Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.
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As a structure, a vtable also has a vtable, which is also a structure. Structures, their vtables, the vtables of the vtables, and so on form a tree of structures. Making a new structure adds a leaf to the tree, and if that structure is a vtable, it may be used to create other leaves.
If you traverse up the tree of vtables, via calling
struct-vtable, eventually you reach a root which is the vtable of
itself:
scheme@(guile-user)> (current-module) 1ドル = #<directory (guile-user) 221b090> scheme@(guile-user)> (struct-vtable 1ドル) 2ドル = #<record-type module> scheme@(guile-user)> (struct-vtable 2ドル) 3ドル = #<<standard-vtable> 12c30a0> scheme@(guile-user)> (struct-vtable 3ドル) 4ドル = #<<standard-vtable> 12c3fa0> scheme@(guile-user)> (struct-vtable 4ドル) 5ドル = #<<standard-vtable> 12c3fa0> scheme@(guile-user)> <standard-vtable> 6ドル = #<<standard-vtable> 12c3fa0>
In this example, we can say that 1ドル is an instance of 2ドル,
2ドル is an instance of 3ドル, 3ドル is an instance of
4ドル, and 4ドル, strangely enough, is an instance of itself.
The value bound to 4ドル in this console session also bound to
<standard-vtable> in the default environment.
A meta-vtable, useful for making new vtables.
All of these values are structures. All but 1ドル are vtables. As
2ドル is an instance of 3ドル, and 3ドル is a vtable, we can
say that 3ドル is a meta-vtable: a vtable that can create
vtables.
With this definition, we can specify more precisely what a vtable is: a vtable is a structure made from a meta-vtable. Making a structure from a meta-vtable runs some special checks to ensure that the first field of the structure is a valid layout. Additionally, if these checks see that the layout of the child vtable contains all the required fields of a vtable, in the correct order, then the child vtable will also be a meta-table, inheriting a magical bit from the parent.
Return #t if obj is a vtable structure: an instance of a
meta-vtable.
<standard-vtable> is a root of the vtable tree. (Normally there
is only one root in a given Guile process, but due to some legacy
interfaces there may be more than one.)
The set of required fields of a vtable is the set of fields in the
<standard-vtable>, and is bound to standard-vtable-fields
in the default environment. It is possible to create a meta-vtable that
with additional fields in its layout, which can be used to create
vtables with additional data:
scheme@(guile-user)> (struct-ref 3ドル vtable-index-layout) 6ドル = pruhsruhpwphuhuhprprpw scheme@(guile-user)> (struct-ref 4ドル vtable-index-layout) 7ドル = pruhsruhpwphuhuh scheme@(guile-user)> standard-vtable-fields 8ドル = "pruhsruhpwphuhuh" scheme@(guile-user)> (struct-ref 2ドル vtable-offset-user) 9ドル = module
In this continuation of our earlier example, 2ドル is a vtable that
has extra fields, because its vtable, 3ドル, was made from a
meta-vtable with an extended layout. vtable-offset-user is a
convenient definition that indicates the number of fields in
standard-vtable-fields.
A string containing the ordered set of fields that a vtable must have.
The first index in a vtable that is available for a user.
Return a structure layout symbol, from a fields string.
fields is as described under make-vtable
(see Vtables). An invalid fields string is an error.
With these definitions, one can define make-vtable in this way:
(define* (make-vtable fields #:optional printer) (make-struct/no-tail <standard-vtable> (make-struct-layout fields) printer))
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