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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Dual ABI</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_macros.html" title="Macros" /><link rel="next" href="using_namespaces.html" title="Namespaces" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Dual ABI</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="using_namespaces.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.abi"></a>Dual ABI</h2></div></div></div><p> In the GCC 5.1 release libstdc++ introduced a new library ABI that
includes new implementations of <code class="classname">std::string</code> and
<code class="classname">std::list</code>. These changes were necessary to conform
to the 2011 C++ standard which forbids Copy-On-Write strings and requires
lists to keep track of their size.
</p><p> In order to maintain backwards compatibility for existing code linked
to libstdc++ the library's soname has not changed and the old
implementations are still supported in parallel with the new ones.
This is achieved by defining the new implementations in an inline namespace
so they have different names for linkage purposes, e.g. the new version of
<code class="classname">std::list<int></code> is actually defined as
<code class="classname">std::__cxx11::list<int></code>. Because the symbols
for the new implementations have different names the definitions for both
versions can be present in the same library.
</p><p> The <span class="symbol">_GLIBCXX_USE_CXX11_ABI</span> macro (see
<a class="xref" href="using_macros.html" title="Macros">Macros</a>) controls whether
the declarations in the library headers use the old or new ABI.
So the decision of which ABI to use can be made separately for each
source file being compiled.
Using the default configuration options for GCC the default value
of the macro is <code class="literal">1</code> which causes the new ABI to be active,
so to use the old ABI you must explicitly define the macro to
<code class="literal">0</code> before including any library headers.
(Be aware that some GNU/Linux distributions configure GCC 5 differently so
that the default value of the macro is <code class="literal">0</code> and users must
define it to <code class="literal">1</code> to enable the new ABI.)
</p><p> Although the changes were made for C++11 conformance, the choice of ABI
to use is independent of the <code class="option">-std</code> option used to compile
your code, i.e. for a given GCC build the default value of the
<span class="symbol">_GLIBCXX_USE_CXX11_ABI</span> macro is the same for all dialects.
This ensures that the <code class="option">-std</code> does not change the ABI, so
that it is straightforward to link C++03 and C++11 code together.
</p><p> Because <code class="classname">std::string</code> is used extensively
throughout the library a number of other types are also defined twice,
including the stringstream classes and several facets used by
<code class="classname">std::locale</code>. The standard facets which are always
installed in a locale may be present twice, with both ABIs, to ensure that
code like
<code class="code">std::use_facet<std::time_get<char>>(locale);</code>
will work correctly for both <code class="classname">std::time_get</code> and
<code class="classname">std::__cxx11::time_get</code> (even if a user-defined
facet that derives from one or other version of
<code class="classname">time_get</code> is installed in the locale).
</p><p> Although the standard exception types defined in
<code class="filename"><stdexcept></code> use strings, most
are not defined twice, so that a <code class="classname">std::out_of_range</code>
exception thrown in one file can always be caught by a suitable handler in
another file, even if the two files are compiled with different ABIs.
</p><p> One exception type does change when using the new ABI, namely
<code class="classname">std::ios_base::failure</code>.
This is necessary because the 2011 standard changed its base class from
<code class="classname">std::exception</code> to
<code class="classname">std::system_error</code>, which causes its layout to change.
Exceptions due to iostream errors are thrown by a function inside
<code class="filename">libstdc++.so</code>, so whether the thrown
exception uses the old <code class="classname">std::ios_base::failure</code> type
or the new one depends on the ABI that was active when
<code class="filename">libstdc++.so</code> was built,
<span class="emphasis"><em>not</em></span> the ABI active in the user code that is using
iostreams.
This means that for a given build of GCC the type thrown is fixed.
In current releases the library throws a special type that can be caught
by handlers for either the old or new type,
but for GCC 7.1, 7.2 and 7.3 the library throws the new
<code class="classname">std::ios_base::failure</code> type,
and for GCC 5.x and 6.x the library throws the old type.
Catch handlers of type <code class="classname">std::ios_base::failure</code>
will only catch the exceptions if using a newer release,
or if the handler is compiled with the same ABI as the type thrown by
the library.
Handlers for <code class="classname">std::exception</code> will always catch
iostreams exceptions, because the old and new type both inherit from
<code class="classname">std::exception</code>.
</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.abi.trouble"></a>Troubleshooting</h3></div></div></div><p> If you get linker errors about undefined references to symbols
that involve types in the <code class="code">std::__cxx11</code> namespace or the tag
<code class="code">[abi:cxx11]</code> then it probably indicates that you are trying to
link together object files that were compiled with different values for the
<span class="symbol">_GLIBCXX_USE_CXX11_ABI</span> macro. This commonly happens when
linking to a third-party library that was compiled with an older version
of GCC. If the third-party library cannot be rebuilt with the new ABI then
you will need to recompile your code with the old ABI.
</p><p> Not all uses of the new ABI will cause changes in symbol names, for
example a class with a <code class="classname">std::string</code> member variable
will have the same mangled name whether compiled with the old or new ABI.
In order to detect such problems the new types and functions are
annotated with the <span class="property">abi_tag</span> attribute, allowing the
compiler to warn about potential ABI incompatibilities in code using them.
Those warnings can be enabled with the <code class="option">-Wabi-tag</code> option.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="using_namespaces.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Macros </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Namespaces</td></tr></table></div></body></html>
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