\chapter{Concepts} \label{ch:concepts}
-%%%--------------------------------------------------------------------------
-\section{Operational model} \label{sec:concepts.model}
-
-The Sod translator runs as a preprocessor, similar in nature to the
-traditional Unix \man{lex}{1} and \man{yacc}{1} tools. The translator reads
-a \emph{module} file containing class definitions and other information, and
-writes C~source and header files. The source files contain function
-definitions and static tables which are fed directly to a C~compiler; the
-header files contain declarations for functions and data structures, and are
-included by source files -- whether hand-written or generated by Sod -- which
-makes use of the classes defined in the module.
-
-Sod is not like \Cplusplus: it makes no attempt to `enhance' the C language
-itself. Sod module files describe classes, messages, methods, slots, and
-other kinds of object-system things, and some of these descriptions need to
-contain C code fragments, but this code is entirely uninterpreted by the Sod
-translator.\footnote{%
- As long as a code fragment broadly follows C's lexical rules, and properly
- matches parentheses, brackets, and braces, the Sod translator will copy it
- into its output unchanged. It might, in fact, be some other kind of C-like
- language, such as Objective~C or \Cplusplus. Or maybe even
- Objective~\Cplusplus, because if having an object system is good, then
- having three must be really awesome.} %
-
-The Sod translator is not a closed system. It is written in Common Lisp, and
-can load extension modules which add new input syntax, output formats, or
-altered behaviour. The interface for writing such extensions is described in
-\xref{p:lisp}. Extensions can change almost all details of the Sod object
-system, so the material in this manual must be read with this in mind: this
-manual describes the base system as provided in the distribution.
-
%%%--------------------------------------------------------------------------
\section{Modules} \label{sec:concepts.modules}
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 exactly one of $C$'s direct superclasses, since otherwise the
+ 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.
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 both $Z$ is
-the link superclass of both $X$ and $Y$. As a consequence of this rule, the
+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}.
\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 function} (see
-\xref{sec:concepts.classes.c}, and \xref{sec:structures.root.sodclass}) which
-sets 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 not be initialized.
+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
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}
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.
+\xref{sec:fixme.compatibility} for more details.)
+
+\subsubsection{Vtables}
+
\subsubsection{Class objects}
In Sod's object system, classes are objects too. Therefore classes are
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{Instance allocation, imprinting, and initialization}
-It is in general not sufficient to declare (or @|malloc|) an object of the
-appropriate class type and fill it in, since the class type only describes an
-instance's layout from the point of view of a single superclass chain. The
-correct type to allocate, to store a direct instance of some class is a
-structure whose tag is the class name suffixed with `@|__ilayout|'; e.g., the
-correct layout structure for a direct instance of @|MyClass| would be
-@|struct MyClass__ilayout|.
-
-Instance layouts may be declared as objects with automatic storage duration
-(colloquially, `allocated on the stack') or allocated dynamically, e.g.,
-using @|malloc|. Sod's runtime system doesn't retain addresses of instances,
-so, for example, Sod doesn't make using a fancy allocator which sometimes
-moves objects around in memory any more difficult than it needs to be.
-
-Once storage for an instance has been allocated, it must be \emph{imprinted}
-before it can be used. 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 possible to send the
-instance messages; however, the instance's slots are uninitialized at this
-point, so most methods are unlikely to do much of any use. So, usually, you
-don't just want to imprint instance storage, but to \emph{initialize} an
-instance. Initialization includes imprinting, but also sets the new
-instance's slots to their initial values, as defined by the class. If
-neither the class nor any of its superclasses defines an initializer for a
-slot then it will not be initialized.
-
-There is currently no facility for providing parameters to the instance
-initialization process (e.g., for use by slot initializer expressions).
-Instance initialization is a complicated matter and for now I want to
-experiment with various approaches before committing to one. My current
-interim approach is to specify slot initializers where appropriate and send
-class-specific messages for more complicated parametrized initialization.
-
-Automatic-duration instances can be conveniently constructed and initialized
-using the @|SOD_DECL| macro (page~\pageref{mac:SOD-DECL}). No special
-support is currently provided for dynamically allocated instances. A simple
-function using @|malloc| might work as follows.
-\begin{prog}
- void *new_instance(const SodClass *c) \\
- \{ \\ \ind
- void *p = malloc(c@->cls.initsz); \\
- if (!p) return (0); \\
- c@->cls.init(p); \\
- return (p); \- \\
- \}
-\end{prog}
-
-\subsubsection{Instance finalization and deallocation}
-There is currently no provided assistance for finalization or deallocation.
-It is the programmer's responsibility to decide and implement an appropriate
-protocol. Note that to free an instance allocated from the heap, one must
-correctly find its base address: the @|SOD_INSTBASE| macro
-(page~\pageref{mac:SOD-INSTBASE}) will do this for you.
-
-The following simple mixin class is suggested.
-\begin{prog}
- [nick = disposable] \\*
- class DisposableObject : SodObject \{ \\*[\jot] \ind
- void release() \{ ; \} \\*
- \quad /\=\+* Release resources held by the receiver. */ \-\- \\*[\jot]
- \} \\[\bigskipamount]
- code c : user \{ \\* \ind
- /\=\+* Free object p's instance storage. If p is a DisposableObject \\*
- {}* then release its resources beforehand. \\*
- {}*/ \- \\*
- void free_instance(void *p) \\*
- \{ \\* \ind
- DisposableObject *d = SOD_CONVERT(DisposableObject, p); \\*
- if (d) DisposableObject_release(d); \\*
- free(d); \- \\*
- \} \- \\*
- \}
-\end{prog}
-
\subsubsection{Conversions}
-Suppose one has a value of type pointer to class type of some class~$C$, and
-wants to convert it to a pointer to class type of some other class~$B$.
+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
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
- @|SOD_XCHAIN| (page~\pageref{mac:SOD-XCHAIN}).
-\item If $B$ is a subclass of~$C$ then the conversion is an \emph{upcast};
+ \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~$B$
- at all. The macro @|SOD_CONVERT| (page~\pageref{mac:SOD-CONVERT}) and the
- function @|sod_convert| (page~\pageref{fun:sod-convert}) perform general
- conversions. They return a null pointer if the conversion fails.
+ 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~$B$
@|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}
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}.
+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 role.
+(\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
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
+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
$M$ is a more (resp.\ less) specific superclass of~$C$ than the class
defining $N$.
-\subsection{The standard method combination}
-\label{sec:concepts.methods.standard}
-
+\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|.
+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:
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.
+follows (see also~\xref{fig:concepts.methods.stdmeth}).
\begin{enumerate}
-\item If any applicable methods have the @|around| role, then the most
+\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.
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. 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.
+ 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| role, then they are all
+\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.
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
+\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'.)
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
+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,
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
+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
@|CALL_NEXT_METHOD| will pass along the updated values, rather than the
original ones.
-\subsection{Aggregating method combinations}
-\label{sec:concepts.methods.aggregating}
+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
+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.
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 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 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}
+%%%--------------------------------------------------------------------------
+\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 --------------------------------------------------
%%% Local variables:
~mdw / sod / blobdiff