website2022/community/counted_body.html

---
title: Counted Body Techniques
copyright: Kevlin Henney 1998-1999.
revised: 
---


Counted Body Techniques



Counted Body Techniques
=======================

[Kevlin Henney](/users/people/kevlin_henney.html)  

 ([kevlin@curbralan.com](mailto:kevlin@curbralan.com))


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....


And then there were none...
---------------------------


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.


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 *Design
 Patterns* [Gamma, Helm, Johnson & Vlissides,
 Addison-Wesley, ISBN 0-201-63361-2]. The intent is given as
> 
> *Provide a surrogate or placeholder for another object
>  to control access to it.*
> 
> 
> 


Coplien [*Advanced C++ Programming Styles and Idioms*,
 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 *Taligent Guide to Designing Programs*
 [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:


* *Hidden*: 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 specialization of LAZY EVALUATION) requires the use of a
 hidden reference counted body.
* *Explicit*: 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.


Attached vs detached
--------------------


For reference counted smart pointers there are two places
 the count can exist, resulting in two different patterns, both
 outlined in *Software Patterns* [Coplien, SIGS, ISBN
 0-884842-50-X]:


* 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.
* 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.


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 -
 `counted_ptr`. *[The Boost name is [`shared_ptr`](/doc/libs/release/libs/smart_ptr/shared_ptr.htm)
 rather than `counted_ptr`.]*


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 specialized
 situations.


A requirements based approach
-----------------------------


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 minimize this and decouple the choice of
 counting mechanism from the smart pointer type used?


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 polymorphism that is based on types, names and function
 signatures.


This requirements based approach can be applied to reference
 counting. The operations we need for a type to be
 *Countable* are loosely:


* An `acquire` operation that registers interest
 in a *Countable* object.
* A `release` operation unregisters interest in
 a *Countable* object.
* An `acquired` query that returns whether or
 not a *Countable* object is currently acquired.
* A `dispose` operation that is responsible for
 disposing of an object that is no longer acquired.


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 of openness comes from a more precise
 specification of the operations.


For a type to be *Countable* it must satisfy the
 following requirements, where `ptr` is a non-null
 pointer to a single object (i.e. not an array) of the type, and
 *`#function`* indicates number of calls to
 `*function(*ptr*)*`:

|  |  |  |
| --- | --- | --- |
| *Expression* | *Return type* | *Semantics and notes* |
| `acquire(ptr)` | no requirement | *post*: `acquired(ptr)` |
| `release(ptr)` | no requirement | *pre*: `acquired(ptr)`
*post*: `acquired(ptr) == #acquire -
 #release` |
| `acquired(ptr)` | convertible to `bool` | *return*: `#acquire > #release` |
| `dispose(ptr, ptr)` | no requirement | *pre*: `!acquired(ptr)`
*post*: `*ptr` no longer usable |


Note that the two arguments to `dispose` 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.


In addition the following requirements must also be
 satisfied, where `null` is a null pointer to the
 *Countable* type:

|  |  |  |
| --- | --- | --- |
| *Expression* | *Return type* | *Semantics and notes* |
| `acquire(null)` | no requirement | *action*: none |
| `release(null)` | no requirement | *action*: none |
| `acquired(null)` | convertible to `bool` | *return*: `false` |
| `dispose(null, null)` | no requirement | *action*: none |


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.


Getting smart
-------------


Given the *Countable* requirements for a type, it is
 possible to define a generic smart pointer type that uses them
 for reference counting:
```

template<typename countable\_type>
class countable\_ptr
{
public: // construction and destruction

    explicit countable\_ptr(countable\_type \*);
    countable\_ptr(const countable\_ptr &);
    ~countable\_ptr();

public: // access

    countable\_type \*operator->() const;
    countable\_type &operator\*() const;
    countable\_type \*get() const;

public: // modification

    countable\_ptr &clear();
    countable\_ptr &assign(countable\_type \*);
    countable\_ptr &assign(const countable\_ptr &);
    countable\_ptr &operator=(const countable\_ptr &);

private: // representation

    countable\_type \*body;

};

```

The interface to this class has been kept intentionally
 simple, e.g. member templates and `throw` specs have
 been omitted, for exposition. The majority of the functions are
 quite simple in implementation, relying very much on the
 `assign` member as a keystone function:
```

template<typename countable\_type>
countable\_ptr<countable\_type>::countable\_ptr(countable\_type \*initial)
  : body(initial)
{
    acquire(body);
}

template<typename countable\_type>
countable\_ptr<countable\_type>::countable\_ptr(const countable\_ptr &other)
  : body(other.body)
{
    acquire(body);
}

template<typename countable\_type>
countable\_ptr<countable\_type>::~countable\_ptr()
{
    clear();
}

template<typename countable\_type>
countable\_type \*countable\_ptr<countable\_type>::operator->() const
{
    return body;
}

template<typename countable\_type>
countable\_type &countable\_ptr<countable\_type>::operator\*() const
{
    return \*body;
}

template<typename countable\_type>
countable\_type \*countable\_ptr<countable\_type>::get() const
{
    return body;
}

template<typename countable\_type>
countable\_ptr<countable\_type> &countable\_ptr<countable\_type>::clear()
{
    return assign(0);
}

template<typename countable\_type>
countable\_ptr<countable\_type> &countable\_ptr<countable\_type>::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<typename countable\_type>
countable\_ptr<countable\_type> &countable\_ptr<countable\_type>::assign(const countable\_ptr &rhs)
{
    return assign(rhs.body);
}

template<typename countable\_type>
countable\_ptr<countable\_type> &countable\_ptr<countable\_type>::operator=(const countable\_ptr &rhs)
{
    return assign(rhs);
}

```

Public accountability
---------------------


Conformance to the requirements means that a type can be
 used with `countable_ptr`. Here is an implementation
 mix-in class (*mix-imp*) that confers countability on its
 derived classes through member functions. This class can be
 used as a class adaptor:
```

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 &);
    countability &operator=(const countability &);

};

```

Notice that the manipulation functions are
 `const` and that the `count` member
 itself is `mutable`. This is because countability is
 not a part of an object's abstract state: memory management
 does not depend on the `const`-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.


So how do we make this class *Countable*? A simple set
 of forwarding functions does the job:
```

void acquire(const countability \*ptr)
{
    if(ptr)
    {
        ptr->acquire();
    }
}

void release(const countability \*ptr)
{
    if(ptr)
    {
        ptr->release();
    }
}

size\_t acquired(const countability \*ptr)
{
    return ptr ? ptr->acquired() : 0;
}

template<class countability\_derived>
void dispose(const countability\_derived \*ptr, const countability \*)
{
    delete ptr;
}

```

Any type that now derives from `countability` may
 now be used with `countable_ptr`:
```

class example : public countability
{
    ...
};

void simple()
{
    countable\_ptr<example> ptr(new example);
    countable\_ptr<example> qtr(ptr);
    ptr.clear(); // set ptr to point to null
}   // allocated object deleted when qtr destructs

```

Runtime mixin
-------------


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 *Countability* on any object, i.e. anything
 pointed to by a `void *`! It goes without saying
 that `void` is perhaps the least committed of any
 type.


The forces to resolve 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:
```

struct countable\_new;
extern const countable\_new countable;

void \*operator new(size\_t, const countable\_new &);
void operator delete(void \*, const countable\_new &);

```

We have overloaded `operator new` with a dummy
 argument to distinguish it from the regular global
 `operator new`. This is comparable to the use of the
 `std::nothrow_t` type and `std::nothrow`
 object in the standard library. The placement `operator
 delete` 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.


The result of a `new` expression using
 `countable` 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 supporting the count and its access from a raw
 pointer:
```

struct count
{
    size\_t value;
};

count \*header(const void \*ptr)
{
    return const\_cast<count \*>(static\_cast<const count \*>(ptr) - 1);
}

```

An important constraint to observe here is the alignment of
 `count` 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
 `union` to coalign `count` 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
 `new` or `malloc`.


Again, note that the count is not considered to be a part of
 the logical state of the object, and hence the conversion from
 `const` to non-`const` -
 `count` is in effect a `mutable`
 type.


The allocator functions themselves are fairly
 straightforward:
```

void \*operator new(size\_t size, const countable\_new &)
{
    count \*allocated = static\_cast<count \*>(::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 &)
{
    ::operator delete(header(ptr));
}

```

Given a correctly allocated header, we now need the
 *Countable* functions to operate on `const void
 *` to complete the picture:
```

void acquire(const void \*ptr)
{
    if(ptr)
    {
        ++header(ptr)->value;
    }
}

void release(const void \*ptr)
{
    if(ptr)
    {
        --header(ptr)->value;
    }
}

size\_t acquired(const void \*ptr)
{
    return ptr ? header(ptr)->value : 0;
}

template<typename countable\_type>
void dispose(const countable\_type \*ptr, const void \*)
{
    ptr->~countable\_type();
    operator delete(const\_cast<countable\_type \*>(ptr), countable);
}

```

The most complex of these is the `dispose`
 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 `const void *`
 distinguishes it from the earlier dispose shown for `const
 countability *`.


Getting smarter
---------------


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 `countable_ptr` we defined earlier? Good
 news: nothing!
```

class example
{
    ...
};

void simple()
{
    countable\_ptr<example> ptr(new(countable) example);
    countable\_ptr<example> qtr(ptr);
    ptr.clear(); // set ptr to point to null
}   // allocated object deleted when qtr destructs

```

The `new(countable)` 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 `delete` on an object allocated
 with `new(countable)`, results in undefined
 behaviour. This is similar to what happens when you mix
 `new[]` with `delete` or
 `malloc` with `delete`. The whole point
 of *Countable* conformance is that *Countable*
 objects are used with `countable_ptr`, and this
 ensures the correct use.


However, accidents will happen, and inevitably you may
 forget to allocate using `new(countable)` and
 instead use `new`. This error and others can be
 detected in most cases by extending the code shown here to add
 a check member to the `count`, validating the check
 on every access. A benefit of ensuring clear separation between
 header and implementation source files mean that you can
 introduce a checking version of this allocator without having
 to recompile your code.


Conclusion
----------


There are two key concepts that this article has
 introduced:


* The use of a generic requirements based approach to
 simplify and adapt the use of the COUNTED BODY pattern.
* The ability, through control of allocation, to
 dynamically and non-intrusively add capabilities to fixed
 types using the RUNTIME MIXIN pattern.


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++.


The complete code for `countable_ptr`,
 `countability`, and the `countable new`
 is also available.
*First published in* [Overload](http://www.accu.org/index.php/overloadonline)
*25, April 1998, ISSN 1354-3172*