doc/concepts.tex: Delete fixme-ref to direct instances.

[sod] / doc / concepts.tex

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\chapter{Concepts} \label{ch:concepts}

%%%--------------------------------------------------------------------------
\section{Modules} \label{sec:concepts.modules}

A \emph{module} is the top-level syntactic unit of input to the Sod
translator.  As described above, given an input module, the translator
generates C source and header files.

A module can \emph{import} other modules.  This makes the type names and
classes defined in those other modules available to class definitions in the
importing module.  Sod's module system is intentionally very simple.  There
are no private declarations or attempts to hide things.

As well as importing existing modules, a module can include a number of
different kinds of \emph{items}:
\begin{itemize}
\item \emph{class definitions} describe new classes, possibly in terms of
  existing classes;
\item \emph{type name declarations} introduce new type names to Sod's
  parser;\footnote{%
    This is unfortunately necessary because C syntax, upon which Sod's input
    language is based for obvious reasons, needs to treat type names
    differently from other kinds of identifiers.} %
  and
\item \emph{code fragments} contain literal C code to be dropped into an
  appropriate place in an output file.
\end{itemize}
Each kind of item, and, indeed, a module as a whole, can have a collection of
\emph{properties} associated with it.  A property has a \emph{name} and a
\emph{value}.  Properties are an open-ended way of attaching additional
information to module items, so extensions can make use of them without
having to implement additional syntax.

%%%--------------------------------------------------------------------------
\section{Classes, instances, and slots} \label{sec:concepts.classes}

For the most part, Sod takes a fairly traditional view of what it means to be
an object system.

An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.  An
object's state is maintained in named \emph{slots}, each of which can store a
C value of an appropriate (scalar or aggregate) type.  An object's behaviour
is stimulated by sending it \emph{messages}.  A message has a name, and may
carry a number of arguments, which are C values; sending a message may result
in the state of receiving object (or other objects) being changed, and a C
value being returned to the sender.

Every object is a (direct) instance of some \emph{class}.  The class
determines which slots its instances have, which messages its instances can
be sent, and which methods are invoked when those messages are received.  The
Sod translator's main job is to read class definitions and convert them into
appropriate C declarations, tables, and functions.  An object cannot
(usually) change its direct class, and the direct class of an object is not
affected by, for example, the static type of a pointer to it.


\subsection{Superclasses and inheritance}
\label{sec:concepts.classes.inherit}

\subsubsection{Class relationships}
Each class has zero or more \emph{direct superclasses}.

A class with no direct superclasses is called a \emph{root class}.  The Sod
runtime library includes a root class named @|SodObject|; making new root
classes is somewhat tricky, and won't be discussed further here.

Classes can have more than one direct superclass, i.e., Sod supports
\emph{multiple inheritance}.  A Sod class definition for a class~$C$ lists
the direct superclasses of $C$ in a particular order.  This order is called
the \emph{local precedence order} of $C$, and the list which consists of $C$
follows by $C$'s direct superclasses in local precedence order is called the
$C$'s \emph{local precedence list}.

The multiple inheritance in Sod works similarly to multiple inheritance in
Lisp-like languages, such as Common Lisp, EuLisp, Dylan, and Python, which is
very different from how multiple inheritance works in \Cplusplus.\footnote{%
  The latter can be summarized as `badly'.  By default in \Cplusplus, an
  instance receives an additional copy of superclass's state for each path
  through the class graph from the instance's direct class to that
  superclass, though this behaviour can be overridden by declaring
  superclasses to be @|virtual|.  Also, \Cplusplus\ offers only trivial
  method combination (\xref{sec:concepts.methods}), leaving programmers to
  deal with delegation manually and (usually) statically.} %

If $C$ is a class, then the \emph{superclasses} of $C$ are
\begin{itemize}
\item $C$ itself, and
\item the superclasses of each of $C$'s direct superclasses.
\end{itemize}
The \emph{proper superclasses} of a class $C$ are the superclasses of $C$
except for $C$ itself.  If a class $B$ is a (direct, proper) superclass of
$C$, then $C$ is a \emph{(direct, proper) subclass} of $B$.  If $C$ is a root
class then the only superclass of $C$ is $C$ itself, and $C$ has no proper
superclasses.

If an object is a direct instance of class~$C$ then the object is also an
(indirect) instance of every superclass of $C$.

If $C$ has a proper superclass $B$, then $B$ must not have $C$ as a direct
superclass.  In different terms, if we construct a directed graph, whose
nodes are classes, and draw an arc from each class to each of its direct
superclasses, then this graph must be acyclic.  In yet other terms, the `is a
superclass of' relation is a partial order on classes.

\subsubsection{The class precedence list}
This partial order is not quite sufficient for our purposes.  For each class
$C$, we shall need to extend it into a total order on $C$'s superclasses.
This calculation is called \emph{superclass linearization}, and the result is
a \emph{class precedence list}, which lists each of $C$'s superclasses
exactly once.  If a superclass $B$ precedes (resp.\ follows) some other
superclass $A$ in $C$'s class precedence list, then we say that $B$ is a more
(resp.\ less) \emph{specific} superclass of $C$ than $A$ is.

The superclass linearization algorithm isn't fixed, and extensions to the
translator can introduce new linearizations for special effects, but the
following properties are expected to hold.
\begin{itemize}
\item The first class in $C$'s class precedence list is $C$ itself; i.e.,
  $C$ is always its own most specific superclass.
\item If $A$ and $B$ are both superclasses of $C$, and $A$ is a proper
  superclass of $B$ then $A$ appears after $B$ in $C$'s class precedence
  list, i.e., $B$ is a more specific superclass of $C$ than $A$ is.
\end{itemize}
The default linearization algorithm used in Sod is the \emph{C3} algorithm,
which has a number of good properties described in~\cite{Barrett:1996:MSL}.
It works as follows.
\begin{itemize}
\item A \emph{merge} of some number of input lists is a single list
  containing each item that is in any of the input lists exactly once, and no
  other items; if an item $x$ appears before an item $y$ in any input list,
  then $x$ also appears before $y$ in the merge.  If a collection of lists
  have no merge then they are said to be \emph{inconsistent}.
\item The class precedence list of a class $C$ is a merge of the local
  precedence list of $C$ together with the class precedence lists of each of
  $C$'s direct superclasses.
\item If there are no such merges, then the definition of $C$ is invalid.
\item Suppose that there are multiple candidate merges.  Consider the
  earliest position in these candidate merges at which they disagree.  The
  \emph{candidate classes} at this position are the classes appearing at this
  position in the candidate merges.  Each candidate class must be a
  superclass of distinct direct superclasses of $C$, since otherwise the
  candidates would be ordered by their common subclass's class precedence
  list.  The class precedence list contains, at this position, that candidate
  class whose subclass appears earliest in $C$'s local precedence order.
\end{itemize}

\begin{figure}
  \centering
  \begin{tikzpicture}[x=7.5mm, y=-14mm, baseline=(current bounding box.east)]
    \node[lit] at ( 0,  0) (R) {SodObject};
    \node[lit] at (-3, +1) (A) {A};     \draw[->] (A) -- (R);
    \node[lit] at (-1, +1) (B) {B};     \draw[->] (B) -- (R);
    \node[lit] at (+1, +1) (C) {C};     \draw[->] (C) -- (R);
    \node[lit] at (+3, +1) (D) {D};     \draw[->] (D) -- (R);
    \node[lit] at (-2, +2) (E) {E};     \draw[->] (E) -- (A);
                                        \draw[->] (E) -- (B);
    \node[lit] at (+2, +2) (F) {F};     \draw[->] (F) -- (A);
                                        \draw[->] (F) -- (D);
    \node[lit] at (-1, +3) (G) {G};     \draw[->] (G) -- (E);
                                        \draw[->] (G) -- (C);
    \node[lit] at (+1, +3) (H) {H};     \draw[->] (H) -- (F);
    \node[lit] at ( 0, +4) (I) {I};     \draw[->] (I) -- (G);
                                        \draw[->] (I) -- (H);
  \end{tikzpicture}
  \quad
  \vrule
  \quad
  \begin{minipage}[c]{0.45\hsize}
    \begin{nprog}
      class A: SodObject \{ \}\quad\=@/* @|A|, @|SodObject| */  \\
      class B: SodObject \{ \}\>@/* @|B|, @|SodObject| */       \\
      class C: SodObject \{ \}\>@/* @|B|, @|SodObject| */       \\
      class D: SodObject \{ \}\>@/* @|B|, @|SodObject| */       \\+
      class E: A, B \{ \}\quad\=@/* @|E|, @|A|, @|B|, \dots */  \\
      class F: A, D \{ \}\>@/* @|F|, @|A|, @|D|, \dots */       \\+
      class G: E, C \{ \}\>@/* @|G|, @|E|, @|A|,
                                @|B|, @|C|, \dots */            \\
      class H: F \{ \}\>@/* @|H|, @|F|, @|A|, @|D|, \dots */    \\+
      class I: G, H \{ \}\>@/* @|I|, @|G|, @|E|, @|H|, @|F|,
                                @|A|, @|B|, @|C|, @|D|, \dots */
    \end{nprog}
  \end{minipage}

  \caption{An example class graph and class precedence lists}
  \label{fig:concepts.classes.cpl-example}
\end{figure}

\begin{example}
  Consider the class relationships shown in
  \xref{fig:concepts.classes.cpl-example}.

  \begin{itemize}

  \item @|SodObject| has no proper superclasses.  Its class precedence list
    is therefore simply $\langle @|SodObject| \rangle$.

  \item In general, if $X$ is a direct subclass only of $Y$, and $Y$'s class
    precedence list is $\langle Y, \ldots \rangle$, then $X$'s class
    precedence list is $\langle X, Y, \ldots \rangle$.  This explains $A$,
    $B$, $C$, $D$, and $H$.

  \item $E$'s list is found by merging its local precedence list $\langle E,
    A, B \rangle$ with the class precedence lists of its direct superclasses,
    which are $\langle A, @|SodObject| \rangle$ and $\langle B, @|SodObject|
    \rangle$.  Clearly, @|SodObject| must be last, and $E$'s local precedence
    list orders the rest, giving $\langle E, A, B, @|SodObject|, \rangle$.
    $F$ is similar.

  \item We determine $G$'s class precedence list by merging the three lists
    $\langle G, E, C \rangle$, $\langle E, A, B, @|SodObject| \rangle$, and
    $\langle C, @|SodObject| \rangle$.  The class precedence list begins
    $\langle G, E, \ldots \rangle$, but the individual lists don't order $A$
    and $C$.  Comparing these to $G$'s direct superclasses, we see that $A$
    is a superclass of $E$, while $C$ is a superclass of -- indeed equal to
    -- $C$; so $A$ must precede $C$, as must $B$, and the final list is
    $\langle G, E, A, B, C, @|SodObject| \rangle$.

  \item Finally, we determine $I$'s class precedence list by merging $\langle
    I, G, H \rangle$, $\langle G, E, A, B, C, @|SodObject| \rangle$, and
    $\langle H, F, A, D, @|SodObject| \rangle$.  The list begins $\langle I,
    G, \ldots \rangle$, and then we must break a tie between $E$ and $H$; but
    $E$ is a superclass of $G$, so $E$ wins.  Next, $H$ and $F$ must precede
    $A$, since these are ordered by $H$'s class precedence list.  Then $B$
    and $C$ precede $D$, since the former are superclasses of $G$, and the
    final list is $\langle I, G, E, H, F, A, B, C, D, @|SodObject| \rangle$.

  \end{itemize}

  (This example combines elements from \cite{Barrett:1996:MSL} and
  \cite{Ducournau:1994:PMM}.)
\end{example}

\subsubsection{Class links and chains}
The definition for a class $C$ may distinguish one of its proper superclasses
as being the \emph{link superclass} for class $C$.  Not every class need have
a link superclass, and the link superclass of a class $C$, if it exists, need
not be a direct superclass of $C$.

Superclass links must obey the following rule: if $C$ is a class, then there
must be no three distinct superclasses $X$, $Y$ and~$Z$ of $C$ such that $Z$
is the link superclass of both $X$ and $Y$.  As a consequence of this rule,
the superclasses of $C$ can be partitioned into linear \emph{chains}, such
that superclasses $A$ and $B$ are in the same chain if and only if one can
trace a path from $A$ to $B$ by following superclass links, or \emph{vice
versa}.

Since a class links only to one of its proper superclasses, the classes in a
chain are naturally ordered from most- to least-specific.  The least specific
class in a chain is called the \emph{chain head}; the most specific class is
the \emph{chain tail}.  Chains are often named after their chain head
classes.


\subsection{Names}
\label{sec:concepts.classes.names}

Classes have a number of other attributes:
\begin{itemize}
\item A \emph{name}, which is a C identifier.  Class names must be globally
  unique.  The class name is used in the names of a number of associated
  definitions, to be described later.
\item A \emph{nickname}, which is also a C identifier.  Unlike names,
  nicknames are not required to be globally unique.  If $C$ is any class,
  then all the superclasses of $C$ must have distinct nicknames.
\end{itemize}


\subsection{Slots} \label{sec:concepts.classes.slots}

Each class defines a number of \emph{slots}.  Much like a structure member, a
slot has a \emph{name}, which is a C identifier, and a \emph{type}.  Unlike
many other object systems, different superclasses of a class $C$ can define
slots with the same name without ambiguity, since slot references are always
qualified by the defining class's nickname.

\subsubsection{Slot initializers}
As well as defining slot names and types, a class can also associate an
\emph{initial value} with each slot defined by itself or one of its
subclasses.  A class $C$ provides an \emph{initialization message} (see
\xref{sec:concepts.lifecycle.birth}, and \xref{sec:structures.root.sodclass})
whose methods set the slots of a \emph{direct} instance of the class to the
correct initial values.  If several of $C$'s superclasses define initializers
for the same slot then the initializer from the most specific such class is
used.  If none of $C$'s superclasses define an initializer for some slot then
that slot will be left uninitialized.

The initializer for a slot with scalar type may be any C expression.  The
initializer for a slot with aggregate type must contain only constant
expressions if the generated code is expected to be processed by a
implementation of C89.  Initializers will be evaluated once each time an
instance is initialized.

Slots are initialized in reverse-precedence order of their defining classes;
i.e., slots defined by a less specific superclass are initialized earlier
than slots defined by a more specific superclass.  Slots defined by the same
class are initialized in the order in which they appear in the class
definition.

The initializer for a slot may refer to other slots in the same object, via
the @|me| pointer: in an initializer for a slot defined by a class $C$, @|me|
has type `pointer to $C$'.  (Note that the type of @|me| depends only on the
class which defined the slot, not the class which defined the initializer.)

A class can also define \emph{class slot initializers}, which provide values
for a slot defined by its metaclass; see \xref{sec:concepts.metaclasses} for
details.


\subsection{C language integration} \label{sec:concepts.classes.c}

For each class~$C$, the Sod translator defines a C type, the \emph{class
type}, with the same name.  This is the usual type used when considering an
object as an instance of class~$C$.  No entire object will normally have a
class type,\footnote{%
  In general, a class type only captures the structure of one of the
  superclass chains of an instance.  A full instance layout contains multiple
  chains.  See \xref{sec:structures.layout} for the full details.} %
so access to instances is almost always via pointers.

\subsubsection{Access to slots}
The class type for a class~$C$ is actually a structure.  It contains one
member for each class in $C$'s superclass chain, named with that class's
nickname.  Each of these members is also a structure, containing the
corresponding class's slots, one member per slot.  There's nothing special
about these slot members: C code can access them in the usual way.

For example, given the definition
\begin{prog}
  [nick = mine]                                                 \\
  class MyClass: SodObject \{                                   \\ \ind
    int x;                                                    \-\\
  \}
\end{prog}
the simple function
\begin{prog}
  int get_x(MyClass *m) \{ return (m@->mine.x); \}
\end{prog}
will extract the value of @|x| from an instance of @|MyClass|.

All of this means that there's no such thing as `private' or `protected'
slots.  If you want to hide implementation details, the best approach is to
stash them in a dynamically allocated private structure, and leave a pointer
to it in a slot.  (This will also help preserve binary compatibility, because
the private structure can grow more members as needed.  See
\xref{sec:concepts.compatibility} for more details.)


\subsubsection{Sending messages}
Sod defines a macro for each message.  If a class $C$ defines a message $m$,
then the macro is called @|$C$_$m$|.  The macro takes a pointer to the
receiving object as its first argument, followed by the message arguments, if
any, and returns the value returned by the object's effective method for the
message (if any).  If you have a pointer to an instance of any of $C$'s
subclasses, then you can send it the message; it doesn't matter whether the
subclass is on the same chain.  Note that the receiver argument is evaluated
twice, so it's not safe to write a receiver expression which has
side-effects.

For example, suppose we defined
\begin{prog}
  [nick = soupy]                                                \\
  class Super: SodObject \{                                     \\ \ind
    void msg(const char *m);                                  \-\\
  \}                                                            \\+
  class Sub: Super \{                                           \\ \ind
    void soupy.msg(const char *m)
      \{ printf("sub sent `\%s'@\\n", m); \}                  \-\\
  \}
\end{prog}
then we can send the message like this:
\begin{prog}
  Sub *sub = /* \dots\ */;                                      \\
  Super_msg(sub, "hello");
\end{prog}

What happens under the covers is as follows.  The structure pointed to by the
instance pointer has a member named @|_vt|, which points to a structure
called a `virtual table', or \emph{vtable}, which contains various pieces of
information about the object's direct class and layout, and holds pointers to
method entries for the messages which the object can receive.  The
message-sending macro in the example above expands to something similar to
\begin{prog}
  sub@->_vt.sub.msg(sub, "Hello");
\end{prog}

The vtable contains other useful information, such as a pointer to the
instance's direct class's \emph{class object} (described below).  The full
details of the contents and layout of vtables are given in
\xref{sec:structures.layout.vtable}.


\subsubsection{Class objects}
In Sod's object system, classes are objects too.  Therefore classes are
themselves instances; the class of a class is called a \emph{metaclass}.  The
consequences of this are explored in \xref{sec:concepts.metaclasses}.  The
\emph{class object} has the same name as the class, suffixed with
`@|__class|'\footnote{%
  This is not quite true.  @|$C$__class| is actually a macro.  See
  \xref{sec:structures.layout.additional} for the gory details.} %
and its type is usually @|SodClass|; @|SodClass|'s nickname is @|cls|.

A class object's slots contain or point to useful information, tables and
functions for working with that class's instances.  (The @|SodClass| class
doesn't define any messages, so it doesn't have any methods other than for
the @|SodObject| lifecycle messages @|init| and @|teardown|; see
\xref{sec:concepts.lifecycle}.  In Sod, a class slot containing a function
pointer is not at all the same thing as a method.)

\subsubsection{Conversions}
Suppose one has a value of type pointer-to-class-type for some class~$C$, and
wants to convert it to a pointer-to-class-type for some other class~$B$.
There are three main cases to distinguish.
\begin{itemize}
\item If $B$ is a superclass of~$C$, in the same chain, then the conversion
  is an \emph{in-chain upcast}.  The conversion can be performed using the
  appropriate generated upcast macro (see below), or by simply casting the
  pointer, using C's usual cast operator (or the \Cplusplus\ @|static_cast<>|
  operator).
\item If $B$ is a superclass of~$C$, in a different chain, then the
  conversion is a \emph{cross-chain upcast}.  The conversion is more than a
  simple type change: the pointer value must be adjusted.  If the direct
  class of the instance in question is not known, the conversion will require
  a lookup at runtime to find the appropriate offset by which to adjust the
  pointer.  The conversion can be performed using the appropriate generated
  upcast macro (see below); the general case is handled by the macro
  \descref{SOD_XCHAIN}{mac}.
\item If $B$ is a subclass of~$C$ then the conversion is a \emph{downcast};
  otherwise the conversion is a~\emph{cross-cast}.  In either case, the
  conversion can fail: the object in question might not be an instance of~$B$
  after all.  The macro \descref{SOD_CONVERT}{mac} and the function
  \descref{sod_convert}{fun} perform general conversions.  They return a null
  pointer if the conversion fails.  (These are therefore your analogue to the
  \Cplusplus\ @|dynamic_cast<>| operator.)
\end{itemize}
The Sod translator generates macros for performing both in-chain and
cross-chain upcasts.  For each class~$C$, and each proper superclass~$B$
of~$C$, a macro is defined: given an argument of type pointer to class type
of~$C$, it returns a pointer to the same instance, only with type pointer to
class type of~$B$, adjusted as necessary in the case of a cross-chain
conversion.  The macro is named by concatenating
\begin{itemize}
\item the name of class~$C$, in upper case,
\item the characters `@|__CONV_|', and
\item the nickname of class~$B$, in upper case;
\end{itemize}
e.g., if $C$ is named @|MyClass|, and $B$'s name is @|SuperClass| with
nickname @|super|, then the macro @|MYCLASS__CONV_SUPER| converts a
@|MyClass~*| to a @|SuperClass~*|.  See
\xref{sec:structures.layout.additional} for the formal description.

%%%--------------------------------------------------------------------------
\section{Keyword arguments} \label{sec:concepts.keywords}

In standard C, the actual arguments provided to a function are matched up
with the formal arguments given in the function definition according to their
ordering in a list.  Unless the (rather cumbersome) machinery for dealing
with variable-length argument tails (@|<stdarg.h>|) is used, exactly the
correct number of arguments must be supplied, and in the correct order.

A \emph{keyword argument} is matched by its distinctive \emph{name}, rather
than by its position in a list.  Keyword arguments may be \emph{omitted},
causing some default behaviour by the function.  A function can detect
whether a particular keyword argument was supplied: so the default behaviour
need not be the same as that caused by any specific value of the argument.

Keyword arguments can be provided in three ways.
\begin{enumerate}
\item Directly, as a variable-length argument tail, consisting (for the most
  part) of alternating keyword names, as pointers to null-terminated strings,
  and argument values, and terminated by a null pointer.  This is somewhat
  error-prone, and the support library defines some macros which help ensure
  that keyword argument lists are well formed.
\item Indirectly, through a @|va_list| object capturing a variable-length
  argument tail passed to some other function.  Such indirect argument tails
  have the same structure as the direct argument tails described above.
  Because @|va_list| objects are hard to copy, the keyword-argument support
  library consistently passes @|va_list| objects \emph{by reference}
  throughout its programming interface.
\item Indirectly, through a vector of @|struct kwval| objects, each of which
  contains a keyword name, as a pointer to a null-terminated string, and the
  \emph{address} of a corresponding argument value.  (This indirection is
  necessary so that the items in the vector can be of uniform size.)
  Argument vectors are rather inconvenient to use, but are the only practical
  way in which a caller can decide at runtime which arguments to include in a
  call, which is useful when writing wrapper functions.
\end{enumerate}

Keyword arguments are provided as a general feature for C functions.
However, Sod has special support for messages which accept keyword arguments
(\xref{sec:concepts.methods.keywords}); and they play an essential rôle in
the instance construction protocol (\xref{sec:concepts.lifecycle.birth}).

%%%--------------------------------------------------------------------------
\section{Messages and methods} \label{sec:concepts.methods}

Objects can be sent \emph{messages}.  A message has a \emph{name}, and
carries a number of \emph{arguments}.  When an object is sent a message, a
function, determined by the receiving object's class, is invoked, passing it
the receiver and the message arguments.  This function is called the
class's \emph{effective method} for the message.  The effective method can do
anything a C function can do, including reading or updating program state or
object slots, sending more messages, calling other functions, issuing system
calls, or performing I/O; if it finishes, it may return a value, which is
returned in turn to the message sender.

The set of messages an object can receive, characterized by their names,
argument types, and return type, is determined by the object's class.  Each
class can define new messages, which can be received by any instance of that
class.  The messages defined by a single class must have distinct names:
there is no `function overloading'.  As with slots
(\xref{sec:concepts.classes.slots}), messages defined by distinct classes are
always distinct, even if they have the same names: references to messages are
always qualified by the defining class's name or nickname.

Messages may take any number of arguments, of any non-array value type.
Since message sends are effectively function calls, arguments of array type
are implicitly converted to values of the corresponding pointer type.  While
message definitions may ascribe an array type to an argument, the formal
argument will have pointer type, as is usual for C functions.  A message may
accept a variable-length argument suffix, denoted @|\dots|.

A class definition may include \emph{direct methods} for messages defined by
it or any of its superclasses.

Like messages, direct methods define argument lists and return types, but
they may also have a \emph{body}, and a \emph{rôle}.

A direct method need not have the same argument list or return type as its
message.  The acceptable argument lists and return types for a method depend
on the message, in particular its method combination
(\xref{sec:concepts.methods.combination}), and the method's rôle.

A direct method body is a block of C code, and the Sod translator usually
defines, for each direct method, a function with external linkage, whose body
contains a copy of the direct method body.  Within the body of a direct
method defined for a class $C$, the variable @|me|, of type pointer to class
type of $C$, refers to the receiving object.


\subsection{Effective methods and method combinations}
\label{sec:concepts.methods.combination}

For each message a direct instance of a class might receive, there is a set
of \emph{applicable methods}, which are exactly the direct methods defined on
the object's class and its superclasses.  These direct methods are combined
together to form the \emph{effective method} for that particular class and
message.  Direct methods can be combined into an effective method in
different ways, according to the \emph{method combination} specified by the
message.  The method combination determines which direct method rôles are
acceptable, and, for each rôle, the appropriate argument lists and return
types.

One direct method, $M$, is said to be more (resp.\ less) \emph{specific} than
another, $N$, with respect to a receiving class~$C$, if the class defining
$M$ is a more (resp.\ less) specific superclass of~$C$ than the class
defining $N$.

\subsubsection{The standard method combination}
The default method combination is called the \emph{standard method
combination}; other method combinations are useful occasionally for special
effects.  The standard method combination accepts four direct method rôles,
called `primary' (the default), @|before|, @|after|, and @|around|.

All direct methods subject to the standard method combination must have
argument lists which \emph{match} the message's argument list:
\begin{itemize}
\item the method's arguments must have the same types as the message, though
  the arguments may have different names; and
\item if the message accepts a variable-length argument suffix then the
  direct method must instead have a final argument of type @|va_list|.
\end{itemize}
Primary and @|around| methods must have the same return type as the message;
@|before| and @|after| methods must return @|void| regardless of the
message's return type.

If there are no applicable primary methods then no effective method is
constructed: the vtables contain null pointers in place of pointers to method
entry functions.

\begin{figure}
  \hbox to\hsize{\hss\hbox{\begin{tikzpicture}
    [order/.append style={color=green!70!black},
     code/.append style={font=\sffamily},
     action/.append style={font=\itshape},
     method/.append style={rectangle, draw=black, thin, fill=blue!30,
                           text height=\ht\strutbox, text depth=\dp\strutbox,
                           minimum width=40mm}]

    \def\delgstack#1#2#3{
      \node (#10) [method, #2] {#3};
      \node (#11) [method, above=6mm of #10] {#3};
      \draw [->] ($(#10.north)!.5!(#10.north west) + (0mm, 1mm)$) --
                 ++(0mm, 4mm)
        node [code, left=4pt, midway] {next_method};
      \draw [<-] ($(#10.north)!.5!(#10.north east) + (0mm, 1mm)$) --
                 ++(0mm, 4mm)
        node [action, right=4pt, midway] {return};
      \draw [->] ($(#11.north)!.5!(#11.north west) + (0mm, 1mm)$) --
                 ++(0mm, 4mm)
        node [code, left=4pt, midway] {next_method}
        node (ld) [above] {$\smash\vdots\mathstrut$};
      \draw [<-] ($(#11.north)!.5!(#11.north east) + (0mm, 1mm)$) --
                 ++(0mm, 4mm)
        node [action, right=4pt, midway] {return}
        node (rd) [above] {$\smash\vdots\mathstrut$};
      \draw [->] ($(ld.north) + (0mm, 1mm)$) -- ++(0mm, 4mm)
        node [code, left=4pt, midway] {next_method};
      \draw [<-] ($(rd.north) + (0mm, 1mm)$) -- ++(0mm, 4mm)
        node [action, right=4pt, midway] {return};
      \node (p) at ($(ld.north)!.5!(rd.north)$) {};
      \node (#1n) [method, above=5mm of p] {#3};
      \draw [->, order] ($(#10.south east) + (4mm, 1mm)$) --
                          ($(#1n.north east) + (4mm, -1mm)$)
        node [midway, right, align=left]
        {Most to \\ least \\ specific};}

    \delgstack{a}{}{@|around| method}
    \draw [<-] ($(a0.south)!.5!(a0.south west) - (0mm, 1mm)$) --
               ++(0mm, -4mm);
    \draw [->] ($(a0.south)!.5!(a0.south east) - (0mm, 1mm)$) --
               ++(0mm, -4mm)
      node [action, right=4pt, midway] {return};

    \draw [->] ($(an.north)!.6!(an.north west) + (0mm, 1mm)$) --
               ++(-8mm, 8mm)
      node [code, midway, left=3mm] {next_method}
      node (b0) [method, above left = 1mm + 4mm and -6mm - 4mm] {};
    \node (b1) [method] at ($(b0) - (2mm, 2mm)$) {};
    \node (bn) [method] at ($(b1) - (2mm, 2mm)$) {@|before| method};
    \draw [->, order] ($(bn.west) - (6mm, 0mm)$) -- ++(12mm, 12mm)
      node [midway, above left, align=center] {Most to \\ least \\ specific};
    \draw [->] ($(b0.north east) + (-10mm, 1mm)$) -- ++(8mm, 8mm)
      node (p) {};

    \delgstack{m}{above right=1mm and 0mm of an.west |- p}{Primary method}
    \draw [->] ($(mn.north)!.5!(mn.north west) + (0mm, 1mm)$) -- ++(0mm, 4mm)
      node [code, left=4pt, midway] {next_method}
      node [above right = 0mm and -8mm]
      {$\vcenter{\hbox{\Huge\textcolor{red}{!}}}
        \vcenter{\hbox{\begin{tabular}[c]{l}
                         \textsf{next_method} \\
                         pointer is null
                       \end{tabular}}}$};

    \draw [->, color=blue, dotted]
        ($(m0.south)!.2!(m0.south east) - (0mm, 1mm)$) --
        ($(an.north)!.2!(an.north east) + (0mm, 1mm)$)
      node [midway, sloped, below] {Return value};

    \draw [<-] ($(an.north)!.6!(an.north east) + (0mm, 1mm)$) --
               ++(8mm, 8mm)
      node [action, midway, right=3mm] {return}
      node (f0) [method, above right = 1mm and -6mm] {};
    \node (f1) [method] at ($(f0) + (-2mm, 2mm)$) {};
    \node (fn) [method] at ($(f1) + (-2mm, 2mm)$) {@|after| method};
    \draw [<-, order] ($(f0.east) + (6mm, 0mm)$) -- ++(-12mm, 12mm)
      node [midway, above right, align=center]
      {Least to \\ most \\ specific};
    \draw [<-] ($(fn.north west) + (6mm, 1mm)$) -- ++(-8mm, 8mm);

  \end{tikzpicture}}\hss}

  \caption{The standard method combination}
  \label{fig:concepts.methods.stdmeth}
\end{figure}

The effective method for a message with standard method combination works as
follows (see also~\xref{fig:concepts.methods.stdmeth}).
\begin{enumerate}

\item If any applicable methods have the @|around| rôle, then the most
  specific such method, with respect to the class of the receiving object, is
  invoked.

  Within the body of an @|around| method, the variable @|next_method| is
  defined, having pointer-to-function type.  The method may call this
  function, as described below, any number of times.

  If there any remaining @|around| methods, then @|next_method| invokes the
  next most specific such method, returning whichever value that method
  returns; otherwise the behaviour of @|next_method| is to invoke the
  @|before| methods (if any), followed by the most specific primary method,
  followed by the @|after| methods (if any), and to return whichever value
  was returned by the most specific primary method, as described in the
  following items.  That is, the behaviour of the least specific @|around|
  method's @|next_method| function is exactly the behaviour that the
  effective method would have if there were no @|around| methods.  Note that
  if the least-specific @|around| method calls its @|next_method| more than
  once then the whole sequence of @|before|, primary, and @|after| methods
  occurs multiple times.

  The value returned by the most specific @|around| method is the value
  returned by the effective method.

\item If any applicable methods have the @|before| rôle, then they are all
  invoked, starting with the most specific.

\item The most specific applicable primary method is invoked.

  Within the body of a primary method, the variable @|next_method| is
  defined, having pointer-to-function type.  If there are no remaining less
  specific primary methods, then @|next_method| is a null pointer.
  Otherwise, the method may call the @|next_method| function any number of
  times.

  The behaviour of the @|next_method| function, if it is not null, is to
  invoke the next most specific applicable primary method, and to return
  whichever value that method returns.

  If there are no applicable @|around| methods, then the value returned by
  the most specific primary method is the value returned by the effective
  method; otherwise the value returned by the most specific primary method is
  returned to the least specific @|around| method, which called it via its
  own @|next_method| function.

\item If any applicable methods have the @|after| rôle, then they are all
  invoked, starting with the \emph{least} specific.  (Hence, the most
  specific @|after| method is invoked with the most `afterness'.)

\end{enumerate}

A typical use for @|around| methods is to allow a base class to set up the
dynamic environment appropriately for the primary methods of its subclasses,
e.g., by claiming a lock, and releasing it afterwards.

The @|next_method| function provided to methods with the primary and
@|around| rôles accepts the same arguments, and returns the same type, as the
message, except that one or two additional arguments are inserted at the
front of the argument list.  The first additional argument is always the
receiving object, @|me|.  If the message accepts a variable argument suffix,
then the second addition argument is a @|va_list|; otherwise there is no
second additional argument; otherwise, In the former case, a variable
@|sod__master_ap| of type @|va_list| is defined, containing a separate copy
of the argument pointer (so the method body can process the variable argument
suffix itself, and still pass a fresh copy on to the next method).

A method with the primary or @|around| rôle may use the convenience macro
@|CALL_NEXT_METHOD|, which takes no arguments itself, and simply calls
@|next_method| with appropriate arguments: the receiver @|me| pointer, the
argument pointer @|sod__master_ap| (if applicable), and the method's
arguments.  If the method body has overwritten its formal arguments, then
@|CALL_NEXT_METHOD| will pass along the updated values, rather than the
original ones.

A primary or @|around| method which invokes its @|next_method| function is
said to \emph{extend} the message behaviour; a method which does not invoke
its @|next_method| is said to \emph{override} the behaviour.  Note that a
method may make a decision to override or extend at runtime.

\subsubsection{Aggregating method combinations}
A number of other method combinations are provided.  They are called
`aggregating' method combinations because, instead of invoking just the most
specific primary method, as the standard method combination does, they invoke
the applicable primary methods in turn and aggregate the return values from
each.

The aggregating method combinations accept the same four rôles as the
standard method combination, and @|around|, @|before|, and @|after| methods
work in the same way.

The aggregating method combinations provided are as follows.
\begin{description} \let\makelabel\code
\item[progn] The message must return @|void|.  The applicable primary methods
  are simply invoked in turn, most specific first.
\item[sum] The message must return a numeric type.\footnote{%
    The Sod translator does not check this, since it doesn't have enough
    insight into @|typedef| names.} %
  The applicable primary methods are invoked in turn, and their return values
  added up.  The final result is the sum of the individual values.
\item[product] The message must return a numeric type.  The applicable
  primary methods are invoked in turn, and their return values multiplied
  together.  The final result is the product of the individual values.
\item[min] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  The final result is the smallest of the
  individual values.
\item[max] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  The final result is the largest of the
  individual values.
\item[and] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  If any method returns zero then the final
  result is zero and no further methods are invoked.  If all of the
  applicable primary methods return nonzero, then the final result is the
  result of the last primary method.
\item[or] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  If any method returns nonzero then the final
  result is that nonzero value and no further methods are invoked.  If all of
  the applicable primary methods return zero, then the final result is zero.
\end{description}

There is also a @|custom| aggregating method combination, which is described
in \xref{sec:fixme.custom-aggregating-method-combination}.


\subsection{Method entries} \label{sec:concepts.methods.entry}

The effective methods for each class are determined at translation time, by
the Sod translator.  For each effective method, one or more \emph{method
entry functions} are constructed.  A method entry function has three
responsibilities.
\begin{itemize}
\item It converts the receiver pointer to the correct type.  Method entry
  functions can perform these conversions extremely efficiently: there are
  separate method entries for each chain of each class which can receive a
  message, so method entry functions are in the privileged situation of
  knowing the \emph{exact} class of the receiving object.
\item If the message accepts a variable-length argument tail, then two method
  entry functions are created for each chain of each class: one receives a
  variable-length argument tail, as intended, and captures it in a @|va_list|
  object; the other accepts an argument of type @|va_list| in place of the
  variable-length tail and arranges for it to be passed along to the direct
  methods.
\item It invokes the effective method with the appropriate arguments.  There
  might or might not be an actual function corresponding to the effective
  method itself: the translator may instead open-code the effective method's
  behaviour into each method entry function; and the machinery for handling
  `delegation chains', such as is used for @|around| methods and primary
  methods in the standard method combination, is necessarily scattered among
  a number of small functions.
\end{itemize}


\subsection{Messages with keyword arguments}
\label{sec:concepts.methods.keywords}

A message or a direct method may declare that it accepts keyword arguments.
A message which accepts keyword arguments is called a \emph{keyword message};
a direct method which accepts keyword arguments is called a \emph{keyword
method}.

While method combinations may set their own rules, usually keyword methods
can only be defined on keyword messages, and all methods defined on a keyword
message must be keyword methods.  The direct methods defined on a keyword
message may differ in the keywords they accept, both from each other, and
from the message.  If two superclasses of some common class both define
keyword methods on the same message, and the methods both accept a keyword
argument with the same name, then these two keyword arguments must also have
the same type.  Different applicable methods may declare keyword arguments
with the same name but different defaults; see below.

The keyword arguments acceptable in a message sent to an object are the
keywords listed in the message definition, together with all of the keywords
accepted by any applicable method.  There is no easy way to determine at
runtime whether a particular keyword is acceptable in a message to a given
instance.

At runtime, a direct method which accepts one or more keyword arguments
receives an additional argument named @|suppliedp|.  This argument is a small
structure.  For each keyword argument named $k$ accepted by the direct
method, @|suppliedp| contains a one-bit-wide bitfield member of type
@|unsigned|, also named $k$.  If a keyword argument named $k$ was passed in
the message, then @|suppliedp.$k$| is one, and $k$ contains the argument
value; otherwise @|suppliedp.$k$| is zero, and $k$ contains the default value
from the direct method definition if there was one, or an unspecified value
otherwise.

%%%--------------------------------------------------------------------------
\section{The object lifecycle} \label{sec:concepts.lifecycle}

\subsection{Creation} \label{sec:concepts.lifecycle.birth}

Construction of a new instance of a class involves three steps.
\begin{enumerate}
\item \emph{Allocation} arranges for there to be storage space for the
  instance's slots and associated metadata.
\item \emph{Imprinting} fills in the instance's metadata, associating the
  instance with its class.
\item \emph{Initialization} stores appropriate initial values in the
  instance's slots, and maybe links it into any external data structures as
  necessary.
\end{enumerate}
The \descref{SOD_DECL}[macro]{mac} handles constructing instances with
automatic storage duration (`on the stack').  Similarly, the
\descref{SOD_MAKE}[macro]{mac} and the \descref{sod_make}{fun} and
\descref{sod_makev}{fun} functions construct instances allocated from the
standard @|malloc| heap.  Programmers can add support for other allocation
strategies by using the \descref{SOD_INIT}[macro]{mac} and the
\descref{sod_init}{fun} and \descref{sod_initv}{fun} functions, which package
up imprinting and initialization.

\subsubsection{Allocation}
Instances of most classes (specifically including those classes defined by
Sod itself) can be held in any storage of sufficient size.  The in-memory
layout of an instance of some class~$C$ is described by the type @|struct
$C$__ilayout|, and if the relevant class is known at compile time then the
best way to discover the layout size is with the @|sizeof| operator.  Failing
that, the size required to hold an instance of $C$ is available in a slot in
$C$'s class object, as @|$C$__class@->cls.initsz|.

It is not in general sufficient to declare, or otherwise allocate, an object
of the class type $C$.  The class type only describes a single chain of the
object's layout.  It is nearly always an error to use the class type as if it
is a \emph{complete type}, e.g., to declare objects or arrays of the class
type, or to enquire about its size or alignment requirements.

Instance layouts may be declared as objects with automatic storage duration
(colloquially, `allocated on the stack') or allocated dynamically, e.g.,
using @|malloc|.  They may be included as members of structures or unions, or
elements of arrays.  Sod's runtime system doesn't retain addresses of
instances, so, for example, Sod doesn't make using fancy allocators which
sometimes move objects around in memory any more difficult than it needs to
be.

There isn't any way to discover the alignment required for a particular
class's instances at runtime; it's best to be conservative and assume that
the platform's strictest alignment requirement applies.

The following simple function correctly allocates and returns space for an
instance of a class given a pointer to its class object @<cls>.
\begin{prog}
  void *allocate_instance(const SodClass *cls)                  \\ \ind
    \{ return malloc(cls@->cls.initsz); \}
\end{prog}

\subsubsection{Imprinting}
Once storage has been allocated, it must be \emph{imprinted} before it can be
used as an instance of a class, e.g., before any messages can be sent to it.

Imprinting an instance stores some metadata about its direct class in the
instance structure, so that the rest of the program (and Sod's runtime
library) can tell what sort of object it is, and how to use it.\footnote{%
  Specifically, imprinting an instance's storage involves storing the
  appropriate vtable pointers in the right places in it.} %
A class object's @|imprint| slot points to a function which will correctly
imprint storage for one of that class's instances.

Once an instance's storage has been imprinted, it is technically possible to
send messages to the instance; however the instance's slots are still
uninitialized at this point, so the applicable methods are unlikely to do
much of any use unless they've been written specifically for the purpose.

The following simple function imprints storage at address @<p> as an instance
of a class, given a pointer to its class object @<cls>.
\begin{prog}
  void imprint_instance(const SodClass *cls, void *p)           \\ \ind
    \{ cls@->cls.imprint(p); \}
\end{prog}

\subsubsection{Initialization}
The final step for constructing a new instance is to \emph{initialize} it, to
establish the necessary invariants for the instance itself and the
environment in which it operates.

Details of initialization are necessarily class-specific, but typically it
involves setting the instance's slots to appropriate values, and possibly
linking it into some larger data structure to keep track of it.  It is
possible for initialization methods to attempt to allocate resources, but
this must be done carefully: there is currently no way to report an error
from object initialization, so the object must be marked as incompletely
initialized, and left in a state where it will be safe to tear down later.

Initialization is performed by sending the imprinted instance an @|init|
message, defined by the @|SodObject| class.  This message uses a nonstandard
method combination which works like the standard combination, except that the
\emph{default behaviour}, if there is no overriding method, is to initialize
the instance's slots, as described below, and to invoke each superclass's
initialization fragments.  This default behaviour may be invoked multiple
times if some method calls on its @|next_method| more than once, unless some
other method takes steps to prevent this.

Slots are initialized in a well-defined order.
\begin{itemize}
\item Slots defined by a more specific superclass are initialized after slots
  defined by a less specific superclass.
\item Slots defined by the same class are initialized in the order in which
  their definitions appear.
\end{itemize}

A class can define \emph{initialization fragments}: pieces of literal code to
be executed to set up a new instance.  Each superclass's initialization
fragments are executed with @|me| bound to an instance pointer of the
appropriate superclass type, immediately after that superclass's slots (if
any) have been initialized; therefore, fragments defined by a more specific
superclass are executed after fragments defined by a less specific
superclass.  A class may define more than one initialization fragment: the
fragments are executed in the order in which they appear in the class
definition.  It is possible for an initialization fragment to use @|return|
or @|goto| for special control-flow effects, but this is not likely to be a
good idea.

The @|init| message accepts keyword arguments
(\xref{sec:concepts.methods.keywords}).  The set of acceptable keywords is
determined by the applicable methods as usual, but also by the
\emph{initargs} defined by the receiving instance's class and its
superclasses, which are made available to slot initializers and
initialization fragments.

There are two kinds of initarg definitions.  \emph{User initargs} are defined
by an explicit @|initarg| item appearing in a class definition: the item
defines a name, type, and (optionally) a default value for the initarg.
\emph{Slot initargs} are defined by attaching an @|initarg| property to a
slot or slot initializer item: the property's value determines the initarg's
name, while the type is taken from the underlying slot type; slot initargs do
not have default values.  Both kinds define a \emph{direct initarg} for the
containing class.

Initargs are inherited.  The \emph{applicable} direct initargs for an @|init|
effective method are those defined by the receiving object's class, and all
of its superclasses.  Applicable direct initargs with the same name are
merged to form \emph{effective initargs}.  An error is reported if two
applicable direct initargs have the same name but different types.  The
default value of an effective initarg is taken from the most specific
applicable direct initarg which specifies a defalt value; if no applicable
direct initarg specifies a default value then the effective initarg has no
default.

All initarg values are made available at runtime to user code --
initialization fragments and slot initializer expressions -- through local
variables and a @|suppliedp| structure, as in a direct method
(\xref{sec:concepts.methods.keywords}).  Furthermore, slot initarg
definitions influence the initialization of slots.

The process for deciding how to initialize a particular slot works as
follows.
\begin{enumerate}
\item If there are any slot initargs defined on the slot, or any of its slot
  initializers, \emph{and} the sender supplied a value for one or more of the
  corresponding effective initargs, then the value of the most specific slot
  initarg is stored in the slot.
\item Otherwise, if there are any slot initializers defined which include an
  initializer expression, then the initializer expression from the most
  specific such slot initializer is evaluated and its value stored in the
  slot.
\item Otherwise, the slot is left uninitialized.
\end{enumerate}
Note that the default values (if any) of effective initargs do \emph{not}
affect this procedure.


\subsection{Destruction}
\label{sec:concepts.lifecycle.death}

Destruction of an instance, when it is no longer required, consists of two
steps.
\begin{enumerate}
\item \emph{Teardown} releases any resources held by the instance and
  disentangles it from any external data structures.
\item \emph{Deallocation} releases the memory used to store the instance so
  that it can be reused.
\end{enumerate}
Teardown alone, for objects which require special deallocation, or for which
deallocation occurs automatically (e.g., instances with automatic storage
duration, or instances whose storage will be garbage-collected), is performed
using the \descref{sod_teardown}[function]{fun}.  Destruction of instances
allocated from the standard @|malloc| heap is done using the
\descref{sod_destroy}[function]{fun}.

\subsubsection{Teardown}
Details of teardown are necessarily class-specific, but typically it
involves releasing resources held by the instance, and disentangling it from
any data structures it might be linked into.

Teardown is performed by sending the instance the @|teardown| message,
defined by the @|SodObject| class.  The message returns an integer, used as a
boolean flag.  If the message returns zero, then the instance's storage
should be deallocated.  If the message returns nonzero, then it is safe for
the caller to forget about instance, but should not deallocate its storage.
This is \emph{not} an error return: if some teardown method fails then the
program may be in an inconsistent state and should not continue.

This simple protocol can be used, for example, to implement a reference
counting system, as follows.
\begin{prog}
  [nick = ref]                                                  \\
  class ReferenceCountedObject: SodObject \{                    \\ \ind
    unsigned nref = 1;                                          \\-
    void inc() \{ me@->ref.nref++; \}                           \\-
    [role = around]                                             \\
    int obj.teardown()                                          \\
    \{                                                          \\ \ind
      if (--\,--me@->ref.nref) return (1);                      \\
      else return (CALL_NEXT_METHOD);                         \-\\
    \}                                                        \-\\
  \}
\end{prog}

The @|teardown| message uses a nonstandard method combination which works
like the standard combination, except that the \emph{default behaviour}, if
there is no overriding method, is to execute the superclass's teardown
fragments, and to return zero.  This default behaviour may be invoked
multiple times if some method calls on its @|next_method| more than once,
unless some other method takes steps to prevent this.

A class can define \emph{teardown fragments}: pieces of literal code to be
executed to shut down an instance.  Each superclass's teardown fragments are
executed with @|me| bound to an instance pointer of the appropriate
superclass type; fragments defined by a more specific superclass are executed
before fragments defined by a less specific superclass.  A class may define
more than one teardown fragment: the fragments are executed in the order in
which they appear in the class definition.  It is possible for an
initialization fragment to use @|return| or @|goto| for special control-flow
effects, but this is not likely to be a good idea.  Similarly, it's probably
a better idea to use an @|around| method to influence the return value than
to write an explicit @|return| statement in a teardown fragment.

\subsubsection{Deallocation}
The details of instance deallocation are obviously specific to the allocation
strategy used by the instance, and this is often orthogonal from the object's
class.

The code which makes the decision to destroy an object may often not be aware
of the object's direct class.  Low-level details of deallocation often
require the proper base address of the instance's storage, which can be
determined using the \descref{SOD_INSTBASE}[macro]{mac}.

%%%--------------------------------------------------------------------------
\section{Metaclasses} \label{sec:concepts.metaclasses}

In Sod, every object is an instance of some class, and -- unlike, say,
\Cplusplus\ -- classes are proper objects.  It follows that, in Sod, every
class~$C$ is itself an instance of some class~$M$, which is called $C$'s
\emph{metaclass}.  Metaclass instances are usually constructed statically, at
compile time, and marked read-only.

As an added complication, Sod classes, and other metaobjects such as
messages, methods, slots and so on, also have classes \emph{at translation
time}.  These translation-time metaclasses are not Sod classes; they are CLOS
classes, implemented in Common Lisp.


\subsection{Runtime metaclasses}
\label{sec:concepts.metaclasses.runtime}

Like other classes, metaclasses can declare messages, and define slots and
methods.  Slots defined by the metaclass are called \emph{class slots}, as
opposed to \emph{instance slots}.  Similarly, messages and methods defined by
the metaclass are termed \emph{class messages} and \emph{class methods}
respectively, though these are used much less frequently.

\subsubsection{The braid}
Every object is an instance of some class.  There are only finitely many
classes.

\begin{figure}
  \centering
  \begin{tikzpicture}
    \node[lit] (obj) {SodObject};
    \node[lit] (cls) [right=10mm of obj] {SodClass};
    \draw [->, dashed] (obj) to[bend right] (cls);
    \draw [->] (cls) to[bend right] (obj);
    \draw [->, dashed] (cls) to[loop right] (cls);
  \end{tikzpicture}
  \qquad
  \fbox{\ \begin{tikzpicture}
    \node (subclass) {subclass of};
    \node (instance) [below=\jot of subclass] {instance of};
    \draw [->] ($(subclass.west) - (10mm, 0)$) -- ++(8mm, 0);
    \draw [->, dashed] ($(instance.west) - (10mm, 0)$) -- ++(8mm, 0);
  \end{tikzpicture}}
  \caption{The Sod braid} \label{fig:concepts.metaclasses.braid}
\end{figure}

Consider the directed graph whose nodes are classes, and where there is an
arc from $C$ to $D$ if and only if $C$ is an instance of $D$.  There are only
finitely many nodes.  Every node has an arc leaving it, because every object
-- and hence every class -- is an instance of some class.  Therefore this
graph must contain at least one cycle.

In Sod, this situation is resolved in the simplest manner possible:
@|SodClass| is the only predefined metaclass, and it is an instance of
itself.  The only other predefined class is @|SodObject|, which is also an
instance of @|SodClass|.  There is exactly one root class, namely
@|SodObject|; consequently, @|SodClass| is a direct subclass of @|SodObject|.

\Xref{fig:concepts.metaclasses.braid} shows a diagram of this situation.

\subsubsection{Class slots and initializers}
Instance initializers were described in \xref{sec:concepts.classes.slots}.  A
class can also define \emph{class initializers}, which provide values for
slots defined by its metaclass.  The initial value for a class slot is
determined as follows.
\begin{itemize}
\item Nonstandard slot classes may be initialized by custom Lisp code.  For
  example, all of the slots defined by @|SodClass| are of this kind.  User
  initializers are not permitted for such slots.
\item If the class or any of its superclasses defines a class initializer for
  the slot, then the class initializer defined by the most specific such
  superclass is used.
\item Otherwise, if the metaclass or one of its superclasses defines an
  instance initializer, then the instance initializer defined by he most
  specific such class is used.
\item Otherwise there is no initializer, and an error will be reported.
\end{itemize}
Initializers for class slots must be constant expressions (for scalar slots)
or aggregate initializers containing constant expressions.

\subsubsection{Metaclass selection and consistency}
Sod enforces a \emph{metaclass consistency rule}: if $C$ has metaclass $M$,
then any subclass $C$ must have a metaclass which is a subclass of $M$.

The definition of a new class can name the new class's metaclass explicitly,
by defining a @|metaclass| property; the Sod translator will verify that the
choice of metaclass is acceptable.

If no @|metaclass| property is given, then the translator will select a
default metaclass as follows.  Let $C_1$, $C_2$, \dots, $C_n$ be the direct
superclasses of the new class, and let $M_1$, $M_2$, \dots, $M_n$ be their
respective metaclasses (not necessarily distinct).  If there exists exactly
one minimal metaclass $M_i$, i.e., there exists an $i$, with $1 \le i \le n$,
such that $M_i$ is a subclass of every $M_j$, for $1 \le j \le n$, then $M_i$
is selected as the new class's metaclass.  Otherwise the situation is
ambiguous and an error will be reported.  Usually, the ambiguity can be
resolved satisfactorily by defining a new class $M^*$ as a direct subclass of
the minimal $M_j$.


\subsection{Translation-time metaobjects}
\label{sec:concepts.metaclasses.compile-time}



\fixme{unwritten}

%%%--------------------------------------------------------------------------
\section{Compatibility considerations} \label{sec:concepts.compatibility}

Sod doesn't make source-level compatibility especially difficult.  As long as
classes, slots, and messages don't change names or dissappear, and slots and
messages retain their approximate types, everything will be fine.

Binary compatibility is much more difficult.  Unfortunately, Sod classes have
rather fragile binary interfaces.\footnote{%
  Research suggestion: investigate alternative instance and vtable layouts
  which improve binary compatibility, probably at the expense of instance
  compactness, and efficiency of slot access and message sending.  There may
  be interesting trade-offs to be made.} %

If instances are allocated \fixme{incomplete}

%%%----- That's all, folks --------------------------------------------------

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