%%%----- Licensing notice ---------------------------------------------------
%%%
-%%% This file is part of the Sensble Object Design, an object system for C.
+%%% This file is part of the Sensible Object Design, an object system for C.
%%%
%%% SOD is free software; you can redistribute it and/or modify
%%% it under the terms of the GNU General Public License as published by
%%% along with SOD; if not, write to the Free Software Foundation,
%%% Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
-\chapter{Concepts}
+\chapter{Concepts} \label{ch:concepts}
-\section{Classes and slots}
+%%%--------------------------------------------------------------------------
+\section{Modules} \label{sec:concepts.modules}
-\section{Messages and methods}
+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.
-\section{Metaclasses}
+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.
-\section{Modules}
+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 $B$ is not allowed to have $C$ has 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.
+
+\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{FIXME:C3}.
+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}
+
+\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 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.)
+
+
+\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, if @|MyClass| has the nickname @|mine|, and defines a slot @|x|
+of type @|int|, then 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:fixme.compatibility} for more details.)
+
+\subsubsection{Vtables}
+
+
+\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. 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. (There 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 role 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{role}.
+
+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 role.
+
+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 roles are
+acceptable, and, for each role, 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 roles,
+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}
+ \begin{tikzpicture}
+ [>=stealth, thick,
+ 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}
+
+ \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| role, 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 @|around| 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| role, 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| role, 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 restore it afterwards.
+
+The @|next_method| function provided to methods with the primary and
+@|around| roles 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| role 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 roles 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{Sending messages in C} \label{sec:concepts.methods.c}
+
+Each instance is associated with its direct class [FIXME]
+
+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, 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 superclasses 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 more 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 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}
+
+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 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 more 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}
+
+%%%--------------------------------------------------------------------------
+\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]
%%%----- That's all, folks --------------------------------------------------
~mdw / sod / blobdiff