CGAL 6.1 - Manual

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Reference Counting and Handle Types

Author
  Stefan Schirra (`stsch.nosp@m.irr@.nosp@m.mpi-s.nosp@m.b.mp.nosp@m.g.de`)

# Reference counting

As of release 2.1, a reference counting scheme is used for the kernel objects in the kernels `CGAL::Cartesian` and `CGAL::Homogeneous`. All copies of an object share a common representation object storing the data associated with a kernel object; see Figure 0.1.

The motivation is to save space by avoiding storing the same data more than once and to save time in the copying procedure. Of course, whether we actually save time and space depends on the size of the data that we would have to copy without sharing representations. The drawback is an indirection in accessing the data. Such an indirection is bad in terms of cache efficiency. Thus there are also non-reference-counting kernels available `CGAL::Simple_cartesian` and `CGAL::Simple_homogeneous`.

Figure 0.1 Objects using reference counting (bottom) share common representation; copying creates a new handle (drawn at the right) pointing to the same representation as the object copied. Without reference counting (top) all data are copied to the new object (drawn at the right);

The reference counting in the kernel objects is not visible to a user and does not affect the interface of the objects. The representation object is often called the body. The object possibly sharing its representation with others is called a handle, because its data consists of a pointer to its representation object only. If the implementation of the member functions is located with the representation object and the functions in the handle just forward calls to the body, the scheme implements the bridge design pattern, which is used to separate an interface from its implementation. The intent of this design pattern is to allow for exchanging implementations of member functions hidden to the user, especially at runtime.

# Handle & Rep

The two classes `Handle` and `Rep` provide reference counting functionality; see Figure 0.2 . By deriving from these classes, the reference counting functionality is inherited. The class `Rep` provides a counter; representation classes derive from this class. The class `Handle` takes care of all the reference-counting related stuff. In particular, it provides appropriate implementations of copy constructor, copy assignment, and destructor. These functions take care of the counter in the common representation. Classes sharing reference-counted representation objects (of a class derived from `Rep`) do not have to worry about the reference counting, with the exception of non-copy-constructors.

There a new representation object must be created and the pointer to the representation object must be set.

If `CGAL_USE_LEDA` is defined and `CGAL_NO_LEDA_HANDLE` is not defined, the types `Handle` and `Rep` are set to the LEDA types `handle_base` and `handle_rep`, respectively (yes, without a `leda_`-prefix). Use of the LEDA class `handle_rep` implies that LEDA memory management is used for the representation types.

typedef handle_base Handle;
typedef handle_rep Rep;
Handle

Scavenging LEDA, we provide the identical functionality in the CGAL classes `Leda_like_handle` and `Leda_like_rep`. If LEDA is not available or `CGAL_NO_LEDA_HANDLE` is set, `Handle` and `Rep` correspond to these types.

typedef Leda_like_handle Handle;
typedef Leda_like_rep Rep;

Figure 0.2 UML class diagram for Handle & Rep scheme.

# Using Handle & Rep

In order to make use of the reference counting provided by the classes `Handle` and `Rep`, your interface class (the class providing the interface of the geometric object) must be derived from `Handle` and your representation class (the class containing the data to be shared) must be derived from `Rep`:

class My_rep : public Rep { /*...*/ };
class My_geo_object : public Handle
{
public:
My_geo_object(const My_geo_object& m);
My_geo_object(Arg1, Arg2);
};

The class `My_geo_object` is responsible for allocating and constructing the `My_rep` object "on the heap". Typically, a constructor call is forwarded to a corresponding constructor of `My_rep`. The address of the new `My_rep` is assigned to `PTR` inherited from `Handle`, e.g.:

My_geo_object::My_geo_object(Arg1 a1, Arg2 a2)
{ PTR = new My_rep(a1, a2); }

The default constructor of `Handle` is called automatically by the compiler and the reference counting is initialized. You always have to define a copy constructor for `My_geo_object`

and to call the copy constructor of `Handle` there:

My_geo_object::My_geo_object(const My_geo_object& m)
: Handle( m)
{}

That's it! There is no need to define a copy assignment operator nor is there a need to define a destructor for the derived class `My_geo_object`! Handle & Rep does the rest for you! You get this functionality by including `CGAL/Handle.h`

It is common practice to add a (protected) member function `ptr()` to the class `My_geo_object`, which casts the `PTR` pointer from `Rep*` to the actual type `My_rep*`.

Note that this scheme is meant for non-modifiable types. You are not allowed to modify data in the representation object, because the data are possibly shared with other `My_geo_object` objects.

# Templated handles

Factoring out the common functionality in base classes enables reuse of the code, but there is also a major drawback. The `Handle` class does not know the type of the representation object. It maintains a `Rep*` pointer. Therefore, this pointer must be cast to a pointer to the actual type in the classes derived from `Handle`. Moreover, since the `Handle` calls the destructor for the representation through a `Rep*`, the destructor of `Rep` must be virtual. Finally, debugging is difficult, again, because the `Handle` class does not know the type of the representation object. Making the actual type of the representation object a template parameter of the handle solves these problems. This is implemented in class template `Handle_for`. This class assumes that the reference-counted class provides the following member functions to manage its internal reference counting:

- `add_reference()`

- `remove_reference()`

- `bool is_referenced()`

- `bool is_shared()`

See the UML class diagram in Figure 0.3. The reference counting functionality and the required interface can be inherited from class `Ref_counted`.

Figure 0.3 UML diagram for templated handles.

Kernel objects have used such a handle/rep scheme since release 2.2.

# Using templated handles

In order to make use of the reference counting provided by the classes `Handle_for` and `Ref_counted`, your representation class, let's say `My_rep` (the class containing the data to be shared), must provide the interface described above (e.g., by deriving from `Ref_counted`), and your interface class (the class providing the interface of the geometric object) must be derived from `Handle_for<My_rep>`. It is assumed that the default constructor of `My_rep` sets the counter to 1 (the default constructor of `Ref_counted` does this, of course):

class My_rep : public Ref_counted { /*...*/ };
class My_geo_object : public Handle_for<My_rep>
{
public:
My_geo_object(const My_geo_object& m);
My_geo_object(Arg1, Arg2);
};

You should also define a copy constructor for `My_rep` as this may be used in in-place creation in the allocator scheme. The constructors of class `My_geo_object` are responsible for constructing the `My_rep` object. Typically, a corresponding constructor call of `My_rep` is forwarded to `Handle_for`.

My_geo_object::My_geo_object(Arg1 a1, Arg2 a2)
: Handle_for<My_rep>( My_rep(a1, a2)) {}

Sometimes, you have to do some calculation first before you can create a representation object in a constructor of `My_rep`.

Then you can use the `initialize_with()` member function of `Handle_for`.

In both cases, `Handle_for` takes care of allocating space for the new object.

If you define a copy constructor for `My_geo_object` you have to call the copy constructor of `Handle_for` there:

My_geo_object::My_geo_object(const My_geo_object& m)
: Handle_for<My_rep>( m)
{}

That's it! Again, there is no need to define a copy assignment operator nor is there a need to define a destructor for the derived class `My_geo_object`! `Handle_for` does the rest for you!

`Handle_for` provides you with an option to modify the data. There is a `copy_on_write()`

that should be called right before every modification of the data. It ensures that only your data are overwritten:

void
My_geo_object::set_x(double d)
{
copy_on_write();
ptr->x = d;
}

You get this functionality by including `CGAL/Handle_for.h`

# Allocation

Class `CGAL::Handle_for` has two template parameters. Besides the type of the stored object, there is also a parameter specifying an allocator. Any concrete argument must be a model for the `Allocator` concept defined in the C++ standard. There is a default value for the second parameter defined as `CGAL_ALLOCATOR(T)`. But you can also choose your own, for example

class My_geo_object : public Handle_for<My_rep, leda_allocator<My_rep> >
{ /* ... */ };

The default allocator is defined in `CGAL/memory.h`

See Chapter Memory Management for more information.