doc/runtime.tex: Suggest asserting that a stack-allocated object tore down.

[sod] / lib / sod.3

.\" -*-nroff-*-
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.\" The Sod runtime library
.\"
.\" (c) 2015 Straylight/Edgeware
.\"
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.\"--------------------------------------------------------------------------
.TH sod 3 "8 September 2015" "Straylight/Edgeware" "Sensible Object Design"
.
.SH NAME
sod \- Sensible Object Design runtime library
.
.\"--------------------------------------------------------------------------
.SH SYNOPSIS
.B #include <sod/sod.h>
.PP
.B void *\c
.B SOD_XCHAIN(\c
.IB chead ,
.BI "const " cls " *" obj );
.br
.B ptrdiff_t
.B SOD_OFFSETDIFF(\c
.IB type ,
.IB mema ,
.IB memb );
.br
.IB cls "__ilayout *" \c
.B SOD_ILAYOUT(\c
.IB cls ,
.IB chead ,
.BI "const void *" obj );
.br
.B void *\c
.B SOD_INSTBASE(\c
.BI "const " cls " *" obj );
.PP
.B const void *\c
.B SOD_CLASSOF(\c
.BI "const " cls " *" obj );
.br
.B int
.B sod_subclassp(\c
.BI "const SodClass *" sub ,
.BI "const SodClass *" super );
.PP
.IB cls " *" \c
.B SOD_CONVERT(\c
.IB cls ,
.BI "const void *" obj );
.br
.B int
.B sod_convert(\c
.BI "const SodClass *" cls ,
.BI "const void *" obj );
.PP
.IB cls " *" \c
.B SOD_INIT(\c
.IB cls ,
.BI "void *" p ,
.IB keywords );
.br
.B void *\c
.B sod_init(\c
.BI "const SodClass *" cls ,
.BI "void *" p ,
.B ...);
.br
.B void *\c
.B sod_initv(\c
.BI "const SodClass *" cls ,
.BI "void *" p ,
.BI "va_list " ap );
.br
.B void
.B sod_teardown(\c
.BI "void *" p );
.br
.B SOD_DECL(\c
.IB cls ,
.IB var );
.br
.IB cls " *" \c
.B SOD_MAKE(\c
.IB cls ,
.IB keywords );
.br
.B void *\c
.B sod_make(\c
.BI "const SodClass *" cls ,
.B ...);
.br
.B void *\c
.B sod_makev(\c
.BI "const SodClass *" cls ,
.BI "va_list " ap );
.br
.B int
.B sod_destroy(\c
.BI "void *" p );
.PP
.
.\"--------------------------------------------------------------------------
.SH DESCRIPTION
.
The functions and macros defined here generally expect that
instances and classes inherit from the standard
.B SodObject
root object.
While the translator can (at some effort) support alternative roots,
they will require different run-time support machinery.
.
.SS Layout utilities
The following macros are useful in
finding one's way around an instance layout structure,
given various levels of information about
what kind of object one is dealing with,
or for computing the tables which are used for
this kind of navigation.
.PP
These macros are mostly intended for use in
code generated by the Sod translator.
Others may find them useful for special effects,
but they can be tricky to understand and use correctly
and can't really be recommended for general use.
.PP
The
.B SOD_OFFSETDIFF
macro returns the signed offset between
two members of a structure or union type.
Given a structure or union type
.IR type ,
and two member names
.I mema
and
.IR memb ,
then
.B SOD_OFFSETDIFF(\c
.IB type ,
.IB mema ,
.IB memb )
gives the difference, in bytes,
between the objects
.IB x . mema
and
.IB x . memb
for any object
.I x
of type
.IR type .
This macro is used internally when generating vtables
and is not expected to be very useful elsewhere.
.PP
The
.B SOD_ILAYOUT
macro recovers the instance layout base address
from a pointer to one of its instance chains.
Specifically, given a class name
.IR cls ,
the nickname
.I chead
of the least specific class in one of
.IR cls 's
superclass chains,
and a pointer
.I obj
to the instance storage for the chain containing
.I chead
within an exact instance of
.I cls
(i.e., not an instance of any proper subclass),
.B SOD_ILAYOUT(\c
.IB cls ,
.IB chead ,
.IB obj )
returns the a pointer to the layout structure containing
.IB obj .
This macro is used internally in effective method bodies
and is not expected to be very useful elsewhere
since it's unusual to have such specific knowledge
about the dynamic type of an instance.
The
.B SOD_INSTBASE
macro (described below) is more suited to general use.
.PP
The
.B SOD_INSTBASE
macro finds the base address of an instance's layout.
Given a pointer
.BI "const " cls " *" obj
to an instance,
.BI SOD_INSTBASE( obj )
returns the base address of the storage allocated to
.IR obj .
This is useful if you want to free a dynamically allocated instance,
for example.
This macro needs to look up an offset in
.IR obj 's
vtable to do its work.
Compare
.B SOD_ILAYOUT
above,
which is faster but requires
precise knowledge of the instance's dynamic class.
.
.SS Classes
The following macros and functions
query the runtime relationhips between
instances and classes.
.PP
The
.B SOD_CLASSOF
macro returns the class object describing an instance's dynamic class.
Given a pointer
.BI "const " cls " *" obj
to an instance,
.BI SOD_CLASSOF( obj )
returns a pointer to
.IR obj 's
dynamic class,
which
(assuming
.I obj
is typed correctly in the first place)
will be a subclass of
.IR cls .
(If you wanted the class object for
.I cls
itself,
it's called
.IB cls __class \fR.)
.PP
The
.B sod_subclassp
function answers whether one class
.I sub
is actually a subclass of another class
.IR super .
.B sod_subclassp(\c
.IB sub ,
.IB super )
returns nonzero if and only if
.I sub
is a subclass of
.IR super .
This involves a run-time trawl through the class structures:
while some effort has been made to make it perform well
it's still not very fast.
.
.SS Conversions
The following macros and functions are used
to convert instance pointers of some (static) type
into instance pointers of other static types
to the same instance.
.PP
The
.B SOD_XCHAIN
macro performs a `cross-chain upcast'.
Given a pointer
.I cls
.BI * obj
to an instance of a class of type
.I cls
and the nickname
.I chead
of the least specific class in one of
.IR cls 's
superclass chains which does not contain
.I cls
itself,
.B SOD_XCHAIN(\c
.IB chead ,
.IB obj )
returns the address of that chain's storage
within the instance layout as a raw
.B void *
pointer.
(Note that
.I cls
is not mentioned explicitly.)
This macro is used by the generated
.IB cls __CONV_ c
conversion macros,
which you are encouraged to use instead where possible.
.PP
The
.B SOD_CONVERT
macro
and
.B sod_convert
function
perform general conversions
(up-, down-, and cross-casts) on instance pointers.
Given a class
.I cls
and a pointer
.BI "const void *" obj
to an instance,
they return an appropriately converted pointer to
.I obj
if
.I obj
is indeed an instance of (some subclass of)
.IR cls ;
otherwise they return a null pointer.
.PP
The
.B SOD_CONVERT
macro expects
.I cls
to be a class name;
the
.B sod_convert
function
expects a pointer to a class object instead.
.PP
This involves a run-time trawl through the class structures:
while some effort has been made to make it perform well
it's still not very fast.
For upcasts (where
.I cls
is a superclass of the static type of
.IR obj )
the automatically defined conversion macros should be used instead,
because they're much faster and can't fail.
.PP
When the target class is known statically,
it's slightly more convenient to use the
.B SOD_CONVERT
macro rather than the
.B sod_convert
function,
since the class object name is longer and uglier,
and the macro returns a pointer of the correct type.
.
.SS Instance lifecycle
The following macros and functions
manage the standard steps along
an instance's lifecycle.
.PP
The
.B SOD_INIT
macro,
and the
.B sod_init
and
.B sod_initv
functions,
imprint and initialize an instance of a class
.I cls
in the storage starting at address
.IR p .
.PP
The direct class for the new instance is specified as
a class name to
.BR SOD_INIT ,
or a pointer to a class object to the functions.
.PP
Keyword arguments for the initialization message may be provided.
The
.B SOD_INIT
macro expects a single preprocessor-time argument
which is a use of one of
.B KWARGS
or
.B NO_KWARGS
(see
.BR keyword (3));
the
.B sod_init
function expects the keywords as
a variable-length argument tail;
and
.B sod_initv
expects the keywords to be passed indirectly,
through the captured argument-tail cursor
.IR ap .
.PP
The return value is an instance pointer for the class
.IR cls ;
the
.B SOD_INIT
macro will have converted it to the correct type,
so it should probably be used where possible.
In fact, this is guaranteed to be equal to
.I p
by the layout rules described in
.BR sod-structs (3).
.PP
The
.B sod_teardown
function tears down an instance of a class,
releasing any resources it holds.
.PP
This function is a very thin wrapper around sending the
.B obj.teardown
message.
It 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.
(For example, the object may maintain a reference count,
and knows that other references remain active.)
.PP
The
.B SOD_DECL
macro declares and initializes an instance
with automatic storage duration.
Given a class name
.I cls
and an identifier
.IR var ,
.B SOD_DECL(\c
.IB cls ,
.IB var )
declares
.I var
to be a pointer to an instance of
.IR cls .
The instance is initialized in the sense that
its vtable and class pointers have been set up,
and slots for which initializers are defined
are set to the appropriate initial values.
The instance has automatic storage duration:
pointers to it will become invalid when control
exits the scope of the declaration.
.PP
Keyword arguments for the initialization message may be provided.
The macro expects a single preprocessor-time argument
which is a use of one of
.B KWARGS
or
.B NO_KWARGS
(see
.BR keyword (3)).
.PP
The instance has automatic storage duration:
pointers to it will become invalid
when control exits the scope of the declaration.
If necessary,
the instance should be torn down before this happens,
using the
.B sod_teardown
function.
It may be appropriate to
.BR assert (3)
that the object is ready for deallocation at this time.
.PP
The
.B SOD_MAKE
macro,
and the
.B sod_make
and
.B sod_makev
functions,
construct and return a pointer to a new instance of a class.
.PP
The direct class for the new instance is specified as a class name to
.BR SOD_MAKE ,
or as a class object to the functions.
.PP
Keyword arguments for the initialization message may be provided.
The
.B SOD_MAKE
macro expects a single preprocessor-time argument
which is a use of one of
.B KWARGS
or
.B NO_KWARGS
(see
.BR keyword (3));
the
.B sod_init
function expects the keywords as
a variable-length argument tail;
and
.B sod_makev
expects the keywords to be passed indirectly,
through the captured argument-tail cursor
.IR ap .
.PP
Storage for the new instance will have been allocated
using the standard
.BR malloc (3)
function.
The easiest way to destroy the instance,
when it is no longer needed,
is probably to call the
.B sod_destroy
function.
.PP
The return value is an instance pointer for the class
.IR cls ;
the
.B SOD_MAKE
macro will have converted it to the correct type,
so it should probably be used where possible.
.PP
The
.B sod_destroy
function tears down and frees an instance allocated using
.BR malloc (3).
.PP
The pointer
.I p
should be an instance pointer,
i.e., a pointer to any of an instance's chains.
The instance is torn down,
by sending it the
.B obj.teardown
message.
If the instance reports itself ready for deallocation,
then its storage is released using
.BR free (3).
The return value is the value returned by the
.B obj.teardown
message.
.
.\"--------------------------------------------------------------------------
.SH SEE ALSO
.BR keyword (3),
.BR sod (1),
.BR sod-structs (3).
.
.\"--------------------------------------------------------------------------
.SH AUTHOR
Mark Wooding, <mdw@distorted.org.uk>
.
.\"----- That's all, folks --------------------------------------------------