X-Git-Url: https://git.distorted.org.uk/~mdw/sod/blobdiff_plain/a42893dda5f4dd2b89fbfe4e497da261159225ca..388ab3827ab7c584c30723f5044c2a38cf6fe55d:/doc/concepts.tex diff --git a/doc/concepts.tex b/doc/concepts.tex index 8151014..b4e80ca 100644 --- a/doc/concepts.tex +++ b/doc/concepts.tex @@ -704,7 +704,7 @@ 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 @. \begin{prog} - void *allocate_instance(const SodClass *cls) \\ \ind + void *allocate_instance(const SodClass *cls) \\ \ind \{ return malloc(cls@->cls.initsz); \} \end{prog} @@ -728,7 +728,7 @@ of any use unless they've been written specifically for the purpose. The following simple function imprints storage at address @

as an instance of a class, given a pointer to its class object @. \begin{prog} - void imprint_instance(const SodClass *cls, void *p) \\ \ind + void imprint_instance(const SodClass *cls, void *p) \\ \ind \{ cls@->cls.imprint(p); \} \end{prog} @@ -745,11 +745,10 @@ 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 using the initializers defined in the class and its -superclasses, 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. +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} @@ -771,19 +770,53 @@ 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. -Note that an initialization fragment defined in a class is copied literally -into each subclass's initialization method. This is fine for simple cases -but wasteful if the initialization logic is complicated. More complex -initialization behaviour should be added either by having an initialization -fragments call functions (necessarily with external linkage), or by defining -@|after| methods on the @|init| message. These will be run after the slot -initializers have been applied, in reverse precedence order. - -Initialization is \emph{parametrized}, so the caller may select from a space -of possible initial states for the new instance, or to inform the new -instance about some other objects known to the caller. Specifically, the -@|init| message accepts keyword arguments (\xref{sec:concepts.keywords}) -which can be defined and used by methods defined on it. +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} @@ -820,16 +853,16 @@ 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 \{ \\ \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); \- \\ - \} \- \\ + [nick = ref] \\ + class ReferenceCountedObject \{ \\ \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}