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<!-- $Id$ -->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Symbol Versioning in ACE</title>
</head>
<body>
<h3>Symbol Versioning in ACE</h3>
<p>
To provide a means for ACE-based application developers to avoid
symbol conflicts when multiple versions of ACE are linked to an
application ACE supports <em>versioned namespaces</em>. When
enabled (disabled by default), ACE's versioned namespace support
causes all ACE symbols (classes, free functions, etc) to be
placed within a C++ namespace of the form "<code>namespace
ACE_5_5_1</code>". For example, the <code>ACE_Reactor</code>
would end up being placed in the versioned namespace like so:
</p>
<blockquote>
<code>
<pre>
namespace ACE_5_5_1
{
class ACE_Reactor
{
...
};
}
using namespace ACE_5_5_1;
</pre>
</code>
</blockquote>
<p>
Notice that a <code>using</code> clause exposes the ACE types
embedded in the versioned namespace back to the global
namespace. This maximizes source code compatibility. ACE
itself does this through the use of two macros:
</p>
<ul>
<li>
<code>ACE_BEGIN_VERSIONED_NAMESPACE_DECL</code><br>
<ul>
<li>
Expands to "<code>namespace ACE_VERSIONED_NAMESPACE NAME
{</code>", where
<code>ACE_VERSIONED_NAMESPACE_NAME</code> by defaults to
namespace name of the form
<code>ACE_<em>major</em>_<em>minor</em>_<em>beta</em></code>.
Users may override the default by defining the
<code>ACE_VERSIONED_NAMESPACE_NAME</code> preprocessor
symbol in their <code><strong>ace/config.h</strong></code>
header file.
</li>
</ul>
</li>
<li>
<code>ACE_END_VERSIONED_NAMESPACE_DECL</code>
<ul>
<li>
Expands to "<code>} using namespace
ACE_VERSIONED_NAMESPACE_NAME;</code>", where
<code>ACE_VERSIONED_NAMESPACE_NAME</code> is described
above.
</li>
</ul>
</li>
</ul>
<h2>Things ACE-based Application Developers Should Know</h2>
<p>
Every effort has been made to make the versioned namespace
support in ACE as transparent as possible, including transparent
versioned symbol support in the ACE_Service_Configurator when
the ACE_Service_Configurator macros, such as <em>e.g.</em>,
<code>ACE_FACTORY_DECLARE</code>, are used appropriately. No
changes to service configurator directives are necessary. For
example, the <code>ACE_Service_Configurator</code> will
transparently mangle the factory function name in a service
configurator directive on-the-fly, meaning it will only load a
"versioned" factory function name. This allows multiple service
object factory functions, for example, to coexist in the same
process space.
</p>
<p>
There is, however, at least one caveat with respect to source
code compatibility: any forward declarations of ACE symbols must
also be placed within the versioned namespace. For example, if
you have a forward declaration for <code>ACE_Reactor</code> in
your application code, you will need to place it within the
configured ACE versioned namespace as follows:
</p>
<blockquote>
<code>
<pre>
ACE_BEGIN_VERSIONED_NAMESPACE_DECL
class ACE_Reactor;
ACE_END_VERSIONED_NAMESPACE_DECL
</pre>
</code>
</blockquote>
<p>
This must only be done once, as these macros hide the versioned
namespace name details from the application. Alternatively, you
could place the forward declaration in a namespace that is an
alias of the ACE versioned namespace, <em>e.g.</em>:
</p>
<blockquote>
<code>
<pre>
namespace Foo = ACE_VERSIONED_NAMESPACE_NAME;</code>
namespace Foo {
class ACE_Reactor;
}
using namespace Foo;
</pre>
</code>
</blockquote>
<p>
Versioned namespace support in ACE may be enabled by adding
<code>versioned_namespace=1</code> to your MPC
<code><strong>default.features</strong></code> file.
</p>
<h2>Things ACE Developers Should Know</h2>
<p>
ACE developers should place all ACE symbols that are potentially
exposed to the user, including forward declarations in a
versioned namespace using the
<code>ACE_BEGIN_VERSIONED_NAMESSPACE_DECL</code> and
<code>ACE_END_VERSIONED_NAMESPACE_DECL</code> macros. Free
functions that are declared to have a C calling convention
(<em>i.e.</em>, <code>extern "C"</code>) should have their names
mangled using the <code>ACE_PREPROC_CONCATENATE</code>
preprocessor. For example:
</p>
<blockquote>
<code>
<pre>
void ACE_func (void) { ... }
...
ACE_func(); // Call ACE_func()
</pre>
</code>
</blockquote>
<p>
becomes:
</p>
<blockquote>
<code>
<pre>
#if (defined (ACE_HAS_VERSIONED_NAMESPACE) \
&& ACE_HAS_VERSIONED_NAMESPACE == 1) \
&& !(defined (_MSC_VER) && _MSC_VER <= 1200)
// MSVC++ 6's preprocessor can't handle macro expansions
// required by the versioned namespace support. *sigh*
# define ACE_FOO_FUNC_NAME ACE_PREPROC_CONCATENATE(ACE_VERSIONED_NAMESPACE_NAME, _ACE_foo_func)
#else
# define ACE_FOO_FUNC_NAME ACE_foo_func
#endif /* ACE_HAS_VERSIONED_NAMESPACE == 1 */
...
void ACE_FOO_FUNC_NAME (void) { ... }
ACE_FOO_FUNC_NAME(); // Call mangled ACE_foo_func().
</pre>
</code>
</blockquote>
<p>
The <code>ACE_PREPROC_CONCATENATE</code> is used rather than a
straight <code>##</code> preprocessor concatenation since in the
latter case preprocessor symbols like
<code>ACE_VERSIONED_NAMESPACE_NAME</code> will not be expanded
if they are concatenated. <code>ACE_PREPROCE_CONCATENATE</code>
forces the preprocessor to expand them during the argument
prescan by calling a macro that itself calls another that
performs the actual concatenation.
</p>
<h3>General Guidelines</h3>
<ul>
<li>
Versioned namespace macro/support must be added to all new files
added to ACE.
</li>
<li>
Do not place include directives between
<code>ACE_BEGIN_VERSIONED_NAMESPACE_DECL</code> and
<code>ACE_END_VERSIONED_NAMESPACE_DECL</code> macros. Doing
so will cause nested namespace to be created, which is not
the desired affect.
</li>
<li>Be aware of preprocessor conditional blocks when placing the
versioned namespace macros. For example, if you open
versioned namespace within a given preprocessor condition
block, you'll most likely want to close it in the same
block.
</li>
<li>
If necessary, reopen and close the versioned namespace
multiple times in the same file by using the macros multiple
times to address the concerns described in the above two
items.
</li>
<li>
The <code>$ACE_ROOT/bin/fuzz.pl</code> script has a sanity
checking test for versioned namespaces that may be of use when
debugging nested namespace issues, and for detecting
<code>BEGIN</code>/<code>END</code> mismatches.
</li>
</ul>
<p>
Versioned namespace support in ACE may be enabled by adding
<code>versioned_namespace=1</code> to your MPC
<code><strong>default.features</strong></code> file. Additional
information about versioned namespaces is available from the <A
HREF="http://www.riverace.com/newsletters/March2006.htm">Riverace
website</A>.
</p>
<hr>
<address><a href="mailto:ossama@dre.vanderbilt.edu">Ossama Othman</a></address>
<!-- Created: Fri Mar 17 08:35:50 PST 2006 -->
<!-- hhmts start -->
Last modified: Fri Mar 17 20:47:41 PST 2006
<!-- hhmts end -->
</body>
</html>
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