From 3cc520dbf508cb7610a31b1c0786090d5621912d Mon Sep 17 00:00:00 2001 Message-Id: <3cc520dbf508cb7610a31b1c0786090d5621912d.1718299457.git.mdw@distorted.org.uk> From: Mark Wooding Date: Tue, 17 Nov 2015 17:11:08 +0000 Subject: [PATCH] doc/concepts.tex: Provide a conceptual background. Organization: Straylight/Edgeware From: Mark Wooding This is a bit messy still, but I think it's a solid start. --- doc/concepts.tex | 612 ++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 607 insertions(+), 5 deletions(-) diff --git a/doc/concepts.tex b/doc/concepts.tex index b9bf6d5..46d99b1 100644 --- a/doc/concepts.tex +++ b/doc/concepts.tex @@ -23,15 +23,617 @@ %%% 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{Operational model} \label{sec:concepts.model} -\section{Messages and methods} +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. -\section{Metaclasses} +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.} % -\section{Modules} +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} + +A \emph{module} is the top-level syntactic unit of input to the Sod +translator. As described above, given an input module, the translator +generates C source and header files. + +A module can \emph{import} other modules. This makes the type names and +classes defined in those other modules available to class definitions in the +importing module. Sod's module system is intentionally very simple. There +are no private declarations or attempts to hide things. + +As well as importing existing modules, a module can include a number of +different kinds of \emph{items}: +\begin{itemize} +\item \emph{class definitions} describe new classes, possibly in terms of + existing classes; +\item \emph{type name declarations} introduce new type names to Sod's + parser;\footnote{% + This is unfortunately necessary because C syntax, upon which Sod's input + language is based for obvious reasons, needs to treat type names + differently from other kinds of identifiers.} % + and +\item \emph{code fragments} contain literal C code to be dropped into an + appropriate place in an output file. +\end{itemize} +Each kind of item, and, indeed, a module as a whole, can have a collection of +\emph{properties} associated with it. A property has a \emph{name} and a +\emph{value}. Properties are an open-ended way of attaching additional +information to module items, so extensions can make use of them without +having to implement additional syntax. + +%%%-------------------------------------------------------------------------- +\section{Classes, instances, and slots} \label{sec:concepts.classes} + +For the most part, Sod takes a fairly traditional view of what it means to be +an object system. + +An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}. An +object's state is maintained in named \emph{slots}, each of which can store a +C value of an appropriate (scalar or aggregate) type. An object's behaviour +is stimulated by sending it \emph{messages}. A message has a name, and may +carry a number of arguments, which are C values; sending a message may result +in the state of receiving object (or other objects) being changed, and a C +value being returned to the sender. + +Every object is a (direct) instance of some \emph{class}. The class +determines which slots its instances have, which messages its instances can +be sent, and which methods are invoked when those messages are received. The +Sod translator's main job is to read class definitions and convert them into +appropriate C declarations, tables, and functions. An object cannot +(usually) change its direct class, and the direct class of an object is not +affected by, for example, the static type of a pointer to it. + +\subsection{Superclasses and inheritance} +\label{sec:concepts.classes.inherit} + +\subsubsection{Class relationships} +Each class has zero or more \emph{direct superclasses}. + +A class with no direct superclasses is called a \emph{root class}. The Sod +runtime library includes a root class named @|SodObject|; making new root +classes is somewhat tricky, and won't be discussed further here. + +Classes can have more than one direct superclass, i.e., Sod supports +\emph{multiple inheritance}. A Sod class definition for a class~$C$ lists +the direct superclasses of $C$ in a particular order. This order is called +the \emph{local precedence order} of $C$, and the list which consists of $C$ +follows by $C$'s direct superclasses in local precedence order is called the +$C$'s \emph{local precedence list}. + +The multiple inheritance in Sod works similarly to multiple inheritance in +Lisp-like languages, such as Common Lisp, EuLisp, Dylan, and Python, which is +very different from how multiple inheritance works in \Cplusplus.\footnote{% + The latter can be summarized as `badly'. By default in \Cplusplus, an + instance receives an additional copy of superclass's state for each path + through the class graph from the instance's direct class to that + superclass, though this behaviour can be overridden by declaring + superclasses to be @|virtual|. Also, \Cplusplus\ offers only trivial + method combination (\xref{sec:concepts.methods}), leaving programmers to + deal with delegation manually and (usually) statically.} % + +If $C$ is a class, then the \emph{superclasses} of $C$ are +\begin{itemize} +\item $C$ itself, and +\item the superclasses of each of $C$'s direct superclasses. +\end{itemize} +The \emph{proper superclasses} of a class $C$ are the superclasses of $C$ +except for $C$ itself. If a class $B$ is a (direct, proper) superclass of +$C$, then $C$ is a \emph{(direct, proper) subclass} of $B$. If $C$ is a root +class then the only superclass of $C$ is $C$ itself, and $C$ has no proper +superclasses. + +If an object is a direct instance of class~$C$ then the object is also an +(indirect) instance of every superclass of $C$. + +If $C$ has a proper superclass $B$, then $B$ 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 exactly one of $C$'s direct superclasses, 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 both $Z$ 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 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. + +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. + +\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{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{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~$B$. +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 + @|SOD_XCHAIN| (page~\pageref{mac:SOD-XCHAIN}). +\item If $B$ is a subclass of~$C$ then the conversion is an \emph{upcast}; + 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. +\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{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 $N$. + +\subsection{The standard method combination} +\label{sec:concepts.methods.standard} + +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. + +The effective method for a message with standard method combination works as +follows. +\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. 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. + + 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 set up the dynamic environment +appropriately for the primary methods, e.g., by claiming a lock. + +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. + +\subsection{Aggregating method combinations} +\label{sec:concepts.methods.aggregating} + +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 @|int|. 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 a nonzero value. +\item[or] The message must return @|int|. The applicable primary methods are + invoked in turn. If any method returns nonzero then the final result is a + 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}. + +%%%-------------------------------------------------------------------------- +\section{Metaclasses} \label{sec:concepts.metaclasses} %%%----- That's all, folks -------------------------------------------------- -- [mdw]