.
.\"----- 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
-.\" the Free Software Foundation; either version 2 of the License, or
-.\" (at your option) any later version.
+.\" it under the terms of the GNU Library General Public License as
+.\" published by the Free Software Foundation; either version 2 of the
+.\" License, or (at your option) any later version.
.\"
.\" SOD is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
-.\" GNU General Public License for more details.
+.\" GNU Library General Public License for more details.
.\"
-.\" You should have received a copy of the GNU General Public License
-.\" along with SOD; if not, write to the Free Software Foundation,
-.\" Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+.\" You should have received a copy of the GNU Library General Public
+.\" License along with SOD; if not, write to the Free
+.\" Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+.\" MA 02111-1307, USA.
.
+.\" Highlight using terminal escapes, rather than overstriking.
.\"\X'tty: sgr 1'
+.
.\" String definitions and font selection.
.ie t \{\
. ds o \(bu
struct SodObject__vt_obj {
\h'2n'const SodClass *_class;
\h'2n'size_t _base;
+\h'2n'struct SodObject__vtmsgs_obj {
+\h'4n'void (*init)(SodObject *\fIme\fB, ...);
+\h'4n'void (*init__v)(SodObject *\fIme\fB, va_list);
+\h'4n'int (*teardown)(SodObject *\fIme\fB);
+\h'2n'} obj;
};
struct SodObject__ilayout {
struct SodClass__vt_obj {
\h'2n'const SodClass *_class;
\h'2n'size_t _base;
+\h'2n'struct SodClass__vtmsgs_obj {
+\h'4n'void (*init)(SodClass *\fIme\fB, ...);
+\h'4n'void (*init__v)(SodClass *\fIme\fB, va_list);
+\h'4n'int (*teardown)(SodClass *\fIme\fB);
+\h'2n'} obj;
};
-struct SodObject__ilayout {
+struct SodClass__ilayout {
\h'2n'union {
\h'4n'struct SodClass__ichain_obj {
\h'6n'const struct SodClass__vt_obj *_vt;
\h'8n'const char *name;
\h'8n'const char *nick;
\h'8n'size_t initsz;
+\h'8n'size_t align;
\h'8n'void *(*imprint)(void *\fIp\fB);
-\h'8n'void *(*init)(void *\fIp\fB);
\h'8n'size_t n_supers;
\h'8n'const SodClass *const *supers;
\h'8n'size_t n_cpl;
and
.B SodClass
is its own metaclass.
-It is not possible to define root classes because of circularities:
+It is not possible to define root classes in module files
+because of circularities:
.B SodObject
has
.B SodClass
-as its metaclass, and
+as its metaclass,
+and
.B SodClass
is a subclass of
.BR SodObject .
.SS The SodObject class
The
.B SodObject
-class defines no slots or messages.
+class defines no slots.
Because
.B SodObject
has no direct superclasses,
and no inherited slots or messages,
so the single chain contains only a vtable pointer.
.PP
-Since there are no messages,
-and
+Since
.B SodClass
also has only one chain,
the vtable contains only the standard class pointer and offset-to-base
the class pointer contains the address of
.B SodObject__class
and the offset is zero.
+.PP
+The
+.B init
+message is used to initialize a newly allocated instance.
+.PP
+This message uses a custom method combination
+which works like the standard method combination
+except that default behaviour
+specific to the receiver's direct class
+is invoked if no primary or around method overrides.
+This default behaviour may be invoked multiple times
+if some method calls on its
+.B next_method
+function more than once.
+.PP
+This default behaviour is to initialize the instance's slots
+using the defined slot initializers,
+and execute the initialization fragments.
+Each slot is initialized
+using the most specific applicable initializer,
+if any.
+Slots without an initializer
+are left uninitialized.
+.PP
+Slots are initialized and initialization fragments executed together,
+a superclass at a time:
+first, the superclass's slots are initialized (if any);
+then the superclass's initialization fragments (if any) are executed,
+starting with the least specific superclass first.
+Slots and initialization fragments defined by the same class
+are processed in the order in which they appear in the class definition.
+.PP
+There are no standard keyword arguments;
+methods on subclasses are free to
+introduce their own in the usual way.
+.PP
+It is usual to provide complex initialization behaviour as
+.B after
+methods.
+This ensures that slots have been initialized as necessary
+before the method executes.
+.PP
+The
+.B teardown
+message is used to tear down an instance which is no longer required.
+.PP
+The message returns an integer flag.
+A zero value means that the instance is safe to deallocate.
+A nonzero value means that the instance should not be deallocated,
+and that it is safe for the caller to simply forget about it.
+This simple protocol may be used, for example,
+to implement a reference-counting system.
+.PP
+This message uses a custom method combination
+which works like the standard method combination
+except that default behaviour is invoked if
+no primary or around method overrides.
+This default behaviour is to execute
+each superclass's teardown fragments,
+most specific first,
+and then return zero to indicate
+that the object is ready for deallocation.
+Teardown fragments defined by the same class
+are processed in the order in which they appear
+in the class definition.
+.PP
+It is usual to provide complex teardown behaviour as
+.B before
+methods.
+Logic to decide whether to allow deallocation
+is usually implemented as
+.B around
+methods.
.
.SS The SodClass class
The
.B size_t initsz;
The size in bytes required to store an instance of the class.
.TP
+.B size_t align;
+A sufficient alignment for the class's instance storage.
+.TP
.BI "void *(*imprint)(void *" p );
A pointer to a function:
given a pointer
The function returns its argument
.IR p .
.TP
-.BI "void *(*init)(void *" p );
-A pointer to a function:
-given a pointer
-.I p
-to at least
-.I initsz
-bytes of appropriately aligned memory,
-initialize an instance of the class in it:
-all of the vtable and class pointers are initialized,
-as are slots for which initializers are defined.
-Other slots are left untouched.
-The function returns its argument
-.IR p .
-.TP
.B size_t n_supers;
The number of direct superclasses.
(This is zero exactly in the case of
\h'6n'} \fIc\fB;
\h'4n'} \fIc\fB;
\h'4n'\fR...\fB
-\h'4n'struct \fIH\fB__ichain_\fIh\fB \fIh\fB;
+\h'4n'struct \fIA\fB__ichain_\fIh\fB \fIa\fB;
\h'2n'} \fIh\fB;
\h'2n'union \fIB\fB__ichainu_\fIi\fB \fIi\fB;
\h'2n'\fR...\fB
.IB a ;
.PP
for each of
-.IR C 's superclasses
+.IR C 's
+superclasses
.IR A
in the same chain in some (unimportant) order.
-A `pointer to
-.IR C '
-is always assumed
-(and, indeed, defined in C's type system)
-to be a pointer to the
-.B struct
-.IB C __ichain_ h \fR.
+The (somewhat obtuse) purpose of this union is to
+engage the `common initial sequence' rule of
+C99 (clause 6.5.2.3).
.PP
The
.B ichain
.IB C __islots
.IB c ;
.PP
+A `pointer to
+.IR C '
+is always assumed
+(and, indeed, defined in C's type system)
+to be a pointer to the
+.B struct
+.IB C __ichain_ h \fR.
+.PP
Finally, the
.B islots
structure simply contains one member for each slot defined by
.I different
structures.
.PP
-The instance layout split neatly into disjoint chains.
+The instance layout splits neatly into disjoint chains.
This is necessary because
each
.B ichain
must have as a prefix the
.B ichain
-for each superclass in the same chain, and
-each slot must be stored in exactly one place.
+for each superclass in the same chain,
+and each slot must be stored in exactly one place.
The layout of vtables doesn't have this second requirement:
it doesn't matter that there are
multiple method entry pointers
for the same effective method
as long as they all work correctly.
+Indeed, it's essential that they do,
+because each chain's method entry function
+will need to apply a different offset to the receiver pointer
+before invoking the effective method.
.PP
A vtable for a class
.I C
.ft P
.fi
.PP
+In the following,
+let
+.I M
+be the metaclass of
+.IR C .
+.PP
The outer layer is a
-.IP
.B union
.IB C __vtu_ h
-.PP
containing a member
.IP
.B struct
The metaclass
.I R
of
-.IR O .
+.I O
is then the
.I root metaclass
of
in the same chain as
.IR J .
Then, if there is currently no class pointer of type
-.I Q
+.IR Q ,
then add a member
.RS
.IP
.I A
defines any messages,
and there is currently no member
-.I a
+.IR a ,
then add a member
.RS
.IP
.IB an ,
.B ...);
.PP
+or a standard message which takes keyword arguments,
+defined as
+.IP
+.I tr
+.IB m ( \c
+.I t1
+.IB a1 ,
+.RB ... ,
+.I tn
+.IB an ?\&
+.IR tn +1
+.IR kn +1
+.RB [ =
+.IR dn +1] \c
+.B ,
+.I tm
+.I km
+.RB [ =
+.IR dm ] \c
+);
+.PP
two entry points are defined:
the usual `main' entry point
which accepts a variable number of
and a `valist' entry point
which accepts an argument of type
.B va_list
-in place of the variable portion of the argument list.
+in place of the variable portion of the argument list
+or keywords.
.IP
.I tr
.BI (* m )( \c
If
.I m
takes a variable number of arguments,
+or keyword arguments,
the macro is more complicated
and is only available in compilers advertising C99 support,
but the effect is the same.
~mdw / sod / blobdiff