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
+An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.
+(Here, we're using the term `object' in the usual sense of `object-oriented
+programming', rather than that of the ISO~C standard. Once we have defined
+an `instance' below, we shall generally prefer that term, so as to prevent
+further confusion between these two uses of the word.)
+
+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 \emph{direct instance} of exactly one \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.
+If an object~$x$ is a direct instance of some class~$C$, then we say that $C$
+is \emph{the class of}~$x$. Note that the class of an object is a property
+of the object's value at runtime, and not of C's compile-time type system.
+We shall be careful in distinguishing C's compile-time notion of \emph{type}
+from Sod's run-time notion of \emph{class}.
+
\subsection{Superclasses and inheritance}
\label{sec:concepts.classes.inherit}
superclasses.
If an object is a direct instance of class~$C$ then the object is also an
-(indirect) instance of every superclass of $C$.
+(indirect) \emph{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 graph, whose vertices are
-classes, and draw an edge 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.
+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
$\langle C, @|SodObject| \rangle$. 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 subclass of $E$, while $C$ is a subclass of -- indeed equal to --
- $C$; so $A$ must precede $C$, as must $B$, and the final list is $\langle
- G, E, A, B, C, @|SodObject| \rangle$.
+ 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 subclass of $G$, so $E$ wins. Next, $H$ and $F$ must precede
+ $E$ 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 $\langle I, G, E, H, F, A, B, C, D, @|SodObject| \rangle$.
the \emph{chain tail}. Chains are often named after their chain head
classes.
+
\subsection{Names}
\label{sec:concepts.classes.names}
\subsection{C language integration} \label{sec:concepts.classes.c}
+It is very important to distinguish compile-time C \emph{types} from Sod's
+run-time \emph{classes}: see \xref{sec:concepts.classes}.
+
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
chains. See \xref{sec:structures.layout} for the full details.} %
so access to instances is almost always via pointers.
+Usually, a value of type pointer-to-class-type of class~$C$ will point into
+an instance of class $C$. However, clever (or foolish) use of pointer
+conversions can invalidate this relationship.
+
\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
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, if @|MyClass| has the nickname @|mine|, and defines a slot @|x|
-of type @|int|, then the simple function
+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}
the private structure can grow more members as needed. See
\xref{sec:concepts.compatibility} for more details.)
+Slots defined by $C$'s link superclass, or any other superclass in the same
+chain, can be accessed in the same way. Slots defined by other superclasses
+can't be accessed directly: the instance pointer must be \emph{converted} to
+point to a different chain. See the subsection `Conversions' below.
+
\subsubsection{Sending messages}
Sod defines a macro for each message. If a class $C$ defines a message $m$,
\subsection{Method entries} \label{sec:concepts.methods.entry}
-Each instance is associated with its direct class \fixme{direct instances}
-
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
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.
+from the message. If two applicable methods on the same message 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
%%%--------------------------------------------------------------------------
\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}
~mdw / sod / blobdiff