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<H1 ALIGN=CENTER STYLE="margin-top: 0cm; text-decoration: none"><A HREF="http://www.openoffice.org/"><IMG SRC="../images/open_office_org_logo.gif" NAME="Graphic1" ALT="OpenOffice.org" ALIGN=RIGHT WIDTH=126 HEIGHT=53 BORDER=0></A><FONT COLOR="#ffffff"><FONT SIZE=6>Cachemaps</FONT></FONT></H1>
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<H3 ALIGN=LEFT STYLE="margin-top: 0cm; text-decoration: none"><FONT COLOR="#ffffff"><FONT SIZE=4>Contents</FONT></FONT></H3>
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<P STYLE="margin-bottom: 0cm"><A HREF="#Abstract">Abstract</A><BR>
<A HREF="#Solution1">Solution 1 (Single Mutex)</A><BR>
<A HREF="#Solution2">Solution 2 (Weak References)</A><BR>
<A HREF="#Solution3">Solution 3 (Best of Both Worlds)</A><BR>
<A HREF="#Measurements">Performance Measurements</A></P>
<P><BR></P>
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<H3 ALIGN=LEFT STYLE="margin-top: 0cm; text-decoration: none"><A NAME="Abstract"></A>
<FONT COLOR="#ffffff"><FONT SIZE=4>Abstract</FONT></FONT></H3>
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<P STYLE="margin-bottom: 0.1cm">I quite often have the need for a construct I will call a
<I>cachemap.</I> A cachemap is like an ordinary map (or hashmap), in
that it maps from keys (normally strings) to objects, but it also
controls the lifetime of the mapped objects. If a client requests an
object that is not yet instantiated, a (refcounted) instance is
created, inserted into the cachemap, and returned to the client. If
the client no longer needs the object (the refcount of the object
drops to zero), it is automatically removed from the cachemap. And if
a client requests an object that is already present in the cachemap
(because it was requested by a client before, and its refcount is
still positive), the present instance is returned. Skeleton C++ class
definitions for the cachemap and the objects it manages could look
like the following:</P>
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<PRE>class ObjectContainer // the cachemap
{
public:
rtl::Reference&lt; Object &gt; get(rtl::OUString const &amp; rKey);
};
class Object // the objects managed by the cachemap
{
public:
void acquire() SAL_THROW(());
void release() SAL_THROW(());
};</PRE>
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<P STYLE="margin-bottom: 0.1cm">The problems in implementing this are that (1) the cachemap must not
<FONT SIZE=2><FONT FACE="Courier New, monospace">acquire()</FONT></FONT>
the objects it currently has in its map (otherwise, the refcounts of
those objects could never drop to zero, and they would never be
removed from the map), and (2) in a multi-threaded environment there
is a problem when one thread does a final <FONT SIZE=2><FONT FACE="Courier New, monospace">release()</FONT></FONT>
of an object (refcount drops to zero and object is removed from the
map) and at the same time another thread happens to call <FONT SIZE=2><FONT FACE="Courier New, monospace">get()</FONT></FONT>
to request that very object.</P>
<P STYLE="margin-bottom: 0.1cm">Below, I describe three different implementations that solve these
problems. The first solution was my initial idea, but it tends to be
slow. The second is based on weak references, but it turned out to be
slow, too. The third combines ideas from the other two
solutions---and it turned out to be fast. So, if you are interested
in boiler plate code for a fast implementation of the cachemap
pattern, turn to <A HREF="#Solution3">the third solution</A>
directly.</P>
<P STYLE="margin-bottom: 0.1cm">The three solutions all follow the same conventions. They
implement an <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer<I>N</I></FONT></FONT>
(the cachemap, mapping from strings to objects) and a corresponding
<FONT SIZE=2><FONT FACE="Courier New, monospace">Object<I>N</I></FONT></FONT>
(the refcounted objects managed by the cachemap). The source code
presented here is also available within the ucb project
(<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/"><FONT SIZE=2><FONT FACE="Courier New, monospace">ucb/workben/cachemap</FONT></FONT></A>),
and it is presented here without the (somewhat longish) copyright
headers.</P>
<P ALIGN=LEFT><BR></P>
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<H3 ALIGN=LEFT STYLE="margin-top: 0cm; text-decoration: none"><A NAME="Solution1"></A>
<FONT COLOR="#ffffff"><FONT SIZE=4>Solution 1 (Single Mutex)</FONT></FONT></H3>
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<P STYLE="margin-bottom: 0.1cm">The
combined header file for both <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
and <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT> is
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobject1.hxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobject1.hxx</FONT></FONT></A>:</P>
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<PRE>#ifndef <A HREF="#validguardnames">INCLUDED_UCB_CACHEMAPOBJECT1_HXX</A>
#define INCLUDED_UCB_CACHEMAPOBJECT1_HXX
#ifndef _OSL_INTERLOCK_H_
#include &quot;osl/interlck.h&quot;
#endif
#ifndef _OSL_MUTEX_HXX_
#include &quot;osl/mutex.hxx&quot;
#endif
#ifndef _RTL_REF_HXX_
#include &quot;rtl/ref.hxx&quot;
#endif
#ifndef _SAL_TYPES_H_
#include &quot;sal/types.h&quot;
#endif
#ifndef _SALHELPER_SIMPLEREFERENCEOBJECT_HXX_
#include &quot;salhelper/simplereferenceobject.hxx&quot;
#endif
#ifndef <A HREF="#stdguards">INCLUDED_MAP</A>
#include &lt;map&gt;
#define INCLUDED_MAP
#endif
#ifndef INCLUDED_MEMORY
#include &lt;memory&gt;
#define INCLUDED_MEMORY
#endif
namespace rtl { class OUString; }
namespace ucb { namespace cachemap { class Object1; } }
namespace ucb { namespace cachemap {
class ObjectContainer1: <A HREF="#refcountedcontainer">public salhelper::SimpleReferenceObject</A>
{
public:
ObjectContainer1();
virtual ~ObjectContainer1() SAL_THROW(());
rtl::Reference&lt; Object1 &gt; get(rtl::OUString const &amp; rKey);
private:
typedef <A HREF="#map">std::map</A>&lt; rtl::OUString, Object1 * &gt; Map;
Map m_aMap;
osl::Mutex m_aMutex;
void releaseElement(Object1 * pElement) SAL_THROW(());
<A HREF="#friends">friend class Object1;</A> // to access Map, releaseElement()
};
class Object1
{
public:
inline void acquire() SAL_THROW(())
{ osl_incrementInterlockedCount(&amp;m_nRefCount); }
inline void release() SAL_THROW(())
{ m_xContainer-&gt;releaseElement(this); }
private:
rtl::Reference&lt; ObjectContainer1 &gt; m_xContainer;
ObjectContainer1::Map::iterator m_aContainerIt;
oslInterlockedCount m_nRefCount;
inline Object1(rtl::Reference&lt; ObjectContainer1 &gt; const &amp; rContainer);
inline ~Object1() SAL_THROW(());
Object1(Object1 &amp;); // not implemented
void operator =(Object1); // not implemented
friend class ObjectContainer1;
// to access m_aContainerIt, m_nRefCount, Object1(), ~Object1()
#if defined WNT
friend struct std::auto_ptr&lt; Object1 &gt; // to access ~Object
// work around compiler bug...
#else // WNT
<A HREF="#autoptrfriend">friend class std::auto_ptr&lt; Object1 &gt;;</A> // to access ~Object1()
#endif // WNT
};
} }
#endif // INCLUDED_UCB_CACHEMAPOBJECT1_HXX</PRE>
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<P STYLE="margin-bottom: 0.1cm">A few notes:</P>
<UL>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="validguardnames"></A>It is a good idea to use valid
C++ identifiers as include guards for header files. That is, do not
use identifiers that start with an underline followed by an upper
case letter (e.g., <FONT SIZE=2><FONT FACE="Courier New, monospace">_UCB_CACHEMAPOBJECT1_HXX_</FONT></FONT>)
or that contain two underlines in a row, because those identifiers
are reserved for internal use by the compiler and standard library
implementation. (<A HREF="http://cseng.aw.com/book/0,,0201633620,00.html">Lakos</A>,
section 2.4, suggests the <FONT SIZE=2><FONT FACE="Courier New, monospace">INCLUDED_<I>XXX</I></FONT></FONT>
style.)</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="stdguards"></A>Use the convention of (redundant but
compilation time reducing) external include guards for standard
header files, too. For this to work, it is necessary that everybody
use the same identifiers for the various standard header files, of
course. (See <A HREF="http://cseng.aw.com/book/0,,0201633620,00.html">Lakos</A>,
section 2.5.)</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="refcountedcontainer"></A><FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
turns out to be a refcounted object. This was not required in the
introduction of the cachemap pattern, but it is rather a consequence
of the chosen implementation (and it should not hurt any client,
either).</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="map"></A>In <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>,
a <FONT SIZE=2><FONT FACE="Courier New, monospace">std::map</FONT></FONT>
is used instead of a (de facto standard?) <FONT SIZE=2><FONT FACE="Courier New, monospace">std::hash_map</FONT></FONT>
that might be a bit faster. The implementation of <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
relies on the fact that iterators into a <FONT SIZE=2><FONT FACE="Courier New, monospace">std::map</FONT></FONT>
are not invalidated when calling <FONT SIZE=2><FONT FACE="Courier New, monospace">insert()</FONT></FONT>
or <FONT SIZE=2><FONT FACE="Courier New, monospace">erase()</FONT></FONT>.
There seems to be no corresponding guarantee for <FONT SIZE=2><FONT FACE="Courier New, monospace">std::hash_map</FONT></FONT>.</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="friends"></A><FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
and <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>
need to be mutual friends. If one entity is a friend of another,
these entities are strongly coupled, and it seems always best to
keep them together in a single header file (thereby breaking the
rule to have a separate header file for each class).</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="autoptrfriend"></A>Logically, only <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
is allowed to create and destroy instances of <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>,
hence the design decision to make the constructor and destructor of
<FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>
private and make <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
a friend of <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>.
But a <FONT SIZE=2><FONT FACE="Courier New, monospace">std::auto_ptr&lt;
Object1 &gt;</FONT></FONT> is needed at one place in the
implementation, and that <FONT SIZE=2><FONT FACE="Courier New, monospace">std::auto_ptr</FONT></FONT>
also needs access to the destructor of <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>.
It seemed best to simply make <FONT SIZE=2><FONT FACE="Courier New, monospace">std::auto_ptr&lt;
Object1 &gt;</FONT></FONT> a friend of <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>,
too, in this situation (though too many friends can be an indication
of bad design.) Alas, the Microsoft compiler used on the Windows x86 platform
(<FONT SIZE=2><FONT FACE="Courier New, monospace">WNT</FONT></FONT> define)
needs to see <FONT SIZE=2><FONT FACE="Courier New, monospace">struct</FONT></FONT> (or nothing) in this friend declaration,
instead of the correct <FONT SIZE=2><FONT FACE="Courier New, monospace">class</FONT></FONT>, so some
platform-dependent code is needed here (and in retrospect it might seem better to make the destructor of
<FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT> public and get rid of all this).</P>
</UL>
<P STYLE="margin-bottom: 0.1cm">The corresponding implementation file is
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobject1.cxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobject1.cxx</FONT></FONT></A>:</P>
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<PRE>#ifndef INCLUDED_UCB_CACHEMAPOBJECT1_HXX
<A HREF="#orderofincludes">#include &quot;cachemapobject1.hxx&quot;</A>
#endif
#ifndef _OSL_DIAGNOSE_H_
#include &quot;osl/diagnose.h&quot;
#endif
#ifndef _OSL_INTERLOCK_H_
#include &quot;osl/interlck.h&quot;
#endif
#ifndef _OSL_MUTEX_HXX_
#include &quot;osl/mutex.hxx&quot;
#endif
#ifndef _RTL_REF_HXX_
#include &quot;rtl/ref.hxx&quot;
#endif
#ifndef _RTL_USTRING_HXX_
#include &quot;rtl/ustring.hxx&quot;
#endif
#ifndef INCLUDED_MEMORY
#include &lt;memory&gt;
#define INCLUDED_MEMORY
#endif
using ucb::cachemap::Object1;
using ucb::cachemap::ObjectContainer1;
inline
Object1::Object1(rtl::Reference&lt; ObjectContainer1 &gt; const &amp; rContainer):
m_xContainer(rContainer),
m_nRefCount(0)
{
OSL_ASSERT(m_xContainer.is());
}
inline Object1::~Object1() SAL_THROW(())
{}
void ObjectContainer1::releaseElement(Object1 * pElement) SAL_THROW(())
{
OSL_ASSERT(pElement);
bool bDelete = false;
{
osl::MutexGuard aGuard(m_aMutex);
if (osl_decrementInterlockedCount(&amp;pElement-&gt;m_nRefCount) == 0)
{
m_aMap.erase(pElement-&gt;m_aContainerIt);
bDelete = true;
}
}
<A HREF="#minimalmutex">if (bDelete)</A>
delete pElement;
}
ObjectContainer1::ObjectContainer1()
{}
ObjectContainer1::~ObjectContainer1() SAL_THROW(())
{}
rtl::Reference&lt; Object1 &gt; ObjectContainer1::get(rtl::OUString const &amp; rKey)
{
osl::MutexGuard aGuard(m_aMutex);
Map::iterator aIt(m_aMap.find(rKey));
if (aIt == m_aMap.end())
{
<A HREF="#autoptr">std::auto_ptr&lt; Object1 &gt;</A> xElement(new Object1(this));
aIt = m_aMap.insert(Map::value_type(rKey, xElement.get())).first;
aIt-&gt;second-&gt;m_aContainerIt = aIt;
xElement.release();
}
return aIt-&gt;second;
}</PRE>
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<UL>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="orderofincludes"></A>It is always a good idea to
include the header file that belongs to an implementation file first
in that file. That way, there is an automatic check that each header
file is self-contained (for almost every header, there is a
corresponding implementation file, and compilation of that file
fails if the header is not self-contained). (See <A HREF="http://cseng.aw.com/book/0,,0201633620,00.html">Lakos</A>,
section 3.2.)</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="minimalmutex"></A>Note how <FONT SIZE=2><FONT FACE="Courier New, monospace">releaseElement()</FONT></FONT>
deletes the element only after the mutex has been released. It is
good practice to do as little as absolutely necessary during the
time a mutex is locked.</P>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="autoptr"></A>Note how a <FONT SIZE=2><FONT FACE="Courier New, monospace">std::auto_ptr&lt;
Object1 &gt;</FONT></FONT> is used to ensure the newly created
<FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>
is properly deleted in case the <FONT SIZE=2><FONT FACE="Courier New, monospace">insert()</FONT></FONT>
happens to throw an exception.</P>
</UL>
<P STYLE="margin-bottom: 0.1cm">The way the problems mentionend in the introduction are solved
here is that <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>
only holds pointers to instances of <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1</FONT></FONT>,
not refcounting references, and that <FONT SIZE=2><FONT FACE="Courier New, monospace">Object1::release()</FONT></FONT>
is simply forwarded to <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1::releaseElement()</FONT></FONT>.
The <FONT SIZE=2><FONT FACE="Courier New, monospace">releaseElement()</FONT></FONT>
method checks whether the refcount of the object drops to zero and
removes it from the map, if appropriate. The methods
<FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1::releaseElement()</FONT></FONT>
and <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1::get()</FONT></FONT>
share a mutex, so that there can never be the situation that one
thread tries to get an object another thread is about to delete. The
disadvantage of this solution is that every call to
<FONT SIZE=2><FONT FACE="Courier New, monospace">Object1::release()</FONT></FONT>
needs to acquire a mutex, which is quite expensive on typical
platforms.</P>
<P ALIGN=LEFT><BR></P>
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<TD WIDTH=100% BGCOLOR="#666699">
<H3 ALIGN=LEFT STYLE="margin-top: 0cm; text-decoration: none"><A NAME="Solution2"></A>
<FONT COLOR="#ffffff"><FONT SIZE=4>Solution 2 (Weak References)</FONT></FONT></H3>
</TD>
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<TD WIDTH=100%>
<P STYLE="margin-bottom: 0.1cm">The UNO concept of weak references offers itself as another
solution to the problem: <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer2</FONT></FONT>
holds weak references to instances of <FONT SIZE=2><FONT FACE="Courier New, monospace">Object2</FONT></FONT>,
and the mechanism used in the <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer2::get()</FONT></FONT>
method to translate a weak reference to an <FONT SIZE=2><FONT FACE="Courier New, monospace">Object2</FONT></FONT>
into a hard reference (that may then turn out to be null) solves all
the problems for us.</P>
<P STYLE="margin-bottom: 0.1cm">With this solution, only <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer2</FONT></FONT>
needs to know <FONT SIZE=2><FONT FACE="Courier New, monospace">Object2</FONT></FONT>,
and not also the other way around (as it was in the first solution
due to the mutual friend-ness of the two classes). Therefore, there
are individual header files for <FONT SIZE=2><FONT FACE="Courier New, monospace">Object2</FONT></FONT>
and <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer2</FONT></FONT>.</P>
<P STYLE="margin-bottom: 0.1cm">The header file
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobject2.hxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobject2.hxx</FONT></FONT></A>
is nothing more than a skeleton (<FONT SIZE=2><FONT FACE="Courier New, monospace">Object2</FONT></FONT>
is so bare-bones it does not even need an implementation file):</P>
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<PRE>#ifndef INCLUDED_UCB_CACHEMAPOBJECT2_HXX
#define INCLUDED_UCB_CACHEMAPOBJECT2_HXX
#ifndef _CPPUHELPER_WEAK_HXX_
#include &quot;cppuhelper/weak.hxx&quot;
#endif
namespace ucb { namespace cachemap {
class Object2: public cppu::OWeakObject
{};
} }
#endif // INCLUDED_UCB_CACHEMAPOBJECT2_HXX</PRE>
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<P STYLE="margin-bottom: 0.1cm">The header file
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobjectcontainer2.hxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobjectcontainer2.hxx</FONT></FONT></A>
is hardly more interesting:</P>
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<PRE>#ifndef INCLUDED_UCB_CACHEMAPOBJECTCONTAINER2_HXX
#define INCLUDED_UCB_CACHEMAPOBJECTCONTAINER2_HXX
#ifndef _CPPUHELPER_WEAKREF_HXX_
#include &quot;cppuhelper/weakref.hxx&quot;
#endif
#ifndef _OSL_MUTEX_HXX_
#include &quot;osl/mutex.hxx&quot;
#endif
#ifndef _RTL_REF_HXX_
#include &quot;rtl/ref.hxx&quot;
#endif
#ifndef _SAL_TYPES_H_
#include &quot;sal/types.h&quot;
#endif
#ifndef INCLUDED_HASH_MAP
#include &lt;hash_map&gt;
#define INCLUDED_HASH_MAP
#endif
namespace rtl {
class OUString;
struct OUStringHash;
}
namespace ucb { namespace cachemap { class Object2; } }
namespace ucb { namespace cachemap {
class ObjectContainer2
{
public:
ObjectContainer2();
~ObjectContainer2() SAL_THROW(());
rtl::Reference&lt; Object2 &gt; get(rtl::OUString const &amp; rKey);
private:
typedef <A HREF="#hashmap">std::hash_map</A>&lt; rtl::OUString,
com::sun::star::uno::WeakReference&lt; Object2 &gt;,
rtl::OUStringHash &gt;
Map;
ObjectContainer2(ObjectContainer2 &amp;); // not implemented
void operator =(ObjectContainer2); // not implemented
Map m_aMap;
osl::Mutex m_aMutex;
};
} }
#endif // INCLUDED_UCB_CACHEMAPOBJECTCONTAINER2_HXX</PRE>
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<UL>
<LI><P STYLE="margin-bottom: 0.1cm"><A NAME="hashmap"></A>Here, a (de facto standard?)
<FONT SIZE=2><FONT FACE="Courier New, monospace">std::hash_map</FONT></FONT>
is used instead of a <FONT SIZE=2><FONT FACE="Courier New, monospace">std::map</FONT></FONT>
as used for <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer1</FONT></FONT>:
We do not need any guarantees about iterators, so there is no reason
not to use the (potentially faster) <FONT SIZE=2><FONT FACE="Courier New, monospace">std::hash_map</FONT></FONT>.</P>
</UL>
<P STYLE="margin-bottom: 0.1cm">The solution to the implementation problems is hidden in the
implementation of <FONT SIZE=2><FONT FACE="Courier New, monospace">cppu::OWeakObject</FONT></FONT>
and <FONT SIZE=2><FONT FACE="Courier New, monospace">com::sun::star::uno::WeakReference</FONT></FONT>,
so the implementation file
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobjectcontainer2.cxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobjectcontainer2.cxx</FONT></FONT></A>
is quite simple, too:</P>
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<HR SIZE=1 NOSHADE>
</TD>
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<TD WIDTH=100%>
<PRE>#ifndef INCLUDED_UCB_CACHEMAPOBJECTCONTAINER2_HXX
#include &quot;cachemapobjectcontainer2.hxx&quot;
#endif
#ifndef INCLUDED_UCB_CACHEMAPOBJECT2_HXX
#include &quot;cachemapobject2.hxx&quot;
#endif
#ifndef _COM_SUN_STAR_UNO_REFERENCE_HXX_
#include &quot;com/sun/star/uno/Reference.hxx&quot;
#endif
#ifndef _COM_SUN_STAR_UNO_XWEAK_HPP_
#include &quot;com/sun/star/uno/XWeak.hpp&quot;
#endif
#ifndef _CPPUHELPER_WEAKREF_HXX_
#include &quot;cppuhelper/weakref.hxx&quot;
#endif
#ifndef _OSL_MUTEX_HXX_
#include &quot;osl/mutex.hxx&quot;
#endif
#ifndef _RTL_REF_HXX_
#include &quot;rtl/ref.hxx&quot;
#endif
#ifndef _RTL_USTRING_HXX_
#include &quot;rtl/ustring.hxx&quot;
#endif
using namespace com::sun;
using ucb::cachemap::Object2;
using ucb::cachemap::ObjectContainer2;
ObjectContainer2::ObjectContainer2()
{}
ObjectContainer2::~ObjectContainer2() SAL_THROW(())
{}
rtl::Reference&lt; Object2 &gt; ObjectContainer2::get(rtl::OUString const &amp; rKey)
{
rtl::Reference&lt; Object2 &gt; xElement;
{
osl::MutexGuard aGuard(m_aMutex);
Map::iterator aIt(m_aMap.find(rKey));
if (aIt != m_aMap.end())
xElement = static_cast&lt; Object2 * &gt;(
star::uno::Reference&lt; star::uno::XWeak &gt;(
aIt-&gt;second.get(), star::uno::UNO_QUERY).
get());
if (!xElement.is())
{
xElement = new Object2;
m_aMap[rKey]
= star::uno::WeakReference&lt; Object2 &gt;(xElement.get());
}
}
return xElement;
}</PRE>
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<TD WIDTH=100%>
<HR SIZE=1 NOSHADE>
</TD>
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<TR>
<TD WIDTH=100%>
<P STYLE="margin-bottom: 0.1cm">The main trick (hack?) here is to translate from a
<FONT SIZE=2><FONT FACE="Courier New, monospace">com::sun::star::uno::WeakReference&lt;
Object2 &gt;</FONT></FONT> via a <FONT SIZE=2><FONT FACE="Courier New, monospace">com::sun::star::uno::Reference&lt;
com::sun::star::uno::XWeak &gt;</FONT></FONT> to an <FONT SIZE=2><FONT FACE="Courier New, monospace">rtl::Reference&lt;
Object2 &gt;</FONT></FONT> (in general, you must go from a
<FONT SIZE=2><FONT FACE="Courier New, monospace">com::sun::star::uno::WeakReference&lt;
<I>T</I> &gt;</FONT></FONT> to a <FONT SIZE=2><FONT FACE="Courier New, monospace">com::sun::star::uno::Reference&lt;
<I>T</I> &gt;</FONT></FONT>, but it is not possible to have a
<FONT SIZE=2><FONT FACE="Courier New, monospace">com::sun::star::uno::Reference&lt;
Object2 &gt;</FONT></FONT>, so we must go via the base class <FONT SIZE=2><FONT FACE="Courier New, monospace">XWeak</FONT></FONT>
here).</P>
<P STYLE="margin-bottom: 0.1cm">The drawback of this solution is that it is not faster than the
initial one (even slower on some platforms). The overhead of using
the (generic) mechanism of weak references when requesting objects
outweighs the benefits of the faster refcounting.</P>
<P STYLE="margin-bottom: 0.1cm">Another point that can well be a drawback of this solution is that
a weak reference to a destroyed object is not removed from the map;
it is replaced with a weak references to a living object the next
time <FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer2::get()</FONT></FONT>
requests that object, but it is never erased from the map. Therefore,
the map itself keeps growing over the lifetime of the
<FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer2</FONT></FONT>
instance.</P>
<P ALIGN=LEFT><BR></P>
</TD>
</TR>
<TR>
<TD WIDTH=100% BGCOLOR="#666699">
<H3 ALIGN=LEFT STYLE="margin-top: 0cm; text-decoration: none"><A NAME="Solution3"></A>
<FONT COLOR="#ffffff"><FONT SIZE=4>Solution 3 (Best of Both Worlds)</FONT></FONT></H3>
</TD>
</TR>
<TR>
<TD WIDTH=100%>
<P STYLE="margin-bottom: 0.1cm">If exploiting the generic facilities in the second solution did
not give any performance advantage, let us try to become faster with
a hand-crafted version (i.e, combining the hand-crafted framework of
the first solution with the implementation details of the second
one).</P>
<P STYLE="margin-bottom: 0.1cm">The header file
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobject3.hxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobject3.hxx</FONT></FONT></A>
is almost identical to the header file from the first solution
(differences are in <FONT COLOR="#008000">green</FONT>):</P>
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<TD WIDTH=100%>
<HR SIZE=1 NOSHADE>
</TD>
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<PRE>#ifndef INCLUDED_UCB_CACHEMAPOBJECT3_HXX
#define INCLUDED_UCB_CACHEMAPOBJECT3_HXX
#ifndef _OSL_INTERLOCK_H_
#include &quot;osl/interlck.h&quot;
#endif
#ifndef _OSL_MUTEX_HXX_
#include &quot;osl/mutex.hxx&quot;
#endif
#ifndef _RTL_REF_HXX_
#include &quot;rtl/ref.hxx&quot;
#endif
#ifndef _SAL_TYPES_H_
#include &quot;sal/types.h&quot;
#endif
#ifndef _SALHELPER_SIMPLEREFERENCEOBJECT_HXX_
#include &quot;salhelper/simplereferenceobject.hxx&quot;
#endif
#ifndef INCLUDED_MAP
#include &lt;map&gt;
#define INCLUDED_MAP
#endif
#ifndef INCLUDED_MEMORY
#include &lt;memory&gt;
#define INCLUDED_MEMORY
#endif
namespace rtl { class OUString; }
namespace ucb { namespace cachemap { class Object3; } }
namespace ucb { namespace cachemap {
class ObjectContainer3: public salhelper::SimpleReferenceObject
{
public:
ObjectContainer3();
virtual ~ObjectContainer3() SAL_THROW(());
rtl::Reference&lt; Object3 &gt; get(rtl::OUString const &amp; rKey);
private:
typedef std::map&lt; rtl::OUString, Object3 * &gt; Map;
Map m_aMap;
osl::Mutex m_aMutex;
void releaseElement(Object3 * pElement) SAL_THROW(());
friend class Object3; // to access Map, releaseElement()
};
class Object3
{
public:
inline void acquire() SAL_THROW(())
{ osl_incrementInterlockedCount(&amp;m_nRefCount); }
<FONT COLOR="#008000">void release() SAL_THROW(());</FONT>
private:
rtl::Reference&lt; ObjectContainer3 &gt; m_xContainer;
ObjectContainer3::Map::iterator m_aContainerIt;
oslInterlockedCount m_nRefCount;
inline Object3(rtl::Reference&lt; ObjectContainer3 &gt; const &amp; rContainer);
inline ~Object3() SAL_THROW(());
Object3(Object3 &amp;); // not implemented
void operator =(Object3); // not implemented
friend class ObjectContainer3;
// to access m_aContainerIt, m_nRefCount, Object3(), ~Object3()
#if defined WNT
friend struct std::auto_ptr&lt; Object3 &gt;; // to access ~Object3()
#else // WNT
friend class std::auto_ptr&lt; Object3 &gt;; // to access ~Object3()
#endif // WNT
};
} }
#endif // INCLUDED_UCB_CACHEMAPOBJECT3_HXX</PRE>
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<HR SIZE=1 NOSHADE>
</TD>
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<TD WIDTH=100%>
<P STYLE="margin-bottom: 0.1cm">Also, the implementation file
<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemapobject3.cxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemapobject3.cxx</FONT></FONT></A>
has a strong resemblance to the implementation file from the first
solution (again, differences in <FONT COLOR="#008000">green</FONT>):</P>
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<TD WIDTH=100%>
<HR SIZE=1 NOSHADE>
</TD>
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<TD WIDTH=100%>
<PRE>#ifndef INCLUDED_UCB_CACHEMAPOBJECT3_HXX
#include &quot;cachemapobject3.hxx&quot;
#endif
#ifndef _OSL_DIAGNOSE_H_
#include &quot;osl/diagnose.h&quot;
#endif
#ifndef _OSL_INTERLOCK_H_
#include &quot;osl/interlck.h&quot;
#endif
#ifndef _OSL_MUTEX_HXX_
#include &quot;osl/mutex.hxx&quot;
#endif
#ifndef _RTL_REF_HXX_
#include &quot;rtl/ref.hxx&quot;
#endif
#ifndef _RTL_USTRING_HXX_
#include &quot;rtl/ustring.hxx&quot;
#endif
#ifndef INCLUDED_MEMORY
#include &lt;memory&gt;
#define INCLUDED_MEMORY
#endif
using ucb::cachemap::Object3;
using ucb::cachemap::ObjectContainer3;
inline
Object3::Object3(rtl::Reference&lt; ObjectContainer3 &gt; const &amp; rContainer):
m_xContainer(rContainer),
m_nRefCount(0)
{
OSL_ASSERT(m_xContainer.is());
}
inline Object3::~Object3() SAL_THROW(())
{}
<FONT COLOR="#008000">void Object3::release() SAL_THROW(())</FONT>
<FONT COLOR="#008000">{</FONT>
<FONT COLOR="#008000"> if (osl_decrementInterlockedCount(&amp;m_nRefCount) == 0)</FONT>
<FONT COLOR="#008000"> {</FONT>
<FONT COLOR="#008000"> m_xContainer-&gt;releaseElement(this);</FONT>
<FONT COLOR="#008000"> delete this;</FONT>
<FONT COLOR="#008000"> }</FONT>
<FONT COLOR="#008000">}</FONT>
void ObjectContainer3::releaseElement(Object3 * pElement) SAL_THROW(())
{
OSL_ASSERT(pElement);
<FONT COLOR="#008000">osl::MutexGuard aGuard(m_aMutex);</FONT>
<FONT COLOR="#008000"> if (pElement-&gt;m_aContainerIt != m_aMap.end())</FONT>
<FONT COLOR="#008000"> m_aMap.erase(pElement-&gt;m_aContainerIt);</FONT>
}
ObjectContainer3::ObjectContainer3()
{}
ObjectContainer3::~ObjectContainer3() SAL_THROW(())
{}
rtl::Reference&lt; Object3 &gt; ObjectContainer3::get(rtl::OUString const &amp; rKey)
{
osl::MutexGuard aGuard(m_aMutex);
Map::iterator aIt(m_aMap.find(rKey));
<FONT COLOR="#008000"> if (aIt == m_aMap.end())</FONT>
<FONT COLOR="#008000"> {</FONT>
<FONT COLOR="#008000"> std::auto_ptr< Object3 > xElement(new Object3(this));</FONT>
<FONT COLOR="#008000"> aIt = m_aMap.insert(Map::value_type(rKey, xElement.get())).first;</FONT>
<FONT COLOR="#008000"> aIt->second->m_aContainerIt = aIt;</FONT>
<FONT COLOR="#008000"> xElement.release();</FONT>
<FONT COLOR="#008000"> return aIt->second;</FONT>
<FONT COLOR="#008000"> }</FONT>
<FONT COLOR="#008000"> else if (osl_incrementInterlockedCount(&aIt->second->m_nRefCount) > 1)</FONT>
<FONT COLOR="#008000"> {</FONT>
<FONT COLOR="#008000"> rtl::Reference< Object3 > xElement(aIt->second);</FONT>
<FONT COLOR="#008000"> osl_decrementInterlockedCount(&aIt->second->m_nRefCount);</FONT>
<FONT COLOR="#008000"> return xElement;</FONT>
<FONT COLOR="#008000"> }</FONT>
<FONT COLOR="#008000"> else</FONT>
<FONT COLOR="#008000"> {</FONT>
<FONT COLOR="#008000"> osl_decrementInterlockedCount(&aIt->second->m_nRefCount);</FONT>
<FONT COLOR="#008000"> aIt->second->m_aContainerIt = m_aMap.end();</FONT>
<FONT COLOR="#008000"> aIt->second = new Object3(this);</FONT>
<FONT COLOR="#008000"> aIt->second->m_aContainerIt = aIt;</FONT>
<FONT COLOR="#008000"> return aIt->second;</FONT>
<FONT COLOR="#008000"> }</FONT>
}</PRE>
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<HR SIZE=1 NOSHADE>
</TD>
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<TR>
<TD WIDTH=100%>
<P STYLE="margin-bottom: 0.1cm">This implementation does not need to lock a mutex in every call to
<FONT SIZE=2><FONT FACE="Courier New, monospace">Object3::release()</FONT></FONT>.
Only if the refcount drops to zero is the mutex of the
<FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer3</FONT></FONT>
locked and the object removed from the map. It is possible that an
<FONT SIZE=2><FONT FACE="Courier New, monospace">ObjectContainer3::get()</FONT></FONT>
gets in the way between dropping the refcount to zero and locking the
mutex. But the <FONT SIZE=2><FONT FACE="Courier New, monospace">get()</FONT></FONT>
method notices that (it goes through a cycle of incrementing and
decrementing the refcount itself, and if the refcount is not larger
than 1, the object is about to be deleted) and leaves alone the
to-be-deleted object and inserts into the map a fresh instance
instead.</P>
<P STYLE="margin-bottom: 0.1cm">Note that this implementation also does not have the drawback of
an ever-growing map, because each object is removed from the map as
soon as its refcount drops to zero.</P>
<P ALIGN=LEFT><BR></P>
</TD>
</TR>
<TR>
<TD WIDTH=100% BGCOLOR="#666699">
<H3 ALIGN=LEFT STYLE="margin-top: 0cm; text-decoration: none"><A NAME="Measurements"></A>
<FONT COLOR="#ffffff"><FONT SIZE=4>Performance Measurements</FONT></FONT></H3>
</TD>
</TR>
<TR>
<TD WIDTH=100%>
<P STYLE="margin-bottom: 0.1cm">A simple test file
(<A HREF="http://www.openoffice.org/source/browse/ucb/ucb/workben/cachemap/cachemaptest.cxx"><FONT SIZE=2><FONT FACE="Courier New, monospace">cachemaptest.cxx</FONT></FONT></A>)
makes 200000 <FONT SIZE=2><FONT FACE="Courier New, monospace">get()</FONT></FONT>
requests, half of which find objects already present in the map. The
refcount of each requested object is incremented and decremented 50
times in a row. Only 100 different identifiers are used for the
objects, so that the second solution does not suffer from the problem
of an ever-growing map. The measurements for two of our production
platforms (Solaris Sparc with Forte compiler and Windows x86 with
Microsoft compiler) are as follows (all times in milliseconds; because
of different machines used, only comparisons within each column are
meaningful):</P>
<TABLE WIDTH=100% BORDER=1 CELLPADDING=4 CELLSPACING=3 STYLE="page-break-inside: avoid">
<COL WIDTH=85*>
<COL WIDTH=85*>
<COL WIDTH=85*>
<TR VALIGN=TOP>
<TD WIDTH=33%>
<P><BR>
</P>
</TD>
<TD WIDTH=33%>
<P>unxsols3.pro</P>
</TD>
<TD WIDTH=33%>
<P>wntmsci7.pro</P>
</TD>
</TR>
<TR>
<TD WIDTH=33% VALIGN=TOP>
<P>Solution 1</P>
</TD>
<TD WIDTH=33% VALIGN=BOTTOM SDVAL="9137" SDNUM="1031;">
<P ALIGN=RIGHT>9137</P>
</TD>
<TD WIDTH=33% VALIGN=BOTTOM SDVAL="3846" SDNUM="1031;">
<P ALIGN=RIGHT>3846</P>
</TD>
</TR>
<TR>
<TD WIDTH=33% VALIGN=TOP>
<P>Solution 2</P>
</TD>
<TD WIDTH=33% VALIGN=BOTTOM SDVAL="8634" SDNUM="1031;">
<P ALIGN=RIGHT>8634</P>
</TD>
<TD WIDTH=33% VALIGN=BOTTOM SDVAL="5598" SDNUM="1031;">
<P ALIGN=RIGHT>5598</P>
</TD>
</TR>
<TR>
<TD WIDTH=33% VALIGN=TOP>
<P>Solution 3</P>
</TD>
<TD WIDTH=33% VALIGN=BOTTOM SDVAL="3166" SDNUM="1031;">
<P ALIGN=RIGHT>3166</P>
</TD>
<TD WIDTH=33% VALIGN=BOTTOM SDVAL="2704" SDNUM="1031;">
<P ALIGN=RIGHT>2704</P>
</TD>
</TR>
</TABLE>
<P ALIGN=LEFT><BR></P>
</TD>
</TR>
<TR>
<TD WIDTH=100%>
<HR SIZE=1 NOSHADE>
</TD>
</TR>
<TR>
<TD WIDTH=100% BGCOLOR="#666699">
<P ALIGN=LEFT><FONT COLOR="#ffffff">Author: <A HREF="mailto:bergmann@sun.com">Stephan
Bergmann</A> ($Date: 2001/06/14 14:49:37 $)<BR><I>Copyright 2001
OpenOffice.org Foundation. All Rights Reserved.</I></FONT>
</P>
</TD>
</TR>
<TR>
<TD WIDTH=100%>
<HR SIZE=1 NOSHADE>
</TD>
</TR>
</TABLE>
<HR SIZE=3 NOSHADE>
</body>
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