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:wrapper FLI Type Descriptor


Allows the specification of automatic conversion functions between Lisp and an instance of a FLI type.




:wrapper foreign-type &key lisp-to-foreign foreign-to-lisp

The underlying type to wrap.
Code specifying how to convert between Lisp and the FLI.
Code specifying how to convert between the FLI and Lisp.

The FLI type :wrapper allows for an extra level of conversion between Lisp and a foreign language through the FLI. With the :wrapper type you can use lisp-to-foreign and foreign-to-lisp to specify conversion functions from and to an instance of another type foreign-type. Whenever data is passed to the object, or received from the object it is passed through the conversion function. See below for an example of a use of :wrapper to pass values to an :int as strings, and to receive them back as strings when the pointer to the :int is dereferenced.


In the following example an :int is allocated with a wrapper to allow the :int to be accessed as a string.

(setq wrap (fli:allocate-foreign-object
            :type '(:wrapper :int 
                    :lisp-to-foreign read-from-string
                    :foreign-to-lisp prin1-to-string)))

The object pointed to by wrap, although consisting of an underlying :int, is set with dereference by passing a string, which is automatically converted using the Lisp function read-from-string. Similarly, when wrap is dereferenced, the value stored as an :int is converted using prin1-to-string to a Lisp string, which is the returned. The following two commands demonstrate this.

(setf (fli:dereference wrap) "#x100")
(fli:dereference wrap)

The first command sets the value stored at wrap to be 256 (100 in hex), by passing a string to it. The second command dereferences the value at wrap, but returns it as a string. The pointer wrap can be coerced to return the value as an actual :int as follows:

(fli:dereference wrap :type :int)
See also

2.4 Encapsulated types

Foreign Language Interface User Guide and Reference Manual - 01 Dec 2021 19:34:59