.. mode: -*- rst -*-

Protocol inheritance
====================

:Tag: design.mps.protocol
:Author: Tony Mann
:Date: 1998-10-12
:Status: incomplete design
:Revision: $Id$
:Copyright: See `C. Copyright and License`_.
:Index terms: pair: protocol inheritance; design
:Readership: MPS developers


Introduction
------------

_`.intro`: This document explains the design of the support for class
inheritance in MPS.

_`.readership`: This document is intended for any MPS developer.


Purpose
-------

_`.purpose.code-maintain`: The purpose of the protocol inheritance
design is to ensure that the MPS code base can make use of the
benefits of object-oriented class inheritance to maximize code reuse,
minimize code maintenance and minimize the use of boilerplate code.

_`.purpose.related`: For related discussion, see
`mail.tony.1998-08-28.16-26`_, `mail.tony.1998-09-01.11-38`_,
`mail.tony.1998-10-06.11-03`_ and other messages in the same threads.

.. _mail.tony.1998-10-06.11-03: https://info.ravenbrook.com/project/mps/mail/1998/10/06/11-03/0.txt
.. _mail.tony.1998-09-01.11-38: https://info.ravenbrook.com/project/mps/mail/1998/09/01/11-38/0.txt
.. _mail.tony.1998-08-28.16-26: https://info.ravenbrook.com/project/mps/mail/1998/08/28/16-26/0.txt


Requirements
------------

_`.req.implicit`: The object system should provide a means for classes
to inherit the methods of their direct superclasses implicitly for all
functions in the protocol without having to write any explicit code
for each inherited function.

_`.req.override`: There must additionally be a way for classes to
override the methods of their superclasses.

_`.req.next-method`: As a result of `.req.implicit`_, classes cannot
make static assumptions about methods used by direct superclasses. The
object system must provide a means for classes to extend (not just
replace) the behaviour of protocol functions, such as a mechanism for
invoking the "next-method".

_`.req.ideal.extend`: The object system must provide a standard way
for classes to implement the protocol supported by their superclass and
additionally add new methods of their own which can be specialized by
subclasses.

_`.req.ideal.multiple-inheritance`: The object system should support
multiple inheritance such that sub-protocols can be "mixed in" with
several classes which do not themselves support identical protocols.


Overview
--------

_`.overview.inst`: The key concept in the design is the relationship
between an "instance" and its "class".  Every structure that
participates in the protocol system begins with an ``InstStruct``
structure that contains a pointer to an ``InstClassStruct`` that
describes it, like this::

      instance          class

     .----------.      .----------.
     |  class   |----->|  class   |
     ------------      ------------
     |  ...     |      |  sig     |
     ------------      ------------
     |  ...     |      |  name    |
     ------------      ------------
     |  ...     |      |superclass|
     ------------      ------------
     |          |      |   ...    |

_`.overview.prefix`: We make use of the fact that we can cast between
structures with common prefixes, or between structures and their first
members, to provide dynamic typing and subtyping (see
[Kernighan_1988]_, A.8.3).

_`.overview.method`: The ``InstClassStruct`` it itself at the start of
a class structure contains pointers to functions that can be called to
manipulate the instance as an abstract data type.  We refer to these
functions as "methods" to distinguish them from functions not involved
in the object-oriented protocol.  The macro ``Method`` is provided for
calling methods.

_`.overview.subclass`: An instance structure can be extended by using
it as the first field of another structure, and by overriding its
class pointer with a pointer to a "subclass" that provides different
behavior.

_`.overview.inherit`: Classes inherit the methods from their
superclasses when they are initialized, so by default they have the
same methods as the class from which they inherit.  Methods on the
superclass can be re-used, providing polymorphism.

_`.overview.inherit.specialize`: Classes may specialize the behaviour
of their superclass. They do this by by overriding methods or other
fields in the class object.

_`.overview.mixin`: Groups of related overrides are provided by
"mixins", and this provides a limited form of multiple inheritance.

_`.overview.extend`: Classes may extend the protocols supported by
their superclasses by adding new fields for methods or other data.
Extending a class creates a new kind of class.

_`.overview.kind`: Classes are themselves instance objects, and have
classes of their own.  A class of a class is referred to as a "kind",
but is not otherwise special.  Classes which share the same set of
methods (or other class fields) are instances of the same kind.  If a
class is extended, it becomes a member of a different kind.  Kinds
allow subtype checking to be applied to classes as well as instances,
to determine whether methods are available. ::

      instance          class             kind
      (e.g. CBS)        (e.g. CBSClass)   (e.g. LandClassClass)
     .----------.      .----------.      .----------.
     |  class   |----->|  class   |----->|  class   |-->InstClassClass
     ------------      ------------      ------------
     |  ...     |      |  sig     |      |  sig     |
     ------------      ------------      ------------
     |  ...     |      |  name    |      |  name    |
     ------------      ------------      ------------
     |  ...     |      |superclass|-.    |superclass|-->InstClassClass
     ------------      ------------ |    ------------
     |          |      |   ...    | |    |   ...    |
                                    |
                                    |
                         LandClass<-'


_`.overview.sig.inherit`: Instances (and therefore classes) will
contain signatures. Classes must not specialize (override) the
signatures they inherit from their superclasses, as they are used to
check the actual type (not sub- or supertype) of the object they're
in.

_`.overview.sig.extend`: When extending an instance or class, it is
normal policy for the new structure to include a new signature as the
last field.

_`.overview.superclass`: Each class contains a ``superclass`` field.
This enables classes to call "next-method".

_`.overview.next-method`: A specialized method in a class can make use
of an overridden method from a superclass using the ``NextMethod``
macro, statically naming the superclass.

_`.overview.next-method.dynamic`: It is possible to write a method
which does not statically know its superclass, and call the next
method by extracting a class from one of its arguments using
``ClassOfPoly`` and finding the superclass using ``SuperclassPoly``.
Debug pool mixins do this.  However, this is not fully general, and
combining such methods is likely to cause infinite recursion.  Take
care!

_`.overview.access`: Classes must be initialized by calls to
functions, since there is no way to express overrides statically in
C89.  ``DEFINE_CLASS`` defines an "ensure" function that initializes
and returns the canonical copy of the class. The canonical copy may
reside in static storage, but no MPS code may refer to that static
storage by name.

_`.overview.init`: In addition to the "ensure" function, each class
must provide an "init" function, which initialises its argument as a
fresh copy of the class.  This allows subclasses to derive their
methods and other fields from superclasses.

_`.overview.naming`: There are some strict naming conventions which
must be followed when defining and using classes. The use is
obligatory because it is assumed by the macros which support the
definition and inheritance mechanism. For every kind ``Foo``,
we insist upon the following naming conventions:

* ``Foo`` names a type that points to a ``FooStruct``.

* ``FooStruct`` is the type of the instance structure, the first field
  of which is the structure it inherits from (ultimately an
  ``InstStruct``).

* ``FooClass`` names the type that points to a ``FooClassStruct``.

* ``FooClassStruct`` names the structure for the class pointed to by
  ``FooStruct``, containing the methods that operate on ``Foo``.


Interface
---------


Class declaration
.................

``DECLARE_CLASS(kind, className)``

_`.if.declare-class`: Class declaration is performed by the macro
``DECLARE_CLASS``, which declares the existence of the class
definition elsewhere.  It is intended for use in headers.


Class definition
................

``DEFINE_CLASS(kind, className, var)``

_`.if.define-class`: Class definition is performed by the macro
``DEFINE_CLASS``. A call to the macro must be followed by a function
body of initialization code. The parameter ``className`` is used to
name the class being defined. The parameter ``var`` is used to name a
local variable of type of classes of kind ``kind``, which is defined
by the macro; it refers to the canonical storage for the class being
defined.  This variable may be used in the initialization code. (The
macro doesn't just pick a name implicitly because of the danger of a
name clash with other names used by the programmer). A call to the
macro defines the ensure function for the class along with some static
storage for the canonical class object, and some other things to
ensure the class gets initialized at most once.


Class access
............

``CLASS(className)``

_`.if.class`: To get the canonical class object, use the ``CLASS``
macro, e.g. ``CLASS(Land)``.


Single inheritance
..................

``INHERIT_CLASS(this, className, parentName)``

_`.if.inheritance`: Class inheritance details must be provided in the
class initialization code (see `.if.define-class`_). Inheritance is
performed by the macro ``INHERIT_CLASS``. A call to this macro will
make the class being defined a direct subclass of ``parentClassName``
by ensuring that all the fields of the embedded parent class (pointed
to by the ``this`` argument) are initialized as the parent class, and
setting the superclass field of ``this`` to be the canonical parent
class object. The parameter ``this`` must be the same kind as
``parentClassName``.


Specialization
..............

_`.if.specialize`: Fields in the class structure must be assigned
explicitly in the class initialization code (see
`.if.define-class`_). This must happen *after* inheritance details
are given (see `.if.inheritance`_), so that overrides work.


Extension
.........

_`.if.extend`: To extend the protocol when defining a new class, a
new type must be defined for the class structure. This must embed the
structure for the primarily inherited class as the first field of the
structure. Extension fields in the class structure must be assigned
explicitly in the class initialization code (see
`.if.define-class`_).  This should be done *after* the inheritance
details are given for consistency with `.if.inheritance`_.  This is,
in fact, how all the useful classes extend ``Inst``.

_`.if.extend.kind`: In addition, a class must be defined for the new
kind of class.  This is just an unspecialized subclass of the kind of
the class being specialized by the extension.  For example::

    typedef struct LandClassStruct {
      InstClassStruct instClass;  /* inherited class */
      LandInsertMethod insert;
      ...
    } LandClassStruct;

    DEFINE_CLASS(Inst, LandClass, class)
    {
      INHERIT_CLASS(class, LandClass, InstClass);
    }

    DEFINE_CLASS(Land, Land, class)
    {
      INHERIT_CLASS(&class->instClass, Land, Inst);
      class->insert = landInsert;
      ...
    }


Methods
.......

``Method(kind, inst, meth)``

_`.if.method`: To call a method on an instance of a class, use the
``Method`` macro to retrieve the method.  This macro may assert if the
class is not of the kind requested.  For example, to call the
``insert`` method on ``land``::

    res = Method(Land, land, insert)(rangeReturn, land, range);


``NextMethod(kind, className, meth)``

_`.if.next-method`: To call a method from a superclass of a class,
use the ``NextMethod`` macro to retrieve the method.  This macro may
assert if the superclass is not of the kind requested.  For example,
the function to split AMS segments wants to split the segments they
are based on, so does::

    res = NextMethod(Seg, AMSSeg, split)(seg, segHi, base, mid, limit);


Conversion
..........


``IsA(className, inst)``

_`if.isa`: Returns non-zero iff the class of ``inst`` is a member of
the class or any of its subclasses.


``MustBeA(className, inst)``

_`.if.must-be-a`: To convert the C type of an instance to that of a
compatible class (the class of the actual object or any superclass),
use the ``MustBeA`` macro.  In hot varieties this macro performs a
fast dynamic type check and will assert if the class is not
compatible.  It is like C++ "dynamic_cast" with an assert.  In cool
varieties, the class check method is called on the object.  For
example, in a specialized Land method in the CBS class::

    static Res cbsInsert(Range rangeReturn, Land land, Range range)
    {
      CBS cbs = MustBeA(CBS, land);
      ...


``MustBeA_CRITICAL(className, inst)``

_`.if.must-be-a.critical`: When the cost of a type check is too
expensive in hot varieties, use ``MustBeA_CRITICAL`` in place of
``MustBeA``.  This only performs the check in cool varieties.  Compare
with ``AVER_CRITICAL``.


``CouldBeA(className, inst)``

_`.if.could-be-a`: To make an unsafe conversion equivalent to
``MustBeA``, use the ``CouldBeA`` macro.  This is in effect a simple
pointer cast, but it expresses the intention of class compatibility in
the source code.  It is mainly intended for use when initializing an
object, when a class compatibility check would fail, when checking an
object, or in debugging code such as describe methods, where asserting
is inappropriate.  It is intended to be equivalent to the C++
``static_cast``, although since this is C there is no actual static
checking, so in fact it's more like ``reinterpret_cast``.


Introspection
.............

_`.introspect.c-lang`: The design includes a number of introspection
functions for dynamically examining class relationships. These
functions are polymorphic and accept arbitrary subclasses of
``InstClass``. C doesn't support such polymorphism. So although these
have the semantics of functions (and could be implemented as functions
in another language with compatible calling conventions) they are
actually implemented as macros. The macros are named as function-style
macros despite the fact that this arguably contravenes
guide.impl.c.macro.method. The justification for this is that this
design is intended to promote the use of polymorphism, and it breaks
the abstraction for the users to need to be aware of what can and
can't be expressed directly in C function syntax. These functions all
have names ending in ``Poly`` to identify them as polymorphic
functions.


``SuperclassPoly(kind, class)``

_`.if.superclass-poly`: An introspection function which returns the
direct superclass of class object ``class`` as a class of kind
``kind``. This may assert if the superclass is not (a subtype of) the
kind requested.


``ClassOfPoly(kind, inst)``

_`.if.class-of-poly`: An introspection function which returns the
class of which ``inst`` is a direct instance, as a class of kind
``kind``. This may assert if the class is not (a subtype of) the kind
requested.


``SetClassOfPoly(inst, class)``

_`.if.set-class-of-poly`: An initialization function that sets the
class of ``inst`` to be ``class``. This is intended only for use in
initialization functions, to specialize the instance once its fields
have been initialized. Each Init function should call its superclass
init, finally reaching InstInit, and then, once it has set up its
fields, use SetClassOfPoly to set the class and check the instance
with its check method. Compare with `design.mps.sig`_.

.. _`design.mps.sig`: sig

``IsSubclass(sub, super)``

_`.if.is-subclass`: An introspection function which returns a ``Bool``
indicating whether ``sub`` is a subclass of ``super``. That is, it is
a predicate for testing subclass relationships.


Protocol guidelines
...................

_`.guide.fail`: When designing an extensible method which might fail,
the design must permit the correct implementation of the failure-case
code.  Typically, a failure might occur in any method in the chain.
Each method is responsible for correctly propagating failure
information supplied by superclass methods and for managing it's own
failures.  This is not really different from the general MPS
convention for unwinding on error paths.  It implies that the design
of a class must include an anti-method for each method that changes
the state of an instance (e.g. by allocating memory) to allow the
state to be reverted in case of a failure.  See `.example.fail`_
below.


Example
.......

_`.example.inheritance`: The following example class definition shows
both inheritance and specialization. It shows the definition of the
class ``RankBuf``, which inherits from ``SegBuf`` of kind ``Seg``
and has specialized ``varargs`` and ``init`` method. ::

    DEFINE_CLASS(Buffer, RankBuf, class)
    {
      INHERIT_CLASS(class, RankBuf, SegBuf);
      class->varargs = rankBufVarargs;
      class->init = rankBufInit;
    }

_`.example.extension`: The following (hypothetical) example class
definition shows inheritance, specialization and also extension. It
shows the definition of the class ``EPDLDebugPool``, which inherits
from ``EPDLPool`` of kind ``Pool``, but also implements a method for
checking properties of the pool. ::

    typedef struct EPDLDebugPoolClassStruct {
      EPDLPoolClassStruct epdl;
      DebugPoolCheckMethod check;
      Sig sig;
    } EPDLDebugPoolClassStruct;

    typedef EPDLDebugPoolClassStruct *EPDLDebugPoolClass;

    DEFINE_CLASS(Inst, EPDLDebugPoolClass, class)
    {
      INHERIT_CLASS(class, EPDLPoolClass, InstClass);
    }

    DEFINE_CLASS(EPDLDebugPool, EPDLDebugPool, class)
    {
      INHERIT_CLASS(&class->epdl, EPDLDebugPool, EPDLPoolClass);
      class->check = EPDLDebugCheck;
      class->sig = EPDLDebugSig;
    }

_`.example.fail`: The following example shows the implementation of
failure-case code for an "init" method, making use of the "finish"
anti-method to clean-up a subsequent failure. ::

    static Res AMSSegInit(Seg seg, Pool pool,
                          Addr base, Size size,
                          ArgList args)
    {
      AMS ams = MustBeA(AMSPool, pool);
      Arena arena = PoolArena(pool);
      AMSSeg amsseg;
      Res res;

      /* Initialize the superclass fields first via next-method call */
      res = NextMethod(Seg, AMSSeg, init)(seg, pool, base, size, args);
      if (res != ResOK)
        goto failNextMethod;
      amsseg = CouldBeA(AMSSeg, seg);

      amsseg->grains = size >> ams->grainShift;
      amsseg->freeGrains = amsseg->grains;
      amsseg->oldGrains = (Count)0;
      amsseg->newGrains = (Count)0;
      amsseg->marksChanged = FALSE; /* <design/poolams/#marked.unused> */
      amsseg->ambiguousFixes = FALSE;

      res = amsCreateTables(ams, &amsseg->allocTable,
                            &amsseg->nongreyTable, &amsseg->nonwhiteTable,
                            arena, amsseg->grains);
      if (res != ResOK)
        goto failCreateTables;

      /* start off using firstFree, see <design/poolams/#no-bit> */
      amsseg->allocTableInUse = FALSE;
      amsseg->firstFree = 0;
      amsseg->colourTablesInUse = FALSE;

      amsseg->ams = ams;
      RingInit(&amsseg->segRing);
      RingAppend((ams->allocRing)(ams, SegRankSet(seg), size),
                 &amsseg->segRing);

      SetClassOfPoly(seg, CLASS(AMSSeg));
      amsseg->sig = AMSSegSig;
      AVERC(AMSSeg, amsseg);

      return ResOK;

    failCreateTables:
      NextMethod(Seg, AMSSeg, finish)(seg);
    failNextMethod:
      AVER(res != ResOK);
      return res;
    }


Implementation
--------------

_`.impl.define-class.lock`: The ``DEFINE_CLASS`` macro ensures that
each class is initialized at most once (even in multi-threaded
programs) by claiming the global recursive lock (see design.mps.thread-safety.arch.global.recursive_).

.. _design.mps.thread-safety.arch.global.recursive: thread-safety#.arch.global.recursive

_`.impl.derived-names`: The ``DEFINE_CLASS()`` macro derives some
additional names from the class name as part of it's implementation.
These should not appear in the source code, but it may be useful to
know about this for debugging purposes. For each class definition for
class ``SomeClass`` of kind ``SomeKind``, the macro defines the
following:

* ``extern SomeKind SomeClassGet(void);``

  The class ensure function. See `.overview.naming`_.  This function
  handles local static storage for the canonical class object and a
  guardian to ensure the storage is initialized at most once.  This
  function is invoked by the ``CLASS`` macro (`.if.class`_).

* ``static void SomeClassInit(SomeKind);``

  A function called by ``SomeClassGet()``. All the class
  initialization code is actually in this function.

_`.impl.subclass`: The subclass test `.if.is-subclass`_ is implemented
using an array of superclasses [Cohen_1991]_ giving a fast
constant-time test.  (RB_ tried an approach using prime factors
[Gibbs_2004]_ but found that they overflowed in 32-bits too easily to
be useful.)  Each class is assigned a "level" which is the distance
from the root of the class hierarchy.  The ``InstClass`` structure
contains an array of class ids indexed by level, representing the
inheritance of this class.  A class is a subclass of another if and
only if the superclass id is present in the array at the superclass
level.  The level is statically defined using enum constants, and the
id is the address of the canonical class object, so the test is fast
and simple.

.. _RB: https://www.ravenbrook.com/consultants/rb/


Common instance methods
-----------------------

_`.method`: These methods are available on all instances.

``typedef void (*FinishMethod)(Inst inst)``

_`.method.finish`: The ``finish`` method should finish the instance
data structure (releasing any resources that were acquired by the
instance during its lifetime) and then call its superclass method via
the ``NextMethod()`` macro.

``typedef Res (*DescribeMethod)(Inst inst, mps_lib_FILE *stream, Count depth)``

_`.method.describe`: The ``describe`` field should print out a
description of the instance to ``stream`` (by calling ``WriteF()``).


A. References
-------------

.. [Cohen_1991] "Type-Extension Type Tests Can Be Performed In
   Constant Time"; Norman H Cohen; IBM Thomas J Watson Research
   Center; ACM Transactions on Programming Languages and Systems,
   Vol. 13 No. 4, pp. 626-629; 1991-10.

.. [Gibbs_2004] "Fast Dynamic Casting"; Michael Gibbs, Bjarne
   Stroustrup; 2004;
   <http://www.stroustrup.com/fast_dynamic_casting.pdf>.

.. [Kernighan_1988] "The C Programming language 2nd Edition"; Brian W.
   Kernighan, Dennis M. Ritchie; 1988.


B. Document History
-------------------

- 1998-10-12 Tony Mann. Initial draft.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2013-04-14 GDR_ Converted to reStructuredText.

- 2016-04-07 RB_ Removing never-used multiple inheritance speculation.

- 2016-04-08 RB_ Substantial reorgnisation.

- 2016-04-13 RB_ Writing up overview of kinds, with explanation of
  class extension.  Writing up ``Method``, ``NextMethod``,
  ``SetClassOfPoly``, ``MustBeA``, etc. and updating the descriptions
  of some older interface.  Updating the example.

- 2016-04-19 RB_ Miscellaneous clean-up in response to review by GDR_.

.. _GDR: https://www.ravenbrook.com/consultants/gdr/


C. Copyright and License
------------------------

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