SWIG and Allegro Common Lisp

# 16 SWIG and Allegro Common Lisp

- Basics
  - Running Swig
  - Command Line Options
  - Inserting user code into generated files
- Wrapping Overview
  - Function Wrapping
  - Foreign Wrappers
  - FFI Wrappers
  - Non-overloaded Defuns
  - Overloaded Defuns
  - What about constant and variable access?
  - Object Wrapping
- Wrapping Details
  - Namespaces
  - Constants
  - Variables
  - Enumerations
  - Arrays
  - Classes and Structs and Unions (oh my!)
    - CLOS wrapping of
    - CLOS Inheritance
    - Member fields and functions
    - Why not directly access C++ classes using foreign types?
  - Templates
    - Generating wrapper code for templates
    - Implicit Template instantiation
  - Typedef, Templates, and Synonym Types
    - Choosing a primary type
  - Function overloading/Parameter defaulting
  - Operator wrapping and Operator overloading
  - Varargs
  - C++ Exceptions
  - Pass by value, pass by reference
- Typemaps
  - Code Generation in the C++ Wrapper
    - IN Typemap
    - OUT Typemap
    - CTYPE Typemap
  - Code generation in Lisp wrappers
    - LIN Typemap
    - LOUT Typemap
    - FFITYPE Typemap
    - LISPTYPE Typemap
    - LISPCLASS Typemap
  - Modifying SWIG behavior using typemaps
- Identifier Converter functions
  - Creating symbols in the lisp environment
  - Existing identifier-converter functions
    - identifier-convert-null
    - identifier-convert-lispify
    - Default identifier to symbol conversions
  - Defining your own identifier-converter
  - Instructing SWIG to use a particular identifier-converter

This chapter describes SWIG's support of Allegro Common Lisp. Allegro CL is a full-featured implementation of the Common Lisp language standard that includes many vendor-specific enhancements and add-on modules for increased usability.

One such module included in Allegro CL is the Foreign Functions Interface (FFI). This module, tailored primarily toward interfacing with C/C++ and, historically, Fortran, provides a means by which compiled foreign code can be loaded into a running lisp environment and executed. The interface supports the calling of foreign functions and methods, allows for executing lisp routines from foreign code (callbacks), and the passing of data between foreign and lisp code.

The goal of this module is to make it possible to quickly generate the necessary foreign function definitions so one can make use of C/C++ foreign libraries directly from lisp without the tedium of having to code them by hand. When necessary, it will also generate further C/C++ code that will need to be linked with the intended library for proper interfacing from lisp. It has been designed with an eye toward flexibility. Some foreign function calls may release the heap, while other should not. Some foreign functions should automatically convert lisp strings into native strings, while others should not. These adjustments and many more are possible with the current module.

It is significant to note that, while this is a vendor-specific module, we would like to acknowledge the current and ongoing work by developers in the open source lisp community that are working on similar interfaces to implementation-independent foreign function interfaces (UFFI or CFFI, for example). Such work can only benefit the lisp community, and we would not be unhappy to see some enterprising folk use this work to add to it.

## 16.1 Basics

### 16.1.1 Running Swig

If you're reading this, you must have some library you need to generate an interface for. In order for SWIG to do this work, however, it needs a bit of information about how it should go about creating your interface, and what you are interfacing to.

SWIG expects a description of what in the foreign interface you wish to connect to. It must consisting of C/C++ declarations and special SWIG directives. SWIG can be furnished with a header file, but an interface can also be generated without library headers by supplying a simple text file--called the interface file, which is typically named with a `.i` extension--containing any foreign declarations of identifiers you wish to use. The most common approach is to use a an interface file with directives to parse the needed headers. A straight parse of library headers will result in usable code, but SWIG directives provides much freedom in how a user might tailor the generated code to their needs or style of coding.

Note that SWIG does not require any function definitions; the declarations of those functions is all that is necessary. Be careful when tuning the interface as it is quite possible to generate code that will not load or compile.

An example interface file is shown below. It makes use of two SWIG directives, one of which requests that the declarations in a header file be used to generate part of the interface, and also includes an additional declaration to be added.

example.i

    %module example

    %include "header.h"

    int fact(int n);

The contents of header.h are very simple:

header.h

    int fact(char *statement);   // pass it a fact, and it will rate it.

The contents of example.cl will look like this:

example.cl

    (defpackage :example
      (:use :common-lisp :swig :ff :excl))

      ... helper routines for defining the interface ...

    (swig-in-package ())

    (swig-defun ("fact")
      ((PARM0_statement cl:string (* :char) ))
      (:returning (:int )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_statement))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("fact")
      ((PARM0_n cl:integer :int ))
      (:returning (:int )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_n))
      (swig-ff-call SWIG_arg0)))

    (swig-dispatcher ("fact" :type :function :arities (1)))

The generated file contains calls to internal swig helper functions. In this case there are two calls to swig-defun. These calls will expand into code that will make the appropriate definitions using the Allegro FFI. Note also, that this code is erroneous. Function overloading is not supported in C, and this code will not compile even though SWIG did not complain.

In order to generate a C interface to Allegro CL using this code run swig using the `-allegrocl` option, as below:

    % swig -allegrocl example.i

When building an interface to C++ code, include the `-c++` option:

    % swig -allegrocl -c++ example.i

As a result of running one of the above commands, a file named `example.cl` will be generated containing the lisp side of the interface. As well, a file `example_wrap.cxx` is also generated, containing C/C++ wrapper code to facilitate access to C++ methods, enumeration values, and constant values. Wrapper functions are necessary in C++ due to the lack of a standard for mangling the names of symbols across all C++ compilers. These wrapper functions are exported from the shared library as appropriate, using the C name mangling convention. The lisp code that is generated will interface to your foreign library through these wrappers.

It is possible to disable the creation of the .cxx file when generating a C interface by using the -nocwrap command-line argument. For interfaces that don't contain complex enum or constant expressions, contain nested struct/union declarations, or doesn't need to use many of the SWIG customization featuers, this will result in a more streamlined, direct interface to the intended module.

The generated wrapper file is below. It contains very simple wrappers by default, that simply pass the arguments to the actual function.

example_wrap.i

       ... lots of SWIG internals ...

    EXPORT int ACL___fact__SWIG_0 (char *larg1) {
        int lresult = (int)0 ;
        char *arg1 = (char *) 0 ;
        int result;

        arg1 = larg1;
        try {
            result = (int)fact(arg1);

            lresult = result;
            return lresult;
        } catch (...) {
            return (int)0;
        }
    }

    EXPORT int ACL___fact__SWIG_1 (int larg1) {
        int lresult = (int)0 ;
        int arg1 ;
        int result;

        arg1 = larg1;
        try {
            result = (int)fact(arg1);

            lresult = result;
            return lresult;
        } catch (...) {
            return (int)0;
        }
    }

And again, the generated lisp code. Note that it differs from what is generated when parsing C code:

       ...

    (swig-in-package ())

    (swig-defmethod ("fact" "ACL___fact__SWIG_0" :type :function :arity 1)
      ((PARM0_statement cl:string (* :char) ))
      (:returning (:int )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_statement))
      (swig-ff-call SWIG_arg0)))

    (swig-defmethod ("fact" "ACL___fact__SWIG_1" :type :function :arity 1)
      ((PARM0_n cl:integer :int ))
      (:returning (:int )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_n))
      (swig-ff-call SWIG_arg0)))

    (swig-dispatcher ("fact" :type :function :arities (1)))

In this case, the interface generates two swig-defmethod forms and a swig-dispatcher form. This provides a single functional interface for all overloaded routines. A more detailed description of this features is to be found in the section titled Function overloading/Parameter defaulting.

In order to load a C++ interface, you will need to build a shared library from example_wrap.cxx. Be sure to link in the actual library you created the interface for, as well as any other dependent shared libraries. For example, if you intend to be able to call back into lisp, you will also need to link in the Allegro shared library. The library you create from the C++ wrapper will be what you then load into Allegro CL.

### 16.1.2 Command Line Options

There are three Allegro CL specific command-line option:

    swig -allegrocl [ options ] filename

       -identifier-converter [name] - Binds the variable swig:*swig-identifier-convert*
                                      in the generated .cl file to name.
    				  This function is used to generate symbols
    				  for the lisp side of the interface.

       -cwrap - [default] Generate a .cxx file containing C wrapper function when
                wrapping C code. The interface generated is similar to what is
    	    done for C++ code.
       -nocwrap - Explicitly turn off generation of .cxx wrappers for C code. Reasonable
                  for modules with simple interfaces. Can not handle all legal enum
    	      and constant constructs, or take advantage of SWIG customization features.

       -isolate - With this command-line argument, all lisp helper functions are defined
                  in a unique package named swig.<module-name> rather than
    	      swig. This prevents conflicts when the module is
    	      intended to be used with other swig generated interfaces that may,
    	      for instance, make use of different identifier converters.

See Section 17.5 Identifier converter functions for more details.

### 16.1.3 Inserting user code into generated files

It is often necessary to include user-defined code into the automatically generated interface files. For example, when building a C++ interface, example_wrap.cxx will likely not compile unless you add a `#include "header.h"` directive. This can be done using the SWIG `%insert(section) %{ ...code... %}` directive:

    %module example

    %insert("runtime") %{
    #include "header.h"
    %}

    %include "header.h"

    int fact(int n);

Additional sections have been added for inserting into the generated lisp interface file

- `lisphead` - inserts before type declarations
- `lisp` - inserts after type declarations according to where it appears in the .i file

Note that the block `%{ ... %}` is effectively a shortcut for `%insert("runtime") %{ ... %}`.

## 16.2 Wrapping Overview

New users to SWIG are encouraged to read SWIG Basics, and SWIG and C++, for those interested in generating an interface to C++.

### 16.2.1 Function Wrapping

Writing lisp code that directly invokes functions at the foreign function interface level can be cumbersome. Data must often be translated between lisp and foreign types, data extracted from objects, foreign objects allocated and freed upon completion of the foreign call. Dealing with pointers can be unwieldy when it comes to keeping them distinct from other valid integer values.

We make an attempt to ease some of these burdens by making the interface to foreign code much more lisp-like, rather than C like. How this is done is described in later chapters. The layers themselves, appear as follows:

            ______________
           |              |  (foreign side)
           | Foreign Code |  What we're generating an interface to.
           |______________|
                   |
    	       |
            _______v______
           |              |  (foreign side)
           | Wrapper code |  extern "C" wrappers calling C++
           |______________|  functions and methods.
                   |
        .  . . - - + - - . .  .
            _______v______
           |              |  (lisp side)
           |  FFI Layer   |  Low level lisp interface. ff:def-foreign-call,
           |______________|  ff:def-foreign-variable
                   |
    	       +----------------------------
            _______v______              _______v______
           |              |            |              | (lisp side)
           |    Defuns    |            |  Defmethods  | wrapper for overloaded
           |______________|            |______________| functions or those with
            (lisp side)                        |        defaulted arguments
    	Wrapper for non-overloaded         |
    	functions and methods       _______v______
    	                           |              | (lisp side)
    				   |    Defuns    | dispatch function
    				   |______________| to overloads based
    				                    on arity

### 16.2.2 Foreign Wrappers

These wrappers are as generated by SWIG default. The types of function parameters can be transformed in place using the CTYPE typemap. This is use for converting pass-by-value parameters to pass-by-reference where necessary. All wrapper parameters are then bound to local variables for possible transformation of values (see LIN typemap). Return values can be transformed via the OUT typemap.

### 16.2.3 FFI Wrappers

These are the generated ff:def-foreign-call forms. No typemaps are applicable to this layer, but the `%ffargs` directive is available for use in .i files, to specify which keyword arguments should be specified for a given function.

ffargs.i:

    %module ffargs

    %ffargs(strings_convert="nil",call_direct="t") foo;
    %ffargs(strings_convert="nil",release_heap=":never",optimize_for_space="t") bar;

    int foo(float f1, float f2);
    int foo(float f1, char c2);

    void bar(void *lisp_fn);

    char *xxx();

Generates:

ffargs.cl:

    (swig-in-package ())

    (swig-defmethod ("foo" "ACL___foo__SWIG_0" :type :function :arity 2)
      ((PARM0_f1 cl:single-float :float )
       (PARM1_f2 cl:single-float :float ))
      (:returning (:int )
       :call-direct t
       :strings-convert nil)
      (let ((SWIG_arg0 PARM0_f1))
      (let ((SWIG_arg1 PARM1_f2))
      (swig-ff-call SWIG_arg0 SWIG_arg1))))

    (swig-defmethod ("foo" "ACL___foo__SWIG_1" :type :function :arity 2)
      ((PARM0_f1 cl:single-float :float )
       (PARM1_c2 cl:character :char character))
      (:returning (:int )
       :call-direct t
       :strings-convert nil)
      (let ((SWIG_arg0 PARM0_f1))
      (let ((SWIG_arg1 PARM1_c2))
      (swig-ff-call SWIG_arg0 SWIG_arg1))))

    (swig-dispatcher ("foo" :type :function :arities (2)))
    (swig-defun ("bar" "ACL___bar__SWIG_0" :type :function)
      ((PARM0_lisp_fn  (* :void) ))
      (:returning (:void )
       :release-heap :never
       :optimize-for-space t
       :strings-convert nil)
      (let ((SWIG_arg0 PARM0_lisp_fn))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("xxx" "ACL___xxx__SWIG_0" :type :function)
      (:void)
      (:returning ((* :char) )
       :strings-convert t)
      (swig-ff-call))

    %ffargs(strings_convert="t");

Is the only default value specified in `allegrocl.swg` to force the muffling of warnings about automatic string conversion when defining ff:def-foreign-call's.

### 16.2.4 Non-overloaded Defuns

These are simple defuns. There is no typechecking of arguments. Parameters are bound to local variables for possible transformation of values, such as pulling values out of instance slots or allocating temporary stack allocated structures, via the `lin` typemap. These arguments are then passed to the foreign-call (where typechecking may occur). The return value from this function can be manipulated via the `lout` typemap.

### 16.2.5 Overloaded Defuns

In the case of overloaded functions, mulitple layers are generated. First, all the overloads for a given name are separated out into groups based on arity, and are wrapped in defmethods. Each method calls a distinct wrapper function, but are themselves distinguished by the types of their arguments (see `lispclass` typemap). These are further wrapped in a dispatching function (defun) which will invoke the appropriate generic-function based on arity. This provides a single functional interface to all overloads. The return value from this function can be manipulated via the `lout` typemap.

### 16.2.6 What about constant and variable access?

Along with the described functional layering, when creating a .cxx wrapper, this module will generate getter and--if not immutable--setter, functions for variables and constants. If the -nocwrap option is used, `defconstant` and `ff:def-foreign-variable` forms will be generated for accessing constants and global variables. These, along with the `defuns` listed above are the intended API for calling into the foreign module.

### 16.2.7 Object Wrapping

All non-primitive types (Classes, structs, unions, and typedefs involving same) have a corresponding foreign-type defined on the lisp side via ff:def-foreign-type.

All non-primitive types are further represented by a CLOS class, created via defclass. An attempt is made to create the same class hierarchy, with all classes inheriting directly or indirectly from ff:foreign-pointer. Further, wherever it is apparent, all pointers returned from foreign code are wrapped in a CLOS instance of the appropriate class. For ff:def-foreign-calls that have been defined to expect a :foreign-address type as argument, these CLOS instances can legally be passed and the pointer to the C++ object automatically extracted. This is a natural feature of Allegro's foreign function interface.

## 16.3 Wrapping Details

In this section is described how particular C/C++ constructs are translated into lisp.

### 16.3.1 Namespaces

C++ namespaces are translated into Lisp packages by SWIG. The Global namespace is mapped to a package named by the `%module` directive or the `-module` command-line argument. Further namespaces are generated by the `swig-defpackage` utility function and given names based on Allegro CLs nested namespace convention. For example:

foo.i:

    %module foo

    %{
    #include "foo.h"
    %}

    %include "foo.h"

    namespace car {
       ...
       namespace tires {
          int do_something(int n);
       }
    }

Generates the following code.

foo.cl

    (defpackage :foo
      (:use :common-lisp :swig :ff :excl))

    ...

    (swig-defpackage ("car"))
    (swig-defpackage ("car" "tires"))

    ...

    (swig-in-package ("car" "tires"))
    (swig-defun ("do_something" "ACL_car_tires__do_something__SWIG_0" :type :function)
      ((PARM0_n  :int ))
      (:returning (:int )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_n))
      (swig-ff-call SWIG_arg0)))

The above interface file would cause packages foo, foo.car, and foo.car.tires to be created. One would find the function wrapper for do_something defined in the foo.car.tires package(*).

(*) Except for the package named by the module, all namespace names are passed to the identifier-converter-function as strings with a `:type` of `:namespace`. It is the job of this function to generate the desired symbol, accounting for case preferences, additional naming cues, etc.

Note that packages created by `swig-defpackage` do not use the COMMON-LISP or EXCL package. This reduces possible conflicts when defining foreign types via the SWIG interface in all but the toplevel modules package. This may lead to confusion if, for example, the current package is `foo.car.tires` and you attempt to use a common-lisp function such as `(car '(1 2 3)`.

### 16.3.2 Constants

Constants, as declared by the preprocessor #define macro or SWIG `%constant` directive, are included in SWIGs parse tree when it can be determined that they are, or could be reduced to, a literal value. Such values are translated into defconstant forms in the generated lisp wrapper when the -nocwrap command-line options is used. Else, wrapper functions are generated as in the case of variable access (see section below).

Here are examples of simple preprocessor constants when using -nocwrap.

    #define A 1                    => (swig-defconstant "A" 1)
    #define B 'c'                  => (swig-defconstant "B" #\c)
    #define C B                    => (swig-defconstant "C" #\c)
    #define D 1.0e2                => (swig-defconstant "D" 1.0d2)
    #define E 2222                 => (swig-defconstant "E" 2222)
    #define F (unsigned int)2222   => no code generated
    #define G 1.02e2f              => (swig-defconstant "G" 1.02f2)
    #define H foo                  => no code generated

Note that where SWIG is unable to determine if a constant is a literal, no node is added to the SWIG parse tree, and so no values can be generated.

For preprocessor constants containing expressions which can be reduced to literal values, nodes are created, but with no simplification of the constant value. A very very simple infix to prefix converter has been implemented that tries to do the right thing for simple cases, but does not for more complex expressions. If the literal parser determines that something is wrong, a warning will be generated and the literal expression will be included in the generated code, but commented out.

    #define I A + E                => (swig-defconstant "I" (+ 1 2222))
    #define J 1|2                  => (swig-defconstant "J" (logior 1 2))
    #define Y 1 + 2 * 3 + 4        => (swig-defconstant "Y" (* (+ 1 2) (+ 3 4)))
    #define Y1 (1 + 2) * (3 + 4)   => (swig-defconstant "Y1" (* (+ 1 2) (+ 3 4)))
    #define Y2 1 * 2 + 3 * 4       => (swig-defconstant "Y2" (* 1 (+ 2 3) 4))  ;; WRONG
    #define Y3 (1 * 2) + (3 * 4)   => (swig-defconstant "Y3" (* 1 (+ 2 3) 4))  ;; WRONG
    #define Z 1 + 2 - 3 + 4 * 5    => (swig-defconstant "Z" (* (+ 1 (- 2 3) 4) 5)) ;; WRONG

Users are cautioned to get to know their constants before use, or not use the `-nocwrap` command-line option.

### 16.3.3 Variables

For C wrapping, a def-foreign-variable call is generated for access to global variables.

When wrapping C++ code, both global and member variables, getter wrappers are generated for accessing their value, and if not immutable, setter wrappers as well. In the example below, note the lack of a setter wrapper for global_var, defined as const.

vars.h

    namespace nnn {
      int const global_var = 2;
      float glob_float = 2.0;
    }

Generated code:

vars.cl

    (swig-in-package ("nnn"))
    (swig-defun ("global_var" "ACL_nnn__global_var_get__SWIG_0" :type :getter)
      (:void)
      (:returning (:int )
       :strings-convert t)
      (swig-ff-call))

    (swig-defun ("glob_float" "ACL_nnn__glob_float_set__SWIG_0" :type :setter)
      ((PARM0_glob_float  :float ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_glob_float))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("glob_float" "ACL_nnn__glob_float_get__SWIG_0" :type :getter)
      (:void)
      (:returning (:float )
       :strings-convert t)
      (swig-ff-call))

Note also, that where applicable, setter wrappers are implemented as setf methods on the getter function, providing a lispy interface to the foreign code.

    user> (load "globalvar.dll")
    ; Foreign loading globalvar.dll.
    t
    user> (load "globalvar.cl")
    ; Loading c:\mikel\src\swig\test\globalvar.cl
    t
    user>
    globalvar> (globalvar.nnn::global_var)
    2
    globalvar> (globalvar.nnn::glob_float)
    2.0
    globalvar> (setf (globalvar.nnn::glob_float) 3.0)
    3.0
    globalvar> (globalvar.nnn::glob_float)
    3.0

### 16.3.4 Enumerations

In C, an enumeration value is an integer value, while in C++ an enumeration value is implicitly convertible to an integer value, but can also be distinguished by it's enum type. For each enum declaration a def-foreign-type is generated, assigning the enum a default type of :int. Users may adjust the foreign type of enums via SWIG `typemaps`.

Enum values are a bit trickier as they can be initialized using any valid C/C++ expression. In C with the -nocwrap command-line option, we handle the typical cases (simple integer initialization) and generate a defconstant form for each enum value. This has the advantage of it not being necessary to probe into foreign space to retrieve enum values. When generating a .cxx wrapper file, a more general solution is employed. A wrapper variable is created in the module_wrap.cxx file, and a ff:def-foreign-variable call is generated to retrieve it's value into lisp.

For example, the following header file

enum.h:

    enum COL { RED, GREEN, BLUE };
    enum FOO { FOO1 = 10, FOO2, FOO3 };

In -nocwrap mode, generates

enum.cl:

    (swig-def-foreign-type "COL" :int)
    (swig-defconstant "RED" 0)
    (swig-defconstant "GREEN" (+ #.(swig-insert-id "RED" () :type :constant) 1))
    (swig-defconstant "BLUE" (+ #.(swig-insert-id "GREEN" () :type :constant) 1))

    (swig-def-foreign-type "FOO" :int)
    (swig-defconstant "FOO1" 10)
    (swig-defconstant "FOO2" (+ #.(swig-insert-id "FOO1" () :type :constant) 1))
    (swig-defconstant "FOO3" (+ #.(swig-insert-id "FOO2" () :type :constant) 1))

And when generating a .cxx wrapper

enum_wrap.cxx:

    EXPORT const int ACL_ENUM___RED__SWIG_0 = RED;
    EXPORT const int ACL_ENUM___GREEN__SWIG_0 = GREEN;
    EXPORT const int ACL_ENUM___BLUE__SWIG_0 = BLUE;
    EXPORT const int ACL_ENUM___FOO1__SWIG_0 = FOO1;
    EXPORT const int ACL_ENUM___FOO2__SWIG_0 = FOO2;
    EXPORT const int ACL_ENUM___FOO3__SWIG_0 = FOO3;

and

enum.cl:

    (swig-def-foreign-type "COL" :int)
    (swig-defvar "RED" "ACL_ENUM___RED__SWIG_0" :type :constant)
    (swig-defvar "GREEN" "ACL_ENUM___GREEN__SWIG_0" :type :constant)
    (swig-defvar "BLUE" "ACL_ENUM___BLUE__SWIG_0" :type :constant)

    (swig-def-foreign-type "FOO" :int)
    (swig-defvar "FOO1" "ACL_ENUM___FOO1__SWIG_0" :type :constant)
    (swig-defvar "FOO2" "ACL_ENUM___FOO2__SWIG_0" :type :constant)
    (swig-defvar "FOO3" "ACL_ENUM___FOO3__SWIG_0" :type :constant)

### 16.3.5 Arrays

One limitation in the Allegro CL foreign-types module, is that, without macrology, expressions may not be used to specify the dimensions of an array declaration. This is not a horrible drawback unless it is necessary to allocate foreign structures based on the array declaration using ff:allocate-fobject. When it can be determined that an array bound is a valid numeric value, SWIG will include this in the generated array declaration on the lisp side, otherwise the value will be included, but commented out.

Below is a comprehensive example, showing a number of legal C/C++ array declarations and how they are translated into foreign-type specifications in the generated lisp code.

array.h

    #define MAX_BUF_SIZE 1024

    namespace FOO {
      int global_var1[13];
      float global_var2[MAX_BUF_SIZE];

    }

    enum COLOR { RED = 10, GREEN = 20, BLUE, PURPLE = 50, CYAN };

    namespace BAR {
      char global_var3[MAX_BUF_SIZE + 1];
      float global_var4[MAX_BUF_SIZE][13];
      signed short global_var5[MAX_BUF_SIZE + MAX_BUF_SIZE];

      int enum_var5[GREEN];
      int enum_var6[CYAN];

      COLOR enum_var7[CYAN][MAX_BUF_SIZE];
    }

Generates:

array.cl

    (in-package #.*swig-module-name*)

    (swig-defpackage ("FOO"))
    (swig-defpackage ("BAR"))

    (swig-in-package ())
    (swig-def-foreign-type "COLOR" :int)
    (swig-defvar "RED" "ACL_ENUM___RED__SWIG_0" :type :constant)
    (swig-defvar "GREEN" "ACL_ENUM___GREEN__SWIG_0" :type :constant)
    (swig-defvar "BLUE" "ACL_ENUM___BLUE__SWIG_0" :type :constant)
    (swig-defvar "PURPLE" "ACL_ENUM___PURPLE__SWIG_0" :type :constant)
    (swig-defvar "CYAN" "ACL_ENUM___CYAN__SWIG_0" :type :constant)

    (swig-in-package ())

    (swig-defconstant "MAX_BUF_SIZE" 1024)
    (swig-in-package ("FOO"))

    (swig-defun ("global_var1" "ACL_FOO__global_var1_get__SWIG_0" :type :getter)
      (:void)
      (:returning ((* :int) )
       :strings-convert t)
      (make-instance 'ff:foreign-pointer :foreign-address (swig-ff-call)))

    (swig-defun ("global_var2" "ACL_FOO__global_var2_set__SWIG_0" :type :setter)
      ((global_var2  (:array :float 1024) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 global_var2))
      (swig-ff-call SWIG_arg0)))

    (swig-in-package ())
    (swig-in-package ("BAR"))
    (swig-defun ("global_var3" "ACL_BAR__global_var3_set__SWIG_0" :type :setter)
      ((global_var3  (:array :char #|1024+1|#) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 global_var3))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("global_var4" "ACL_BAR__global_var4_set__SWIG_0" :type :setter)
      ((global_var4  (:array (:array :float 13) 1024) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 global_var4))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("global_var4" "ACL_BAR__global_var4_get__SWIG_0" :type :getter)
      (:void)
      (:returning ((* (:array :float 13)) )
       :strings-convert t)
      (make-instance 'ff:foreign-pointer :foreign-address (swig-ff-call)))

    (swig-defun ("global_var5" "ACL_BAR__global_var5_set__SWIG_0" :type :setter)
      ((global_var5  (:array :short #|1024+1024|#) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 global_var5))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("enum_var5" "ACL_BAR__enum_var5_set__SWIG_0" :type :setter)
      ((enum_var5  (:array :int #|GREEN|#) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 enum_var5))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("enum_var6" "ACL_BAR__enum_var6_set__SWIG_0" :type :setter)
      ((enum_var6  (:array :int #|CYAN|#) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 enum_var6))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("enum_var7" "ACL_BAR__enum_var7_set__SWIG_0" :type :setter)
      ((enum_var7  (:array (:array #.(swig-insert-id "COLOR" ()) 1024) #|CYAN|#) ))
      (:returning (:void )
       :strings-convert t)
      (let ((SWIG_arg0 enum_var7))
      (swig-ff-call SWIG_arg0)))

    (swig-defun ("enum_var7" "ACL_BAR__enum_var7_get__SWIG_0" :type :getter)
      (:void)
      (:returning ((* (:array #.(swig-insert-id "COLOR" ()) 1024)) )
       :strings-convert t)
      (make-instance 'ff:foreign-pointer :foreign-address (swig-ff-call)))

### 16.3.6 Classes and Structs and Unions (oh my!)

#### 16.3.6.1 CLOS wrapping of

Classes, unions, and structs are all treated the same way by the interface generator. For any of these objects, a def-foreign-type and a defclass form are generated. For every function that returns an object (or pointer/reference) of C/C++ type `X`, the wrapping defun (or defmethod) on the Lisp side will automatically wrap the pointer returned in an instance of the apropriate class. This makes it much easier to write and debug code than if pointers were passed around as a jumble of integer values.

#### 16.3.6.2 CLOS Inheritance

The CLOS class schema generated by the interface mirrors the inheritance of the classes in foreign code, with the ff:foreign-pointer class at its root. ff:foreign-pointer is a thin wrapper for pointers that is made available by the foreign function interface. It's key benefit is that it may be passed as an argument to any ff:def-foreign-call that is expecting a pointer as the parameter.

#### 16.3.6.3 Member fields and functions

All public fields will have accessor getter/setter functions generated for them, as appropriate. All public member functions will have wrapper functions generated.

We currently ignore anything that isn't `public` (i.e. `private` or `protected`), because the C++ compiler won't allow the wrapper functions to access such fields. Likewise, the interface does nothing for `friend` directives,

#### 16.3.6.4 Why not directly access C++ classes using foreign types?

The def-foreign-type generated by the SWIG interface is currently incomplete. We can reliably generate the object layout of simple structs and unions; they can be allocated via ff:allocate-fobject, and their member variables accessed directly using the various ff:fslot-value-* functions. However, the layout of C++ classes is more complicated. Different compilers adjust class layout based on inheritance patterns, and the presence of virtual member functions. The size of member function pointers vary across compilers as well. As a result, it is recommended that users of any generated interface not attempt to access C++ instances via the foreign type system, but instead use the more robust wrapper functions.

### 16.3.7 Templates

#### 16.3.7.1 Generating wrapper code for templates

SWIG provides support for dealing with templates, but by default, it will not generate any member variable or function wrappers for templated classes. In order to create these wrappers, you need to explicitly tell SWIG to instantiate them. This is done via the `%template` directive.

#### 16.3.7.2 Implicit Template instantiation

While no wrapper code is generated for accessing member variables, or calling member functions, type code is generated to include these templated classes in the foreign-type and CLOS class schema.

### 16.3.8 Typedef, Templates, and Synonym Types

In C/C++ it is possible, via typedef, to have many names refer to the same `type`. In general, this is not a problem, though it can lead to confusion. Assume the below C++ header file:

synonyms.h

    class A {
       int x;
       int y;
    };

    typedef A Foo;

    A *xxx(int i);         /* sets A->x = A->y = i */
    Foo *yyy(int i);       /* sets Foo->x = Foo->y = i */

    int zzz(A *inst = 0);  /* return inst->x + inst->y */

The function `zzz` is an overloaded functions; the foreign function call to it will be wrapped in a generic-function whose argument will be checked against a type of `A`. Assuming a simple implementation, a call to `xxx(1)` will return a pointer to an A object, which will be wrapped in a CLOS instance of class `A`, and a call to `yyy(1)` will result in a CLOS instance of type `Foo` being returned. Without establishing a clear type relationship between `Foo` and `A`, an attempt to call `zzz(yyy(1))` will result in an error.

We resolve this issue, by noting synonym relationships between types while generating the interface. A Primary type is selected (more on this below) from the candidate list of synonyms. For all other synonyms, intead of generating a distinct CLOS class definition, we generate a form that expands to:

`(setf (find-class <synonym>) <primary>)`

The result is that all references to synonym types in foreign code, are wrapped in the same CLOS wrapper, and, in particular, method specialization in wrapping generic functions works as expected.

Given the above header file, synonym.h, a Lisp session would appear as follows:

    CL-USER> (load "synonym.dll")
    ; Foreign loading synonym.dll.
    t
    CL-USER> (load "synonym.cl")
    ; Loading c:\mikel\src\swig\test\synonym.cl
    t
    CL-USER>
    synonym> (setf a (xxx 3))
    #<A nil #x3261a0 @ #x207299da>
    synonym> (setf foo (yyy 10))
    #<A nil #x3291d0 @ #x2072e982>
    synonym> (zzz a)
    6
    synonym> (zzz foo)
    20
    synonym>

#### 16.3.8.1 Choosing a primary type

The choice of a primary type is selected by the following criteria from a set of synonym types.

- If a synonym type has a class definition, it is the primary type.
- If a synonym type is a class template and has been explicitly instantiated via `%template`, it is the primary type.
- For all other sets of synonymous types, the synonym which is parsed first becomes the primary type.

### 16.3.9 Function overloading/Parameter defaulting

For each possible argument combination, a distinct wrapper function is created in the .cxx file. On the Lisp side, a generic functions is defined for each possible arity the overloaded/defaulted call may have. Each distinct wrapper is then called from within a defmethod on the appropriate generic function. These are further wrapped inside a dispatch function that checks the number of arguments it is called with and passes them via apply to the appropriate generic-function. This allows for a single entry point to overloaded functions on the lisp side.

Example:

overload.h:

    class A {
     public:
      int x;
      int y;
    };

    float xxx(int i, int x = 0);   /* return i * x */
    float xxx(A *inst, int x);     /* return x + A->x + A->y */

Creates the following three wrappers, for each of the possible argument combinations

overload_wrap.cxx

    EXPORT void ACL___delete_A__SWIG_0 (A *larg1) {
        A *arg1 = (A *) 0 ;

        arg1 = larg1;
        try {
            delete arg1;

        } catch (...) {

        }
    }

    EXPORT float ACL___xxx__SWIG_0 (int larg1, int larg2) {
        float lresult = (float)0 ;
        int arg1 ;
        int arg2 ;
        float result;

        arg1 = larg1;
        arg2 = larg2;
        try {
            result = (float)xxx(arg1,arg2);

            lresult = result;
            return lresult;
        } catch (...) {
            return (float)0;
        }
    }

    EXPORT float ACL___xxx__SWIG_1 (int larg1) {
        float lresult = (float)0 ;
        int arg1 ;
        float result;

        arg1 = larg1;
        try {
            result = (float)xxx(arg1);

            lresult = result;
            return lresult;
        } catch (...) {
            return (float)0;
        }
    }

    EXPORT float ACL___xxx__SWIG_2 (A *larg1, int larg2) {
        float lresult = (float)0 ;
        A *arg1 = (A *) 0 ;
        int arg2 ;
        float result;

        arg1 = larg1;
        arg2 = larg2;
        try {
            result = (float)xxx(arg1,arg2);

            lresult = result;
            return lresult;
        } catch (...) {
            return (float)0;
        }
    }

And the following foreign-function-call and method definitions on the lisp side:

overload.cl

    (swig-defmethod ("xxx" "ACL___xxx__SWIG_0" :type :function :arity 2)
      ((PARM0_i cl:integer :int )
       (PARM1_x cl:integer :int ))
      (:returning (:float )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_i))
      (let ((SWIG_arg1 PARM1_x))
      (swig-ff-call SWIG_arg0 SWIG_arg1))))

    (swig-defmethod ("xxx" "ACL___xxx__SWIG_1" :type :function :arity 1)
      ((PARM0_i cl:integer :int ))
      (:returning (:float )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_i))
      (swig-ff-call SWIG_arg0)))

    (swig-defmethod ("xxx" "ACL___xxx__SWIG_2" :type :function :arity 2)
      ((PARM0_inst #.(swig-insert-id "A" () :type :class) (* #.(swig-insert-id "A" ())) )
       (PARM1_x cl:integer :int ))
      (:returning (:float )
       :strings-convert t)
      (let ((SWIG_arg0 PARM0_inst))
      (let ((SWIG_arg1 PARM1_x))
      (swig-ff-call SWIG_arg0 SWIG_arg1))))

    (swig-dispatcher ("xxx" :type :function :arities (1 2)))

And their usage in a sample lisp session:

    overload> (setf a (new_A))
    #<A nil #x329268 @ #x206cf612>
    overload> (setf (A_x a) 10)
    10
    overload> (setf (A_y a) 20)
    20
    overload> (xxx 1)
    0.0
    overload> (xxx 3 10)
    30.0
    overload> (xxx a 1)
    31.0
    overload> (xxx a 2)
    32.0
    overload>

### 16.3.10 Operator wrapping and Operator overloading

Wrappers to defined C++ Operators are automatically renamed, using `%rename`, to the following defaults:

    /* name conversion for overloaded operators. */
    #ifdef __cplusplus
    %rename(__add__)	     *::operator+;
    %rename(__pos__)	     *::operator+();
    %rename(__pos__)	     *::operator+() const;

    %rename(__sub__)	     *::operator-;
    %rename(__neg__)	     *::operator-() const;
    %rename(__neg__)	     *::operator-();

    %rename(__mul__)	     *::operator*;
    %rename(__deref__)	     *::operator*();
    %rename(__deref__)	     *::operator*() const;

    %rename(__div__)	     *::operator/;
    %rename(__mod__)	     *::operator%;
    %rename(__logxor__)	     *::operator^;
    %rename(__logand__)	     *::operator&;
    %rename(__logior__)	     *::operator|;
    %rename(__lognot__)	     *::operator~();
    %rename(__lognot__)	     *::operator~() const;

    %rename(__not__)	     *::operator!();
    %rename(__not__)	     *::operator!() const;

    %rename(__assign__)	     *::operator=;

    %rename(__add_assign__)      *::operator+=;
    %rename(__sub_assign__)	     *::operator-=;
    %rename(__mul_assign__)	     *::operator*=;
    %rename(__div_assign__)	     *::operator/=;
    %rename(__mod_assign__)	     *::operator%=;
    %rename(__logxor_assign__)   *::operator^=;
    %rename(__logand_assign__)   *::operator&=;
    %rename(__logior_assign__)   *::operator|=;

    %rename(__lshift__)	     *::operator<<;
    %rename(__lshift_assign__)   *::operator<<=;
    %rename(__rshift__)	     *::operator>>;
    %rename(__rshift_assign__)   *::operator>>=;

    %rename(__eq__)		     *::operator==;
    %rename(__ne__)		     *::operator!=;
    %rename(__lt__)		     *::operator<;
    %rename(__gt__)		     *::operator>;
    %rename(__lte__)	     *::operator<=;
    %rename(__gte__)	     *::operator>=;

    %rename(__and__)	     *::operator&&;
    %rename(__or__)		     *::operator||;

    %rename(__preincr__)	     *::operator++();
    %rename(__postincr__)	     *::operator++(int);
    %rename(__predecr__)	     *::operator--();
    %rename(__postdecr__)	     *::operator--(int);

    %rename(__comma__)	     *::operator,();
    %rename(__comma__)	     *::operator,() const;

    %rename(__member_ref__)      *::operator->;
    %rename(__member_func_ref__) *::operator->*;

    %rename(__funcall__)	     *::operator();
    %rename(__aref__)	     *::operator[];

Name mangling occurs on all such renamed identifiers, so that wrapper name generated by `B::operator=` will be `B___eq__`, i.e. `<class-or-namespace>_` has been added. Users may modify these default names by adding `%rename` directives in their own .i files.

Operator overloading can be achieved by adding functions based on the mangled names of the function. In the following example, a class B is defined with a Operator== method defined. The swig `%extend` directive is used to add an overload method on Operator==.

opoverload.h

    class B {
     public:
      int x;
      int y;
      bool operator==(B const& other) const;
    };

and

opoverload.i

    %module opoverload

    %{
    #include <fstream>
    #include "opoverload.h"
    %}

    %{
    bool B___eq__(B const *inst, int const x)
    {
      // insert the function definition into the wrapper code before
      // the wrapper for it.
      // ... do stuff ...
    }
    %}

    %include "opoverload.h"

    %extend B {
     public:
      bool __eq__(int const x) const;
    };

Either operator can be called via a single call to the dispatch function:

    opoverload> (B___eq__ x1 x2)
    nil
    opoverload> (B___eq__ x1 3)
    nil
    opoverload>

### 16.3.11 Varargs

Variable length argument lists are not supported, by default. If such a function is encountered, a warning will generated to stderr. Varargs are supported via the SWIG `%vararg` directive. This directive allows you to specify a (finite) argument list which will be inserted into the wrapper in place of the variable length argument indicator. As an example, consider the function `printf()`. It's declaration would appear as follows:

See the following section on Variable Length arguments provides examples on how `%vararg` can be used, along with other ways such functions can be wrapped.

### 16.3.12 C++ Exceptions

Each C++ wrapper includes a handler to catch any exceptions that may be thrown while in foreign code. This helps prevent simple C++ errors from killing the entire lisp process. There is currently no mechanism to have these exceptions forwarded to the lisp condition system, nor has any explicit support of the exception related SWIG typemaps been implemented.

### 16.3.13 Pass by value, pass by reference

Allegro CL does not support the passing of non-primitive foreign structures by value. As a result, SWIG must automatically detect and convert function parameters and return values to pointers whenever necessary. This is done via the use of `typemaps`, and should not require any fine tuning by the user, even for newly defined types.

## 16.4 Typemaps

SWIG Typemaps provide a powerful tool for automatically generating code to handle various menial tasks required of writing an interface to foreign code. The purpose of this section is to describe each of the typemaps used by the Allegro CL module. Please read the chapter on Typemaps for more information.

### 16.4.1 Code Generation in the C++ Wrapper

Every C++ wrapper generated by SWIG takes the following form:

    return-val wrapper-name(parm0, parm1, ..., parmN)
    {
       return-val lresult;   /* return value from wrapper */
       <local-declaration>
       ... results;          /* return value from function call */

       <binding locals to parameters>

       try {
          result = function-name(local0, local1, ..., localN);

          <convert and bind result to lresult>

          return lresult;
       catch (...) {
          return (int)0;
       }

#### 16.4.1.1 IN Typemap

the `in` typemap is used to generate code to convert parameters passed to C++ wrapper functions into the arguments desired for the call being wrapped. That is, it fills in the code for the `<binding locals to parameters>` section above. We use this map to automatically convert parameters passed by reference to the wrapper function into by-value arguments for the wrapped call, and also to convert boolean values, which are passed as integers from lisp (by default), into the appropriate type for the language of code being wrapped.

These are the default specifications for the IN typemap. Here, `$input` refers to the parameter code is being generated for, and `$1` is the local variable to which it is being assigned. The default settings of this typemap are as follows:

    %typemap(in) bool                          "$1 = (bool)$input;";
    %typemap(in) char, unsigned char, signed char,
                 short, signed short, unsigned short,
                 int, signed int, unsigned int,
                 long, signed long, unsigned long,
                 float, double, long double, char *, void *, void,
                 enum SWIGTYPE, SWIGTYPE *,
                 SWIGTYPE[ANY], SWIGTYPE &     "$1 = $input;";
    %typemap(in) SWIGTYPE                      "$1 = *$input;";

#### 16.4.1.2 OUT Typemap

The `out` typemap is used to generate code to form the return value of the wrapper from the return value of the wrapped function. This code is placed in the <convert and bind result to lresult> section of the above code diagram. It's default mapping is as follows:

    %typemap(out) bool                          "$result = (int)$1;";
    %typemap(out) char, unsigned char, signed char,
                  short, signed short, unsigned short,
                  int, signed int, unsigned int,
                  long, signed long, unsigned long,
                  float, double, long double, char *, void *, void,
                  enum SWIGTYPE, SWIGTYPE *,
                  SWIGTYPE[ANY], SWIGTYPE &    "$result = $1;";
    %typemap(out) SWIGTYPE                     "$result = new $1_type($1);";

#### 16.4.1.3 CTYPE Typemap

This typemap is not used for code generation, but purely for the transformation of types in the parameter list of the wrapper function. It's primary use is to handle by-value to by-reference conversion in the wrappers parameter list. Its default settings are:

    %typemap(ctype) bool                       "int";
    %typemap(ctype) char, unsigned char, signed char,
                    short, signed short, unsigned short,
                    int, signed int, unsigned int,
                    long, signed long, unsigned long,
                    float, double, long double, char *, void *, void,
                    enum SWIGTYPE, SWIGTYPE *,
                    SWIGTYPE[ANY], SWIGTYPE &  "$1_ltype";
    %typemap(ctype) SWIGTYPE                   "$&1_type";

These three typemaps are specifically employed by the the Allegro CL interface generator. SWIG also implements a number of other typemaps that can be used for generating code in the C/C++ wrappers. You can read about these common typemaps here.

### 16.4.2 Code generation in Lisp wrappers

A number of custom typemaps have also been added to facilitate the generation of code in the lisp side of the interface. These are described below. The basic code generation structure is applied as a series of nested expressions, one for each parameter, then one for manipulating the return value, and last, the foreign function call itself.

Note that the typemaps below use fully qualified symbols where necessary. Users writing their own typemaps should do likewise. See the explanation in the last paragraph of 16.3.1 Namespaces for details.

#### 16.4.2.1 LIN Typemap

The LIN typemap allows for the manipulating the lisp objects passed as arguments to the wrapping defun before passing them to the foreign function call. For example, when passing lisp strings to foreign code, it is often necessary to copy the string into a foreign structure of type (:char *) of appropriate size, and pass this copy to the foreign call. Using the LIN typemap, one could arrange for the stack-allocation of a foreign char array, copy your string into it, and not have to worry about freeing the copy after the function returns.

The LIN typemap accepts the following `$variable` references.

- `$in` - expands to the name of the parameter being applied to this typemap
- `$out` - expands to the name of the local variable assigned to this typemap
- `$in_fftype` - the foreign function type of the C type.
- `$*in_fftype` - the foreign function type of the C type with one pointer removed. If there is no pointer, then $*in_fftype is the same as $in_fftype.
- `$body` - very important. Instructs SWIG where subsequent code generation steps should be inserted into the current typemap. Leaving out a `$body` reference will result in lisp wrappers that do very little by way of calling into foreign code. Not recommended.

    %typemap(lin)	SWIGTYPE 	"(cl:let (($out $in))\n  $body)";

#### 16.4.2.2 LOUT Typemap

The LOUT typemap is the means by which we effect the wrapping of foreign pointers in CLOS instances. It is applied after all LIN typemaps, and immediately before the actual foreign-call.

The LOUT typemap uses the following $variable

- `$lclass` - Expands to the CLOS class that represents foreign-objects of the return type matching this typemap.
- `$body` - Same as for the LIN map. Place this variable where you want the foreign-function call to occur.
- `$ldestructor` - Expands to the symbol naming the destructor for this class ($lclass) of object. Allows you to insert finalization or automatic garbage collection into the wrapper code (see default mappings below).

    %typemap(lout) bool, char, unsigned char, signed char,
                   short, signed short, unsigned short,
                   int, signed int, unsigned int,
                   long, signed long, unsigned long,
                   float, double, long double, char *, void *, void,
                   enum SWIGTYPE    "$body";
    %typemap(lout) SWIGTYPE[ANY], SWIGTYPE *,
                   SWIGTYPE &       "(cl:make-instance '$lclass :foreign-address $body)";
    %typemap(lout) SWIGTYPE    "(cl:let* ((address $body)\n
                                (ACL_result (cl:make-instance '$lclass :foreign-address address)))\n
                                (cl:unless (cl::zerop address)\n
                                (excl:schedule-finalization ACL_result #'$ldestructor))\n
                                 ACL_result)";

#### 16.4.2.3 FFITYPE Typemap

The FFITYPE typemap works as a helper for a body of code that converts C/C++ type specifications into Allegro CL foreign-type specifications. These foreign-type specifications appear in ff:def-foreing-type declarations, and in the argument list and return values of ff:def-foreign-calls. You would modify this typemap if you want to change how the FFI passes through arguments of a given type. For example, if you know that a particular compiler represents booleans as a single byte, you might add an entry for:

    %typemap(ffitype) bool ":unsigned-char";

Note that this typemap is pure type transformation, and is not used in any code generations step the way the LIN and LOUT typemaps are. The default mappings for this typemap are:

    %typemap(ffitype) bool ":int";
    %typemap(ffitype) char ":char";
    %typemap(ffitype) unsigned char ":unsigned-char";
    %typemap(ffitype) signed char ":char";
    %typemap(ffitype) short, signed short ":short";
    %typemap(ffitype) unsigned short ":unsigned-short";
    %typemap(ffitype) int, signed int ":int";
    %typemap(ffitype) unsigned int ":unsigned-int";
    %typemap(ffitype) long, signed long ":long";
    %typemap(ffitype) unsigned long ":unsigned-long";
    %typemap(ffitype) float ":float";
    %typemap(ffitype) double ":double";
    %typemap(ffitype) char * "(* :char)";
    %typemap(ffitype) void * "(* :void)";
    %typemap(ffitype) void ":void";
    %typemap(ffitype) enum SWIGTYPE ":int";
    %typemap(ffitype) SWIGTYPE & "(* :void)";

#### 16.4.2.4 LISPTYPE Typemap

This is another type only transformation map, and is used to provide the lisp-type, which is the optional third argument in argument specifier in a ff:def-foreign-call form. Specifying a lisp-type allows the foreign call to perform type checking on the arguments passed in. The default entries in this typemap are:

    %typemap(lisptype) bool "cl:boolean";
    %typemap(lisptype) char "cl:character";
    %typemap(lisptype) unsigned char "cl:integer";
    %typemap(lisptype) signed char "cl:integer";

#### 16.4.2.5 LISPCLASS Typemap

The LISPCLASS typemap is used to generate the method signatures for the generic-functions which wrap overloaded functions and functions with defaulted arguments. The default entries are:

    %typemap(lispclass) bool "t";
    %typemap(lispclass) char "cl:character";
    %typemap(lispclass) unsigned char, signed char,
                        short, signed short, unsigned short,
                        int, signed int, unsigned int,
                        long, signed long, unsigned long,
                        enum SWIGTYPE       "cl:integer";
    %typemap(lispclass) float "cl:single-float";
    %typemap(lispclass) double "cl:double-float";
    %typemap(lispclass) char * "cl:string";

### 16.4.3 Modifying SWIG behavior using typemaps

The following example shows how we made use of the above typemaps to add support for the wchar_t type.

    %typecheck(SWIG_TYPECHECK_UNICHAR) wchar_t { $1 = 1; };

    %typemap(in)        wchar_t "$1 = $input;";
    %typemap(lin)       wchar_t "(cl:let (($out (cl:char-code $in)))\n  $body)";
    %typemap(lin)       wchar_t* "(excl:with-native-string
                                             ($out $in
                                              :external-format #+little-endian :fat-le
                                                               #-little-endian :fat)\n
                                     $body)"

    %typemap(out)       wchar_t "$result = $1;";
    %typemap(lout)      wchar_t "(cl:code-char $body)";
    %typemap(lout)      wchar_t* "(excl:native-to-string $body
                                              :external-format #+little-endian :fat-le
                                                               #-little-endian :fat)";

    %typemap(ffitype)   wchar_t ":unsigned-short";
    %typemap(lisptype)  wchar_t "";
    %typemap(ctype)     wchar_t "wchar_t";
    %typemap(lispclass) wchar_t "cl:character";
    %typemap(lispclass) wchar_t* "cl:string";

## 16.5 Identifier Converter functions

### 16.5.1 Creating symbols in the lisp environment

Various symbols must be generated in the lisp environment to which class definitions, functions, constants, variables, etc. must be bound. Rather than force a particular convention for naming these symbols, an identifier (to symbol) conversion function is used. A user-defined identifier-converter can then implement any symbol naming, case-modifying, scheme desired.

In generated SWIG code, whenever some interface object must be referenced by its lisp symbol, a macro is inserted that calls the identifier-converter function to generate the appropriate symbol reference. It is therefore expected that the identifier-converter function reliably return the same (eq) symbol given the same set of arguments.

### 16.5.2 Existing identifier-converter functions

Two basic identifier routines have been defined.

#### 16.5.2.1 identifier-convert-null

No modification of the identifier string is performed. Based on other arguments, the identifier may be concatenated with other strings, from which a symbol will be created.

#### 16.5.2.2 identifier-convert-lispify

All underscores in the identifier string are converted to hyphens. Otherwise, identifier-convert-lispify performs the same symbol transformations.

#### 16.5.2.3 Default identifier to symbol conversions

Check the definitions of the above two default identifier-converters in `Lib/allegrocl/allegrocl.swg` for default naming conventions.

### 16.5.3 Defining your own identifier-converter

A user-defined identifier-converter function should conform to the following specification:

    (defun identifier-convert-fn (id &key type class arity) ...body...)
    result ==> symbol or (setf symbol)

The `ID` argument is a string representing an identifier in the foreign environment.

The :type keyword argument provides more information on the type of identifier. It's value is a symbol. This allows the identifier-converter to apply different heuristics when mapping different types of identifiers to symbols. SWIG will generate calls to your identifier-converter using the following types.

- :class - names a CLOS class.
- :constant - names a defconstant
- :constructor - names a function for creating a foreign object
- :destructor - names a function for freeing a foreign object
- :function - names a CLOS wrapping defmethod or defun.
- :ff-operator - names a foreign call defined via ff:def-foreign-call
- :getter - getter function
- :namespace - names a C++ namespace
- :setter - names a setter function. May return a (setf symbol) reference
- :operator - names a C++ operator, such as Operator=, Operator*.
- :slot - names a slot in a struct/class/union declaration.
- :type - names a foreign-type defined via ff:def-foreign-type.
- :variable - names a variable defined via ff:def-foreign-variable.

The :class keyword argument is a string naming a foreign class. When non-nil, it indicates that the current identifier has scope in the specified class.

The :arity keyword argument only appears in swig:swig-defmethod forms generated for overloaded functions. It's value is an integer indicating the number of arguments passed to the routine indicated by this identifier.

### 16.5.4 Instructing SWIG to use a particular identifier-converter

By default, SWIG will use identifier-converter-null. To specify another convert function, use the `-identifier-converter` command-line argument. The value should be a string naming the function you wish the interface to use instead, when generating symbols. ex:

    % swig -allegrocl -c++ -module mymodule -identifier-converter my-identifier-converter