6.7.10.1 Vtables

A vtable is a structure type, specifying its layout, and other information. A vtable is actually itself a structure, but there’s no need to worry about that initially (see section Vtable Contents.)

Scheme Procedure: make-vtable fields [print]

Create a new vtable.

fields is a string describing the fields in the structures to be created. Each field is represented by two characters, a type letter and a permissions letter, for example "pw". The types are as follows.

• p – a Scheme value. “p” stands for “protected” meaning it’s protected against garbage collection.
• u – an arbitrary word of data (an scm_t_bits). At the Scheme level it’s read and written as an unsigned integer. “u” stands for “uninterpreted” (it’s not treated as a Scheme value), or “unprotected” (it’s not marked during GC), or “unsigned long” (its size), or all of these things.
• s – a self-reference. Such a field holds the SCM value of the structure itself (a circular reference). This can be useful in C code where you might have a pointer to the data array, and want to get the Scheme SCM handle for the structure. In Scheme code it has no use.

The second letter for each field is a permission code,

• w – writable, the field can be read and written.
• r – read-only, the field can be read but not written.
• o – opaque, the field can be neither read nor written at the Scheme level. This can be used for fields which should only be used from C code.

Here are some examples. See section Tail Arrays, for information on the legacy tail array facility.

(make-vtable "pw")      ;; one writable field
(make-vtable "prpw")    ;; one read-only and one writable
(make-vtable "pwuwuw")  ;; one scheme and two uninterpreted

The optional print argument is a function called by display and write (etc) to give a printed representation of a structure created from this vtable. It’s called (print struct port) and should look at struct and write to port. The default print merely gives a form like ‘#<struct ADDR:ADDR>’ with a pair of machine addresses.

The following print function for example shows the two fields of its structure.

(make-vtable "prpw"
             (lambda (struct port)
               (format port "#<~a and ~a>"
                       (struct-ref struct 0)
                       (struct-ref struct 1))))

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