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<h1>Counted Body Techniques</h1>
</div>
<div class="section-body">
<p style="text-align: center"><a href=
"/users/people/kevlin_henney.html">Kevlin Henney</a><br />
(<a href=
"mailto:kevlin@curbralan.com">kevlin@curbralan.com</a>)</p>
<p>Reference counting techniques? Nothing new, you might think.
Every good C++ text that takes you to an intermediate or
advanced level will introduce the concept. It has been explored
with such thoroughness in the past that you might be forgiven
for thinking that everything that can be said has been said.
Well, let's start from first principles and see if we can
unearth something new....</p>
<h2>And then there were none...</h2>
<p>The principle behind reference counting is to keep a running
usage count of an object so that when it falls to zero we know
the object is unused. This is normally used to simplify the
memory management for dynamically allocated objects: keep a
count of the number of references held to that object and, on
zero, delete the object.</p>
<p>How to keep a track of the number of users of an object?
Well, normal pointers are quite dumb, and so an extra level of
indirection is required to manage the count. This is
essentially the PROXY pattern described in <i>Design
Patterns</i> [Gamma, Helm, Johnson &amp; Vlissides,
Addison-Wesley, ISBN 0-201-63361-2]. The intent is given as</p>
<blockquote>
<p><i>Provide a surrogate or placeholder for another object
to control access to it.</i></p>
</blockquote>
<p>Coplien [<i>Advanced C++ Programming Styles and Idioms</i>,
Addison-Wesley, ISBN 0-201-56365-7] defines a set of idioms
related to this essential separation of a handle and a body
part. The <i>Taligent Guide to Designing Programs</i>
[Addison-Wesley, ISBN 0-201-40888-0] identifies a number of
specific categories for proxies (aka surrogates). Broadly
speaking they fall into two general categories:</p>
<ul>
<li><i>Hidden</i>: The handle is the object of interest,
hiding the body itself. The functionality of the handle is
obtained by delegation to the body, and the user of the
handle is unaware of the body. Reference counted strings
offer a transparent optimisation. The body is shared between
copies of a string until such a time as a change is needed,
at which point a copy is made. Such a COPY ON WRITE pattern
(a specialisation of LAZY EVALUATION) requires the use of a
hidden reference counted body.</li>
<li><i>Explicit</i>: Here the body is of interest and the
handle merely provides intelligence for its access and
housekeeping. In C++ this is often implemented as the SMART
POINTER idiom. One such application is that of reference
counted smart pointers that collaborate to keep a count of an
object, deleting it when the count falls to zero.</li>
</ul>
<h2>Attached vs detached</h2>
<p>For reference counted smart pointers there are two places
the count can exist, resulting in two different patterns, both
outlined in <i>Software Patterns</i> [Coplien, SIGS, ISBN
0-884842-50-X]:</p>
<ul>
<li>COUNTED BODY or ATTACHED COUNTED HANDLE/BODY places the
count within the object being counted. The benefits are that
countability is a part of the object being counted, and that
reference counting does not require an additional object. The
drawbacks are clearly that this is intrusive, and that the
space for the reference count is wasted when the object is
not heap based. Therefore the reference counting ties you to
a particular implementation and style of use.</li>
<li>DETACHED COUNTED HANDLE/BODY places the count outside the
object being counted, such that they are handled together.
The clear benefit of this is that this technique is
completely unintrusive, with all of the intelligence and
support apparatus in the smart pointer, and therefore can be
used on classes created independently of the reference
counted pointer. The main disadvantage is that frequent use
of this can lead to a proliferation of small objects, i.e.
the counter, being created on the heap.</li>
</ul>
<p>Even with this simple analysis, it seems that the DETACHED
COUNTED HANDLE/BODY approach is ahead. Indeed, with the
increasing use of templates this is often the favourite, and is
the principle behind the common - but not standard -
<code>counted_ptr</code>. <i>[The Boost name is <a href=
"/doc/libs/release/libs/smart_ptr/shared_ptr.htm"><code>shared_ptr</code></a>
rather than <code>counted_ptr</code>.]</i></p>
<p>A common implementation of COUNTED BODY is to provide the
counting mechanism in a base class that the counted type is
derived from. Either that, or the reference counting mechanism
is provided anew for each class that needs it. Both of these
approaches are unsatisfactory because they are quite closed,
coupling a class into a particular framework. Added to this the
non-cohesiveness of having the count lying dormant in a
non-counted object, and you get the feeling that excepting its
use in widespread object models such as COM and CORBA the
COUNTED BODY approach is perhaps only of use in specialised
situations.</p>
<h2>A requirements based approach</h2>
<p>It is the question of openness that convinced me to revisit
the problems with the COUNTED BODY idiom. Yes, there is a
certain degree of intrusion expected when using this idiom, but
is there anyway to minimise this and decouple the choice of
counting mechanism from the smart pointer type used?</p>
<p>In recent years the most instructive body of code and
specification for constructing open general purpose components
has been the Stepanov and Lee's STL (Standard Template
Library), now part of the C++ standard library. The STL
approach makes extensive use of compile time polymorphism based
on well defined operational requirements for types. For
instance, each container, contained and iterator type is
defined by the operations that should be performable on an
object of that type, often with annotations describing
additional constraints. Compile time polymorphism, as its name
suggests, resolves functions at compile time based on function
name and argument usage, i.e. overloading. This is less
intrusive, although less easily diagnosed if incorrect, than
runtime poymorphism that is based on types, names and function
signatures.</p>
<p>This requirements based approach can be applied to reference
counting. The operations we need for a type to be
<i>Countable</i> are loosely:</p>
<ul>
<li>An <code>acquire</code> operation that registers interest
in a <i>Countable</i> object.</li>
<li>A <code>release</code> operation unregisters interest in
a <i>Countable</i> object.</li>
<li>An <code>acquired</code> query that returns whether or
not a <i>Countable</i> object is currently acquired.</li>
<li>A <code>dispose</code> operation that is responsible for
disposing of an object that is no longer acquired.</li>
</ul>
<p>Note that the count is deduced as a part of the abstract
state of this type, and is not mentioned or defined in any
other way. The openness of this approach derives in part from
the use of global functions, meaning that no particular member
functions are implied; a perfect way to wrap up an existing
counted body class without modifying the class itself. The
other aspect to the openness comes from a more precise
specification of the operations.</p>
<p>For a type to be <i>Countable</i> it must satisfy the
following requirements, where <code>ptr</code> is a non-null
pointer to a single object (i.e. not an array) of the type, and
<i><code>#function</code></i> indicates number of calls to
<code><i>function(</i>ptr<i>)</i></code>:</p>
<table border="1" cellspacing="2" cellpadding="2" summary="">
<tr>
<td><i>Expression</i></td>
<td><i>Return type</i></td>
<td><i>Semantics and notes</i></td>
</tr>
<tr>
<td><code>acquire(ptr)</code></td>
<td>no requirement</td>
<td><i>post</i>: <code>acquired(ptr)</code></td>
</tr>
<tr>
<td><code>release(ptr)</code></td>
<td>no requirement</td>
<td><i>pre</i>: <code>acquired(ptr)<br /></code>
<i>post</i>: <code>acquired(ptr) == #acquire -
#release</code></td>
</tr>
<tr>
<td><code>acquired(ptr)</code></td>
<td>convertible to <code>bool</code></td>
<td><i>return</i>: <code>#acquire &gt; #release</code></td>
</tr>
<tr>
<td><code>dispose(ptr, ptr)</code></td>
<td>no requirement</td>
<td><i>pre</i>: <code>!acquired(ptr)<br /></code>
<i>post</i>: <code>*ptr</code> no longer usable</td>
</tr>
</table>
<p>Note that the two arguments to <code>dispose</code> are to
support selection of the appropriate type safe version of the
function to be called. In the general case the intent is that
the first argument determines the type to be deleted, and would
typically be templated, while the second selects which template
to use, e.g. by conforming to a specific base class.</p>
<p>In addition the following requirements must also be
satisfied, where <code>null</code> is a null pointer to the
<i>Countable</i> type:</p>
<table border="1" summary="">
<tr>
<td><i>Expression</i></td>
<td><i>Return type</i></td>
<td><i>Semantics and notes</i></td>
</tr>
<tr>
<td><code>acquire(null)</code></td>
<td>no requirement</td>
<td><i>action</i>: none</td>
</tr>
<tr>
<td><code>release(null)</code></td>
<td>no requirement</td>
<td><i>action</i>: none</td>
</tr>
<tr>
<td><code>acquired(null)</code></td>
<td>convertible to <code>bool</code></td>
<td><i>return</i>: <code>false</code></td>
</tr>
<tr>
<td><code>dispose(null, null)</code></td>
<td>no requirement</td>
<td><i>action</i>: none</td>
</tr>
</table>
<p>Note that there are no requirements on these functions in
terms of exceptions thrown or not thrown, except that if
exceptions are thrown the functions themselves should be
exception safe.</p>
<h2>Getting smart</h2>
<p>Given the <i>Countable</i> requirements for a type, it is
possible to define a generic smart pointer type that uses them
for reference counting:</p>
<pre>
template&lt;typename countable_type&gt;
class countable_ptr
{
public: // construction and destruction
explicit countable_ptr(countable_type *);
countable_ptr(const countable_ptr &amp;);
~countable_ptr();
public: // access
countable_type *operator-&gt;() const;
countable_type &amp;operator*() const;
countable_type *get() const;
public: // modification
countable_ptr &amp;clear();
countable_ptr &amp;assign(countable_type *);
countable_ptr &amp;assign(const countable_ptr &amp;);
countable_ptr &amp;operator=(const countable_ptr &amp;);
private: // representation
countable_type *body;
};
</pre>
<p>The interface to this class has been kept intentionally
simple, e.g. member templates and <code>throw</code> specs have
been omitted, for exposition. The majority of the functions are
quite simple in implementation, relying very much on the
<code>assign</code> member as a keystone function:</p>
<pre>
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt;::countable_ptr(countable_type *initial)
: body(initial)
{
acquire(body);
}
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt;::countable_ptr(const countable_ptr &amp;other)
: body(other.body)
{
acquire(body);
}
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt;::~countable_ptr()
{
clear();
}
template&lt;typename countable_type&gt;
countable_type *countable_ptr&lt;countable_type&gt;::operator-&gt;() const
{
return body;
}
template&lt;typename countable_type&gt;
countable_type &amp;countable_ptr&lt;countable_type&gt;::operator*() const
{
return *body;
}
template&lt;typename countable_type&gt;
countable_type *countable_ptr&lt;countable_type&gt;::get() const
{
return body;
}
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt; &amp;countable_ptr&lt;countable_type&gt;::clear()
{
return assign(0);
}
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt; &amp;countable_ptr&lt;countable_type&gt;::assign(countable_type *rhs)
{
// set to rhs (uses Copy Before Release idiom which is self assignment safe)
acquire(rhs);
countable_type *old_body = body;
body = rhs;
// tidy up
release(old_body);
if(!acquired(old_body))
{
dispose(old_body, old_body);
}
return *this;
}
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt; &amp;countable_ptr&lt;countable_type&gt;::assign(const countable_ptr &amp;rhs)
{
return assign(rhs.body);
}
template&lt;typename countable_type&gt;
countable_ptr&lt;countable_type&gt; &amp;countable_ptr&lt;countable_type&gt;::operator=(const countable_ptr &amp;rhs)
{
return assign(rhs);
}
</pre>
<h2>Public accountability</h2>
<p>Conformance to the requirements means that a type can be
used with <code>countable_ptr</code>. Here is an implementation
mix-in class (<i>mix-imp</i>) that confers countability on its
derived classes through member functions. This class can be
used as a class adaptor:</p>
<pre>
class countability
{
public: // manipulation
void acquire() const;
void release() const;
size_t acquired() const;
protected: // construction and destruction
countability();
~countability();
private: // representation
mutable size_t count;
private: // prevention
countability(const countability &amp;);
countability &amp;operator=(const countability &amp;);
};
</pre>
<p>Notice that the manipulation functions are
<code>const</code> and that the <code>count</code> member
itself is <code>mutable</code>. This is because countability is
not a part of an object's abstract state: memory management
does not depend on the <code>const</code>-ness or otherwise of
an object. I won't include the definitions of the member
functions here as you can probably guess them: increment,
decrement and return the current count, respectively for the
manipulation functions. In a multithreaded environment you
should ensure that such read and write operations are
atomic.</p>
<p>So how do we make this class <i>Countable</i>? A simple set
of forwarding functions does the job:</p>
<pre>
void acquire(const countability *ptr)
{
if(ptr)
{
ptr-&gt;acquire();
}
}
void release(const countability *ptr)
{
if(ptr)
{
ptr-&gt;release();
}
}
size_t acquired(const countability *ptr)
{
return ptr ? ptr-&gt;acquired() : 0;
}
template&lt;class countability_derived&gt;
void dispose(const countability_derived *ptr, const countability *)
{
delete ptr;
}
</pre>
<p>Any type that now derives from <code>countability</code> may
now be used with <code>countable_ptr</code>:</p>
<pre>
class example : public countability
{
...
};
void simple()
{
countable_ptr&lt;example&gt; ptr(new example);
countable_ptr&lt;example&gt; qtr(ptr);
ptr.clear(); // set ptr to point to null
} // allocated object deleted when qtr destructs
</pre>
<h2>Runtime mixin</h2>
<p>The challenge is to apply COUNTED BODY in a non-intrusive
fashion, such that there is no overhead when an object is not
counted. What we would like to do is confer this capability on
a per object rather than on a per class basis. Effectively we
are after <i>Countability</i> on any object, i.e. anything
pointed to by a <code>void *</code>! It goes without saying
that <code>void</code> is perhaps the least committed of any
type.</p>
<p>The forces to resolve on this are quite interesting, to say
the least. Interesting, but not insurmountable. Given that the
class of a runtime object cannot change dynamically in any well
defined manner, and the layout of the object must be fixed, we
have to find a new place and time to add the counting state.
The fact that this must be added only on heap creation suggests
the following solution:</p>
<pre>
struct countable_new;
extern const countable_new countable;
void *operator new(size_t, const countable_new &amp;);
void operator delete(void *, const countable_new &amp;);
</pre>
<p>We have overloaded <code>operator new</code> with a dummy
argument to distinguish it from the regular global
<code>operator new</code>. This is comparable to the use of the
<code>std::nothrow_t</code> type and <code>std::nothrow</code>
object in the standard library. The placement <code>operator
delete</code> is there to perform any tidy up in the event of
failed construction. Note that this is not yet supported on all
that many compilers.</p>
<p>The result of a <code>new</code> expression using
<code>countable</code> is an object allocated on the heap that
has a header block that holds the count, i.e. we have extended
the object by prefixing it. We can provide a couple of features
in an anonymous namespace (not shown) in the implementation
file for for supporting the count and its access from a raw
pointer:</p>
<pre>
struct count
{
size_t value;
};
count *header(const void *ptr)
{
return const_cast&lt;count *&gt;(static_cast&lt;const count *&gt;(ptr) - 1);
}
</pre>
<p>An important constraint to observe here is the alignment of
<code>count</code> should be such that it is suitably aligned
for any type. For the definition shown this will be the case on
almost all platforms. However, you may need to add a padding
member for those that don't, e.g. using an anonymous
<code>union</code> to coalign <code>count</code> and the most
aligned type. Unfortunately, there is no portable way of
specifying this such that the minimum alignment is also
observed - this is a common problem when specifying your own
allocators that do not directly use the results of either
<code>new</code> or <code>malloc</code>.</p>
<p>Again, note that the count is not considered to be a part of
the logical state of the object, and hence the conversion from
<code>const</code> to non-<code>const</code> -
<code>count</code> is in effect a <code>mutable</code>
type.</p>
<p>The allocator functions themselves are fairly
straightforward:</p>
<pre>
void *operator new(size_t size, const countable_new &amp;)
{
count *allocated = static_cast&lt;count *&gt;(::operator new(sizeof(count) + size));
*allocated = count(); // initialise the header
return allocated + 1; // adjust result to point to the body
}
void operator delete(void *ptr, const countable_new &amp;)
{
::operator delete(header(ptr));
}
</pre>
<p>Given a correctly allocated header, we now need the
<i>Countable</i> functions to operate on <code>const void
*</code> to complete the picture:</p>
<pre>
void acquire(const void *ptr)
{
if(ptr)
{
++header(ptr)-&gt;value;
}
}
void release(const void *ptr)
{
if(ptr)
{
--header(ptr)-&gt;value;
}
}
size_t acquired(const void *ptr)
{
return ptr ? header(ptr)-&gt;value : 0;
}
template&lt;typename countable_type&gt;
void dispose(const countable_type *ptr, const void *)
{
ptr-&gt;~countable_type();
operator delete(const_cast&lt;countable_type *&gt;(ptr), countable);
}
</pre>
<p>The most complex of these is the <code>dispose</code>
function that must ensure that the correct type is destructed
and also that the memory is collected from the correct offset.
It uses the value and type of first argument to perform this
correctly, and the second argument merely acts as a strategy
selector, i.e. the use of <code>const void *</code>
distinguishes it from the earlier dispose shown for <code>const
countability *</code>.</p>
<h2>Getting smarter</h2>
<p>Now that we have a way of adding countability at creation
for objects of any type, what extra is needed to make this work
with the <code>countable_ptr</code> we defined earlier? Good
news: nothing!</p>
<pre>
class example
{
...
};
void simple()
{
countable_ptr&lt;example&gt; ptr(new(countable) example);
countable_ptr&lt;example&gt; qtr(ptr);
ptr.clear(); // set ptr to point to null
} // allocated object deleted when qtr destructs
</pre>
<p>The <code>new(countable)</code> expression defines a
different policy for allocation and deallocation and, in common
with other allocators, any attempt to mix your allocation
policies, e.g. call <code>delete</code> on an object allocated
with <code>new(countable)</code>, results in undefined
behaviour. This is similar to what happens when you mix
<code>new[]</code> with <code>delete</code> or
<code>malloc</code> with <code>delete</code>. The whole point
of <i>Countable</i> conformance is that <i>Countable</i>
objects are used with <code>countable_ptr</code>, and this
ensures the correct use.</p>
<p>However, accidents will happen, and inevitably you may
forget to allocate using <code>new(countable)</code> and
instead use <code>new</code>. This error and others can be
detected in most cases by extending the code shown here to add
a check member to the <code>count</code>, validating the check
on every access. A benefit of ensuring clear separation between
header and implementation source files means that you can
introduce a checking version of this allocator without having
to recompile your code.</p>
<h2>Conclusion</h2>
<p>There are two key concepts that this article has
introduced:</p>
<ul>
<li>The use of a generic requirements based approach to
simplify and adapt the use of the COUNTED BODY pattern.</li>
<li>The ability, through control of allocation, to
dynamically and non-intrusively add capabilities to fixed
types using the RUNTIME MIXIN pattern.</li>
</ul>
<p>The application of the two together gives rise to a new
variant of the essential COUNTED BODY pattern, UNINTRUSIVE
COUNTED BODY. You can take this theme even further and contrive
a simple garbage collection system for C++.</p>
<p>The complete code for <code>countable_ptr</code>,
<code>countability</code>, and the <code>countable new</code>
is also available.</p>
<div style="text-align: right">
<i>First published in</i> <a href=
"http://www.accu.org/index.php/overloadonline">Overload</a>
<i>25, April 1998, ISSN 1354-3172</i>
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