New headers replacing C-headers</a>
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</dt>
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<dt>4.4.2. <a href="#sec-fstream-header">
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<fstream> does
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not define std::cout,
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std::cin etc.</a>
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</dt>
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</dl></dd>
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<dt>5. <a href="#sec-iterators">Iterators</a>
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</dt>
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<dt>6. <a href="#sec-macros">
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Libc-macros (i.e. isspace from
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<cctype>)</a>
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</dt>
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<dt>7. <a href="#sec-stream-state">State of streams</a>
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</dt>
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<dt>8. <a href="#sec-vector-at">vector::at is missing (i.e. gcc 2.95.x)</a>
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</dt>
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<dt>9. <a href="#sec-eof">Using std::char_traits<char>::eof()</a>
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</dt>
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<dt>10. <a href="#sec-string-clear">Using string::clear()/string::erase()</a>
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</dt>
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<dt>11. <a href="#sec-scan-form">GNU Extensions ostream::form and istream::scan</a>
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</dt>
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<dt>12. <a href="#sec-stringstream">Using stringstreams</a>
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</dt>
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<dt>13. <a href="#sec-about">About...</a>
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</dt>
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</dl>
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</div>
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<p>
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In the following, when I say portable, I will refer to "portable among ISO
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14882-implementations". On the other hand, if I say "backportable" or
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"conservative", I am talking about "compiles with older
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libstdc++-implementations".
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</p>
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<div class="section">
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<div class="titlepage"><div><h2 class="title" style="clear: both">
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<a name="sec-nsstd"></a>Namespace std::</h2></div></div>
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<p>
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The latest C++-standard (ISO-14882) requires that the standard
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C++-library is defined in namespace std::. Thus, in order to use
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classes from the standard C++-library, you can do one of three
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things:
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<div class="itemizedlist"><ul type="disc">
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<li><p>wrap your code in <b>namespace std {
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... }</b> => This is not an option because only symbols
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from the standard c++-library are defined in namespace std::.
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</p></li>
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<li><p>put a kind of
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<span class="emphasis"><i>using-declaration</i></span> in your source (either
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<b>using namespace std;</b> or i.e. <b>using
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std::string;</b>) => works well for source-files, but
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cannot be used in header-files.
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</p></li>
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<li><p>use a <span class="emphasis"><i>fully qualified name</i></span> for
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each libstdc++-symbol (i.e. <b>std::string</b>,
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<b>std::cout</b>) => can always be used
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</p></li>
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</ul></div>
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</p>
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<p>
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Because there are many compilers which still use an implementation
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that does not have the standard C++-library in namespace
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<b>std::</b>, some care is required to support these as
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well.
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</p>
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<p>
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Namespace back-portability-issues are generally not a problem with
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g++, because versions of g++ that do not have libstdc++ in
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<b>std::</b> use <b>-fno-honor-std</b>
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(ignore <b>std::</b>, <b>:: = std::</b>) by
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default. That is, the responsibility for enabling or disabling
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<b>std::</b> is on the user; the maintainer does not have
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to care about it. This probably applies to some other compilers as
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well.
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</p>
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<p>
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The following sections list some possible solutions to support compilers
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that cannot ignore std::.
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</p>
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<div class="section">
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<div class="titlepage"><div><h3 class="title">
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<a name="sec-gtkmm-hack"></a>Using <span class="emphasis"><i>namespace
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composition</i></span> if the project uses a separate
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namespace</h3></div></div>
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<p>
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<a href="http://gtkmm.sourceforge.net" target="_top">Gtk--</a> defines
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most of its classes in namespace Gtk::. Thus, it was possible to
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adapt Gtk-- to namespace std:: by using a C++-feature called
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<span class="emphasis"><i>namespace composition</i></span>. This is what happens if
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you put a <span class="emphasis"><i>using</i></span>-declaration into a
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namespace-definition: the imported symbol(s) gets imported into the
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currently active namespace(s). For example:
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<pre class="programlisting">
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namespace Gtk {
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using std::string;
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class Window { ... }
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}
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</pre>
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In this example, <b>std::string</b> gets imported into
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namespace Gtk::. The result is that you don't have to use
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<b>std::string</b> in this header, but still
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<b>std::string</b> does not get imported into
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the global namespace (::) unless the user does
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<b>using namespace Gtk;</b> (which is not recommended
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practice for Gtk--, so it is not a problem). Additionally, the
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<b>using</b>-declarations are wrapped in macros that
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are set based on autoconf-tests to either "" or i.e. <b>using
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std::string;</b> (depending on whether the system has
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libstdc++ in <b>std::</b> or not). (ideas from
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<tt><<a href="mailto:llewelly@dbritsch.dsl.xmission.com">llewelly@dbritsch.dsl.xmission.com</a>></tt>, Karl Nelson
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<tt><<a href="mailto:kenelson@ece.ucdavis.edu">kenelson@ece.ucdavis.edu</a>></tt>)
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><h3 class="title">
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<a name="sec-emptyns"></a>Defining an empty namespace std</h3></div></div>
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<p>
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By defining an (empty) namespace <b>std::</b> before
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using it, you avoid getting errors on systems where no part of the
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library is in namespace std:
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<pre class="programlisting">
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namespace std { }
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using namespace std;
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</pre>
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><h3 class="title">
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<a name="sec-avoidfqn"></a>Avoid to use fully qualified names
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(i.e. std::string)</h3></div></div>
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<p>
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If some compilers complain about <b>using
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std::string;</b>, and if the "hack" for gtk-- mentioned above
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does not work, then I see two solutions:
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<div class="itemizedlist"><ul type="disc">
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<li><p>
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Define <b>std::</b> as a macro if the compiler
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doesn't know about <b>std::</b>.
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<pre class="programlisting">
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#ifdef OLD_COMPILER
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#define std
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#endif
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</pre>
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(thanks to Juergen Heinzl who posted this solution on
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gnu.gcc.help)
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</p></li>
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<li><p>
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Define a macro NS_STD, which is defined to
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either "" or "std"
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based on an autoconf-test. Then you should be able to use
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<b>NS_STD::string</b>, which will evaluate to
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<b>::string</b> ("string in the global namespace") on
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systems that do not put string in std::. (This is untested)
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</p></li>
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</ul></div>
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><h3 class="title">
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<a name="sec-osprojects"></a>How some open-source-projects deal
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with this</h3></div></div>
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<p>
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This information was gathered around May 2000. It may not be correct
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by the time you read this.
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</p>
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<div class="table">
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<p><b>Table 1. Namespace std:: in Open-Source programs</b></p>
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<table summary="Namespace std:: in Open-Source programs" border="1">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<tbody>
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<tr>
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<td><a href="http://www.clanlib.org" target="_top">clanlib</a></td>
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<td>usual</td>
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</tr>
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<tr>
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<td><a href="http://pingus.seul.org" target="_top">pingus</a></td>
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<td>usual</td>
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</tr>
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<tr>
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<td><a href="http://www.mozilla.org" target="_top">mozilla</a></td>
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<td>usual</td>
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</tr>
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<tr>
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<td><a href="http://libsigc.sourceforge.net" target="_top">
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libsigc++</a></td>
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<td>conservative-impl</td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="table">
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<p><b>Table 2. Notations for categories</b></p>
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<table summary="Notations for categories" border="1">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<tbody>
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<tr>
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<td>usual</td>
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<td>mostly fully qualified names and some
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using-declarations (but not in headers)</td>
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</tr>
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<tr>
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<td>none</td>
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<td>no namespace std at all</td>
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</tr>
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<tr>
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<td>conservative-impl</td>
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<td>wrap all
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namespace-handling in macros to support compilers without
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namespace-support (no libstdc++ used in headers)</td>
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</tr>
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</tbody>
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</table>
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</div>
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<p>
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As you can see, this currently lacks an example of a project
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which uses libstdc++-symbols in headers in a back-portable way
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(except for Gtk--: see the <a href="#sec-gtkmm-hack" title="Using namespace
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composition if the project uses a separate
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namespace">section on the gtkmm-hack</a>).
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</p>
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</div>
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</div>
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<div class="section">
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<div class="titlepage"><div><h2 class="title" style="clear: both">
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<a name="sec-nocreate"></a>there is no ios::nocreate/ios::noreplace
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in ISO 14882</h2></div></div>
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<p>
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I have seen <b>ios::nocreate</b> being used for
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input-streams, most probably because the author thought it would be
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more correct to specify nocreate "explicitly". So you can simply
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leave it out for input-streams.
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</p>
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<p>
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For output streams, "nocreate" is probably the default, unless you
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specify <b>std::ios::trunc</b> ? To be safe, you can open
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the file for reading, check if it has been opened, and then decide
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whether you want to create/replace or not. To my knowledge, even
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older implementations support <b>app</b>,
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<b>ate</b> and <b>trunc</b> (except for
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<b>app</b> ?).
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><h2 class="title" style="clear: both">
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<a name="sec-stream::attach"></a><b>stream::attach(int
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fd)</b> is not in the standard any more</h2></div></div>
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<p>
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Phil Edwards <tt><<a href="mailto:pedwards@disaster.jaj.com">pedwards@disaster.jaj.com</a>></tt> writes:
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It was considered and rejected. Not all environments use file
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descriptors. Of those that do, not all of them use integers to represent
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them.
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</p>
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<p>
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When using libstdc++-v3, you can use
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<div class="funcsynopsis">
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<pre class="funcsynopsisinfo">
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#include <fstream>
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</pre>
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<p><code><code class="funcdef">
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<b class="fsfunc">basic_filebuf<...>::basic_filebuf<...>
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</b>
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</code>(<var class="pdparam">file</var>, <var class="pdparam">mode</var>, <var class="pdparam">size</var>);<br>__c_file_type* <var class="pdparam">file</var>;<br>ios_base::open_mode <var class="pdparam">mode</var>;<br>int <var class="pdparam">size</var>;</code></p>
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</div>
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but the the signature of this constructor has changed often, and
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it might change again. For the current state of this, check
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<a href="../ext/howto.html" target="_top">the howto for extensions</a>.
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</p>
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<p>
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For a portable solution (among systems which use
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filedescriptors), you need to implement a subclass of
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<b>std::streambuf</b> (or
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<b>std::basic_streambuf<..></b>) which opens a file
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given a descriptor, and then pass an instance of this to the
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stream-constructor. For an example of this, refer to
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<a href="http://www.josuttis.com/cppcode/fdstream.html" target="_top">fdstream example</a>
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by Nicolai Josuttis.
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</p>
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</div>
|
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<div class="section">
|
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<div class="titlepage"><div><h2 class="title" style="clear: both">
|
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<a name="sec-headers"></a>The new headers</h2></div></div>
|
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<p>
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All new headers can be seen in this <a href="headers_cc.txt" target="_top">
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source-code</a>.
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</p>
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<p>
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The old C++-headers (iostream.h etc.) are available, but gcc generates
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a warning that you are using deprecated headers.
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</p>
|
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<div class="section">
|
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<div class="titlepage"><div><h3 class="title">
|
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<a name="sec-cheaders"></a>New headers replacing C-headers</h3></div></div>
|
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<p>
|
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You should not use the C-headers (except for system-level
|
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headers) from C++ programs. Instead, you should use a set of
|
|
headers that are named by prepending 'c' and, as usual,
|
|
omitting the extension (.h). For example, instead of using
|
|
<tt><math.h></tt>, you
|
|
should use <tt><cmath></tt>. In some cases this has
|
|
the advantage that the C++-header is more standardized than
|
|
the C-header (i.e. <tt><ctime></tt> (almost)
|
|
corresponds to either <tt><time.h></tt> or <tt><sys/time.h></tt>).
|
|
|
|
The standard specifies that if you include the C-style header
|
|
(<tt><math.h></tt> in
|
|
this case), the symbols will be available both in the global
|
|
namespace and in namespace <b>std::</b> (but
|
|
libstdc++ does not yet have fully compliant headers) On the
|
|
other hand, if you include only the new header (i.e. <tt><cmath></tt>), the symbols
|
|
will only be defined in namespace <b>std::</b>
|
|
(and macros will be converted to inline-functions).
|
|
</p>
|
|
<p>
|
|
For more information on this, and for information on how the
|
|
GNU C++ implementation might reuse ("shadow") the C
|
|
library-functions, have a look at <a href="http://www.cantrip.org/cheaders.html" target="_top">
|
|
www.cantrip.org</a>.
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h3 class="title">
|
|
<a name="sec-fstream-header"></a>
|
|
<tt><fstream></tt> does
|
|
not define <b>std::cout</b>,
|
|
<b>std::cin</b> etc.</h3></div></div>
|
|
<p>
|
|
In earlier versions of the standard,
|
|
<tt><fstream.h></tt>,
|
|
<tt><ostream.h></tt>
|
|
and <tt><istream.h></tt>
|
|
used to define
|
|
<b>cout</b>, <b>cin</b> and so on. Because
|
|
of the templatized iostreams in libstdc++-v3, you need to include
|
|
<tt><iostream></tt>
|
|
explicitly to define these.
|
|
</p>
|
|
</div>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-iterators"></a>Iterators</h2></div></div>
|
|
<p>
|
|
The following are not proper uses of iterators, but may be working
|
|
fixes for existing uses of iterators.
|
|
<div class="itemizedlist"><ul type="disc">
|
|
<li><p>you cannot do
|
|
<b>ostream::operator<<(iterator)</b> to
|
|
print the address of the iterator => use
|
|
<b>operator<< &*iterator</b> instead ?
|
|
</p></li>
|
|
<li><p>you cannot clear an iterator's reference
|
|
(<b>iterator = 0</b>) => use
|
|
<b>iterator = iterator_type();</b> ?
|
|
</p></li>
|
|
<li><p>
|
|
<b>if (iterator)</b> won't work any
|
|
more => use <b>if (iterator != iterator_type())</b>
|
|
?</p></li>
|
|
</ul></div>
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-macros"></a>
|
|
Libc-macros (i.e. <b>isspace</b> from
|
|
<tt><cctype></tt>)</h2></div></div>
|
|
<p>
|
|
Glibc 2.0.x and 2.1.x define the
|
|
<tt><ctype.h></tt>
|
|
-functionality as macros (isspace, isalpha etc.). Libstdc++-v3
|
|
"shadows" these macros as described in the <a href="#sec-cheaders" title="New headers replacing C-headers">section about
|
|
c-headers</a>.
|
|
</p>
|
|
<p>
|
|
Older implementations of libstdc++ (g++-2 for egcs 1.x and g++-3
|
|
for gcc 2.95.x), however, keep these functions as macros, and so it
|
|
is not back-portable to use fully qualified names. For example:
|
|
<pre class="programlisting">
|
|
#include <cctype>
|
|
int main() { std::isspace('X'); }
|
|
</pre>
|
|
will result in something like this (unless using g++-v3):
|
|
<pre class="programlisting">
|
|
std:: (__ctype_b[(int) ( ( 'X' ) )] & (unsigned short int)
|
|
_ISspace ) ;
|
|
</pre>
|
|
</p>
|
|
<p>
|
|
One solution I can think of is to test for -v3 using
|
|
autoconf-macros, and define macros for each of the C-functions
|
|
(maybe that is possible with one "wrapper" macro as well ?).
|
|
</p>
|
|
<p>
|
|
Another solution which would fix g++ is to tell the user to modify a
|
|
header-file so that g++-2 (egcs 1.x) and g++-3 (gcc 2.95.x) define a
|
|
macro which tells <tt><ctype.h></tt> to define functions
|
|
instead of macros:
|
|
<pre class="programlisting">
|
|
// This keeps isalnum, et al from being propagated as macros.
|
|
#if __linux__
|
|
#define __NO_CTYPE 1
|
|
#endif
|
|
|
|
[ now include <ctype.h> ]
|
|
</pre>
|
|
</p>
|
|
<p>
|
|
Another problem arises if you put a <b>using namespace
|
|
std;</b> declaration at the top, and include <tt><ctype.h></tt>. This will result in
|
|
ambiguities between the definitions in the global namespace
|
|
(<tt><ctype.h></tt>) and the
|
|
definitions in namespace <b>std::</b>
|
|
(<b><cctype></b>).
|
|
</p>
|
|
<p>
|
|
The solution to this problem was posted to the libstdc++-v3
|
|
mailing-list:
|
|
Benjamin Kosnik <tt><<a href="mailto:bkoz@redhat.com">bkoz@redhat.com</a>></tt> writes:
|
|
‘
|
|
--enable-cshadow-headers is currently broken. As a result, shadow
|
|
headers are not being searched....
|
|
’
|
|
This is now outdated, but gcc 3.0 still does not have fully
|
|
compliant "shadow headers".
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-stream-state"></a>State of streams</h2></div></div>
|
|
<p>
|
|
At least some older implementations don't have
|
|
<b>std::ios_base</b>, so you should use
|
|
<b>std::ios::badbit</b>, <b>std::ios::failbit</b>
|
|
and <b>std::ios::eofbit</b> and
|
|
<b>std::ios::goodbit</b>.
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-vector-at"></a>vector::at is missing (i.e. gcc 2.95.x)</h2></div></div>
|
|
<p>
|
|
One solution is to add an autoconf-test for this:
|
|
<pre class="programlisting">
|
|
AC_MSG_CHECKING(for container::at)
|
|
AC_TRY_COMPILE(
|
|
[
|
|
#include <vector>
|
|
#include <deque>
|
|
#include <string>
|
|
|
|
using namespace std;
|
|
],
|
|
[
|
|
deque<int> test_deque(3);
|
|
test_deque.at(2);
|
|
vector<int> test_vector(2);
|
|
test_vector.at(1);
|
|
string test_string("test_string");
|
|
test_string.at(3);
|
|
],
|
|
[AC_MSG_RESULT(yes)
|
|
AC_DEFINE(HAVE_CONTAINER_AT)],
|
|
[AC_MSG_RESULT(no)])
|
|
</pre>
|
|
If you are using other (non-GNU) compilers it might be a good idea
|
|
to check for <b>string::at</b> separately.
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-eof"></a>Using std::char_traits<char>::eof()</h2></div></div>
|
|
<p>
|
|
<pre class="programlisting">
|
|
#ifdef HAVE_CHAR_TRAITS
|
|
#define CPP_EOF std::char_traits<char>::eof()
|
|
#else
|
|
#define CPP_EOF EOF
|
|
#endif
|
|
</pre>
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-string-clear"></a>Using string::clear()/string::erase()</h2></div></div>
|
|
<p>
|
|
There are two functions for deleting the contents of a string:
|
|
<b>clear</b> and <b>erase</b> (the latter
|
|
returns the string).
|
|
<pre class="programlisting">
|
|
void
|
|
clear() { _M_mutate(0, this->size(), 0); }
|
|
</pre>
|
|
<pre class="programlisting">
|
|
basic_string&
|
|
erase(size_type __pos = 0, size_type __n = npos)
|
|
{
|
|
return this->replace(_M_check(__pos), _M_fold(__pos, __n),
|
|
_M_data(), _M_data());
|
|
}
|
|
</pre>
|
|
The implementation of <b>erase</b> seems to be more
|
|
complicated (from libstdc++-v3), but <b>clear</b> is not
|
|
implemented in gcc 2.95.x's libstdc++, so you should use
|
|
<b>erase</b> (which is probably faster than
|
|
<b>operator=(charT*)</b>).
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-scan-form"></a>GNU Extensions ostream::form and istream::scan</h2></div></div>
|
|
<p>
|
|
These are not supported any more - use
|
|
<a href="#sec-stringstream" title="Using stringstreams">
|
|
stringstreams</a> instead.
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-stringstream"></a>Using stringstreams</h2></div></div>
|
|
<p>
|
|
Libstdc++-v3 provides the new
|
|
<b>i/ostringstream</b>-classes, (<tt><sstream></tt>), but for compatibility
|
|
with older implementations you still have to use
|
|
<b>i/ostrstream</b> (<tt><strstream></tt>):
|
|
<pre class="programlisting">
|
|
#ifdef HAVE_SSTREAM
|
|
#include <sstream>
|
|
#else
|
|
#include <strstream>
|
|
#endif
|
|
</pre>
|
|
<div class="itemizedlist"><ul type="disc">
|
|
<li><p> <b>strstream</b> is considered to be
|
|
deprecated
|
|
</p></li>
|
|
<li><p> <b>strstream</b> is limited to
|
|
<b>char</b>
|
|
</p></li>
|
|
<li><p> with <b>ostringstream</b> you don't
|
|
have to take care of terminating the string or freeing its
|
|
memory
|
|
</p></li>
|
|
<li><p> <b>istringstream</b> can be re-filled
|
|
(clear(); str(input);)
|
|
</p></li>
|
|
</ul></div>
|
|
</p>
|
|
<p>
|
|
You can then use output-stringstreams like this:
|
|
<pre class="programlisting">
|
|
#ifdef HAVE_SSTREAM
|
|
std::ostringstream oss;
|
|
#else
|
|
std::ostrstream oss;
|
|
#endif
|
|
oss << "Name=" << m_name << ", number=" << m_number << std::endl;
|
|
...
|
|
#ifndef HAVE_SSTREAM
|
|
oss << std::ends; // terminate the char*-string
|
|
#endif
|
|
// str() returns char* for ostrstream and a string for ostringstream
|
|
// this also causes ostrstream to think that the buffer's memory
|
|
// is yours
|
|
m_label.set_text(oss.str());
|
|
#ifndef HAVE_SSTREAM
|
|
// let the ostrstream take care of freeing the memory
|
|
oss.freeze(false);
|
|
#endif
|
|
</pre>
|
|
</p>
|
|
<p>
|
|
Input-stringstreams can be used similarly:
|
|
<pre class="programlisting">
|
|
std::string input;
|
|
...
|
|
#ifdef HAVE_SSTREAM
|
|
std::istringstream iss(input);
|
|
#else
|
|
std::istrstream iss(input.c_str());
|
|
#endif
|
|
int i;
|
|
iss >> i;
|
|
</pre>
|
|
One (the only?) restriction is that an istrstream cannot be re-filled:
|
|
<pre class="programlisting">
|
|
std::istringstream iss(numerator);
|
|
iss >> m_num;
|
|
// this is not possible with istrstream
|
|
iss.clear();
|
|
iss.str(denominator);
|
|
iss >> m_den;
|
|
</pre>
|
|
If you don't care about speed, you can put these conversions in
|
|
a template-function:
|
|
<pre class="programlisting">
|
|
template <class X>
|
|
void fromString(const string& input, X& any)
|
|
{
|
|
#ifdef HAVE_SSTREAM
|
|
std::istringstream iss(input);
|
|
#else
|
|
std::istrstream iss(input.c_str());
|
|
#endif
|
|
X temp;
|
|
iss >> temp;
|
|
if (iss.fail())
|
|
throw runtime_error(..)
|
|
any = temp;
|
|
}
|
|
</pre>
|
|
Another example of using stringstreams is in <a href="../21_strings/howto.html" target="_top">this howto</a>.
|
|
</p>
|
|
<p>
|
|
I have read the Josuttis book on Standard C++, so some information
|
|
comes from there. Additionally, there is information in
|
|
"info iostream", which covers the old implementation that gcc 2.95.x
|
|
uses.
|
|
</p>
|
|
</div>
|
|
<div class="section">
|
|
<div class="titlepage"><div><h2 class="title" style="clear: both">
|
|
<a name="sec-about"></a>About...</h2></div></div>
|
|
<p>
|
|
Please send any experience, additions, corrections or questions to
|
|
<a href="mailto:fnatter@gmx.net" target="_top">fnatter@gmx.net</a> or for
|
|
discussion to the libstdc++-v3-mailing-list.
|
|
</p>
|
|
</div>
|
|
</div></body>
|
|
</html>
|