# 16 SWIG and Ocaml

- Preliminaries
  - Running SWIG
  - Getting the right header files
  - Compiling the code
  - The camlp4 module
  - Current thoughts on best practice for Ocaml
  - Using your module
  - Compilation problems and compiling with C++
- The low-level Ocaml/C interface
  - The generated module
  - Enums
  - C++ Classes
    - STL vector and string Example
    - C++ Class Example
    - Compiling the example
    - Sample Session
  - Director Classes
  - Exceptions

This chapter describes SWIG's support of Ocaml. Ocaml is a relatively recent addition to the ML family, and is a recent addition to SWIG. It's the second compiled, typed language to be added. Ocaml has widely acknowledged benefits for engineers, mostly derived from a sophistocated type system, compile-time checking which eliminates several classes of common programming errors, and good native performance. While all of this is wonderful, there are well-written C and C++ libraries that Ocaml users will want to take advantage of as part of their arsenal (such as SSL and gdbm), as well as their own mature C and C++ code. SWIG allows this code to be used in a natural, type-safe way with Ocaml, by providing the necessary, but repetetive glue code which creates and uses Ocaml values to communicate with C and C++ code. In addition, SWIG also produces the needed Ocaml source that binds variants, functions, classes, etc.

## 16.1 Preliminaries

SWIG 1.3 works with Ocaml 3.04 and above. Given the choice, you should use the latest stable release. The SWIG Ocaml module has been tested on Linux (m68k,MIPS,Intel) and Cygwin on Windows. The best way to determine whether your system will work is to compile the examples and test-suite which come with SWIG. You can do this by running `make check` from the SWIG root directory after installing SWIG. The Ocaml module has been tested using the system's dynamic linking (the usual -lxxx against libxxx.so, as well as with Gerd Stolpmann's Dl package . The ocaml_dynamic and ocaml_dynamic_cpp targets in the file Examples/Makefile illustrate how to compile and link SWIG modules that will be loaded dynamically. This has only been tested on Linux so far.

### 16.1.1 Running SWIG

The basics of getting a SWIG Ocaml module up and running can be seen from one of SWIG's example Makefiles, but is also described here. To build an Ocaml module, run SWIG using the `-ocaml` option.

>     %swig -ocaml example.i

This will produce 3 files. The file `example_wrap.c` contains all of the C code needed to build an Ocaml module. To build the module, you will compile the file `example_wrap.c` with `ocamlc` or `ocamlopt` to create the needed .o file. You will need to compile the resulting .ml and .mli files as well, and do the final link with -custom (not needed for native link).

### 16.1.2 Getting the right header files

You may need the libswigocaml.h file that comes with the distribution to be included. It provides several useful functions that almost all programs that use SWIG will need. It is located in $(prefix)/include/libswigocaml.h where $(prefix) is usually /usr/local, but could be /usr. This is set at configure time. The functions most frequently used in here are isnull and nullptr for creating and checking null pointers.

### 16.1.3 Compiling the code

Use `ocamlc` or `ocamlopt` to compile your SWIG interface like:

>     % ocamlc -c -ccopt "-I/usr/include/foo -I/usr/local/include" example_wrap.c
>     % ocamlc -c example.mli
>     % ocamlc -c example.ml

`ocamlc` is aware of .c files and knows how to handle them. Unfortunately, it does not know about .cxx, .cc, or .cpp files, so when SWIG is invoked in C++ mode, you must:

>     % cp example_wrap.cxx example_wrap.cxx.c
>     % ocamlc -c ... -ccopt -xc++ example_wrap.cxx.c
>     % ...

### 16.1.4 The camlp4 module

The camlp4 module (swig.ml -> swig.cmo) contains a simple rewriter which makes C++ code blend more seamlessly with objective caml code. It's use is optional. The source file is included in the Lib/ocaml directory of the SWIG source distribution.

The basic principle of the module is to recognize certain non-caml expressions and convert them for use with C++ code as interfaced by SWIG. The camlp4 module is written to work with generated SWIG interfaces, and probably isn't great to use with anything else.

Here are the main rewriting rules:

Input | Rewritten to
--- | ---
f'( ... ) as in atoi'("0") or _exit'(0) | f(C_list [ ... ]) as in atoi (C_list [ C_string "0" ]) or _exit (C_list [ C_int 0 ])
object -> "method" ( ... ) | (invoke object) "method" (C_list [ ... ])
object 'binop argument as in a '+= b | (invoke object) "+=" argument as in (invoke a) "+=" b |
Note that because camlp4 always recognizes << and >>, they are replaced by lsl and lsr in operator names.
'unop object as in '! a | (invoke a) "!" C_void
Smart pointer access like this object '-> "method" ( args ) | (invoke (invoke object "->" C_void))
Invoke syntax object . '( ... ) | (invoke object) "()" (C_list [ ... ])
Array syntax object '[ 10 ] | (invoke object) "[]" (C_int 10)

### 16.1.5 Current thoughts on best practice for Ocaml

Because the VC compiler (cl) needs link options specified after all compiler options, and ocamlc doesn't really understand that, I think that this is the best way to link ocaml code with C++ code. I formulated this method to make it easy for co-workers who rely on MSDev to create GUIs, etc.. to live in harmony with the ocaml parts of the application.

Let's say you have ocaml sources foo.ml and bar.ml and interface frob.i;

>     swig -ocaml -c++ frob.i
>     ocamlc -custom -c frob.mli
>     ocamlc -custom -c frob.ml
>     cp frob_wrap.cxx frob_wrap.c
>     ocamlc -custom -c -I$(FROBLIB)/include frob_wrap.c
>     ocamlc -custom -c foo.ml
>     ocamlc -custom -c bar.ml
>     ocamlc -pack -o foobar.cmo foo.cmo bar.cmo frob.cmo
>     ocamlc -custom -output-obj -o foobar.obj foobar.cmo

At this point, foobar.obj can be included in your MSVC project and linked against other code. This is how you link it:

>     link /OUT:big_program.exe \
>         other1.obj other2.obj foobar.obj frob_wrap.obj \
>         $(OCAMLLIB)/ocamlrun.lib $(FROBLIB)/lib/frob.lib

### 16.1.6 Using your module

You can test-drive your module by building a toplevel ocaml interpreter. Consult the ocaml manual for details.

When linking any ocaml bytecode with your module, use the -custom option to build your functions into the primitive list. This option is not needed when you build native code.

### 16.1.7 Compilation problems and compiling with C++

As mentioned above, .cxx files need special handling to be compiled with `ocamlc`. Other than that, C code that uses `class` as a non-keyword, and C code that is too liberal with pointer types may not compile under the C++ compiler. Most code meant to be compiled as C++ will not have problems.

## 16.2 The low-level Ocaml/C interface

In order to provide access to overloaded functions, and provide sensible outputs from them, all C entites are represented as members of the c_obj type:

In the code as seen by the typemap writer, there is a value, swig_result, that always contains the current return data. It is a list, and must be appended with the caml_list_append function, or with functions and macros provided by objective caml.

>     type c_obj =
>         C_void
>       | C_bool of bool
>       | C_char of char
>       | C_uchar of char
>       | C_short of int
>       | C_ushort of int
>       | C_int of int
>       | C_uint of int32
>       | C_int32 of int32
>       | C_int64 of int64
>       | C_float of float
>       | C_double of float
>       | C_ptr of int64 * int64
>       | C_array of c_obj array
>       | C_list of c_obj list
>       | C_obj of (string -> c_obj -> c_obj)
>       | C_string of string
>       | C_enum of c_enum_t

A few functions exist which generate and return these:

- caml_ptr_val receives a c_obj and returns a void *. This should be used for all pointer purposes.
- caml_long_val receives a c_obj and returns a long. This should be used for most integral purposes.
- caml_val_ptr receives a void * and returns a c_obj.
- caml_val_bool receives a C int and returns a c_obj representing it's bool value.
- caml_val_(u)?(char|short|int|long|float|double) receives an appropriate C value and returns a c_obj representing it.
- caml_val_string receives a char * and returns a string value.
- caml_val_string_len receives a char * and a length and returns a string value.
- caml_val_obj receives a void * and an object type and returns a C_obj, which contains a closure giving method access.

Because of this style, a typemap can return any kind of value it wants from a function. This enables out typemaps and inout typemaps to work well. The one thing to remember about outputting values is that you must append them to the return list with swig_result = caml_list_append(swig_result,v).

This function will return a new list that has your element appended. Upon return to caml space, the fnhelper function beautifies the result. A list containing a single item degrades to only that item (i.e. [ C_int 3 ] -> C_int 3), and a list containing more than one item is wrapped in C_list (i.e. [ C_char 'a' ; C_char 'b' -> C_list [ C_char 'a' ; C_char b ]). This is in order to make return values easier to handle when functions have only one return value, such as constructors, and operators. In addition, string, pointer, and object values are interchangable with respect to caml_ptr_val, so you can allocate memory as caml strings and still use the resulting pointers for C purposes, even using them to construct simple objects on. Note, though, that foreign C++ code does not respect the garbage collector, although the SWIG interface does.

The wild card type that you can use in lots of different ways is C_obj. It allows you to wrap any type of thing you like as an object using the same mechanism that the ocaml module does. When evaluated in caml_ptr_val, the returned value is the result of a call to the object's "&" operator, taken as a pointer.

You should only construct values using objective caml, or using the functions caml_val_* functions provided as static functions to a SWIG ocaml module, as well as the caml_list_* functions. These functions provide everything a typemap needs to produce values. In addition, value items pass through directly, but you must make your own type signature for a function that uses value in this way.

### 16.2.1 The generated module

The SWIG `%module` directive specifies the name of the Ocaml module to be generated. If you specified ``%module example`', then your Ocaml code will be accessible in the module Example. The module name is always capitalized as is the ocaml convention. Note that you must not use any Ocaml keyword to name your module. Remember that the keywords are not the same as the C++ ones.

You can introduce extra code into the output wherever you like with SWIG. These are the places you can introduce code:

"header" | This code is inserted near the beginning of the C wrapper file, before any function definitions.
"wrapper" | This code is inserted in the function definition section.
"runtime" | This code is inserted near the end of the C wrapper file.
"mli" | This code is inserted into the caml interface file. Special signatures should be inserted here.
"ml" | This code is inserted in the caml code defining the interface to your C code. Special caml code, as well as any initialization which should run when the module is loaded may be inserted here.

### 16.2.2 Enums

SWIG will wrap enumerations as polymorphic variants in the output Ocaml code, as above in C_enum. In order to support all C++-style uses of enums, the function int_to_enum and enum_to_int are provided for ocaml code to produce and consume these values as integers. Other than that, correct uses of enums will not have a problem. Since enum labels may overlap between enums, the enum_to_int and int_to_enum functions take an enum type label as an argument. Example:

>     %module enum_test
>     %{
>     enum c_enum_type { a = 1, b, c = 4, d = 8 };
>     %}
>     enum c_enum_type { a = 1, b, c = 4, d = 8 };

The output mli contains:

>     type c_enum_type = [
>       `unknown
>     | `c_enum_type
>     ]
>     type c_enum_tag = [
>       `int of int
>     | `a
>     | `b
>     | `c
>     | `d
>     ]
>     val int_to_enum c_enum_type -> int -> c_obj
>     val enum_to_int c_enum_type -> c_obj -> c_obj

So it's possible to do this:

>     bash-2.05a$ ocamlmktop -custom enum_test_wrap.o enum_test.cmo -o enum_test_top
>     bash-2.05a$ ./enum_test_top
>             Objective Caml version 3.04

>     # open Enum_test ;;
>     # let x = C_enum `a ;;
>     val x : Enum_test.c_obj = C_enum `a
>     # enum_to_int `c_enum_type x ;;
>     - : Enum_test.c_obj = C_int 1
>     # int_to_enum `c_enum_type 4 ;;
>     - : Enum_test.c_obj = C_enum `c

### 16.2.3 C++ Classes

C++ classes, along with structs and unions are represented by C_obj (string -> c_obj -> c_obj) wrapped closures. These objects contain a method list, and a type, which allow them to be used like C++ objects. When passed into typemaps that use pointers, they degrade to pointers through their "&" method. Every method an object has is represented as a string in the object's method table, and each method table exists in memory only once. In addition to any other operators an object might have, certain builtin ones are provided by SWIG: (all of these take no arguments (C_void))

"~" | Delete this object
"&" | Return an ordinary C_ptr value representing this object's address
":methods" | Returns a list of strings containing the names of the methods this object contains
":classof" | Returns the name of the class this object belongs to.
":parents" | Returns a list of all direct parent classes which have been wrapped by SWIG.
"::[parent-class]" | Returns a view of the object as the indicated parent class. This is mainly used internally by the SWIG module, but may be useful to client programs.
"[member-variable]" | Each member variable is wrapped as a method with an optional parameter. Called with one argument, the member variable is set to the value of the argument. With zero arguments, the value is returned.

Note that this string belongs to the wrapper object, and not the underlying pointer, so using create_[x]_from_ptr alters the returned value for the same object.

#### 16.2.3.1 STL vector and string Example

Standard typemaps are now provided for STL vector and string. More are in the works. STL strings are passed just like normal strings, and returned as strings. STL string references don't mutate the original string, (which might be surprising), because Ocaml strings are mutable but have fixed length. Instead, use multiple returns, as in the argout_ref example.

example.i

    %module example
    %{
    #include "example.h"
    %}

    %include stl.i

    namespace std {
            %template(StringVector) std::vector<string>;
    };

    %include example.h

This example is in Examples/ocaml/stl

Since there's a makefile in that directory, the example is easy to build.

Here's a sample transcript of an interactive session using a string vector after making a toplevel like this:

    ocamlmktop -custom -cclib -g -ccopt -g -g \
       example_wrap.o example.cmo -o example_top -cclib -lstdc++

>     bash-2.05a$ ./example_top
>             Objective Caml version 3.06

>     # open Example ;;
>     # let x = new_StringVector C_void ;;
>     val x : Example.c_obj = C_obj <fun>
>     # (invoke x) ":methods" C_void ;;
>     - : Example.c_obj =
>     C_list
>      [C_string "nop"; C_string "size"; C_string "empty"; C_string "clear";
>       C_string "push_back"; C_string "[]"; C_string "="; C_string "set";
>       C_string "~"; C_string "&"; C_string ":parents"; C_string ":classof";
>       C_string ":methods"]
>     # (invoke x) "push_back" (C_string "foo") ;;
>     - : Example.c_obj = C_void
>     # (invoke x) "push_back" (C_string "bar") ;;
>     - : Example.c_obj = C_void
>     # (invoke x) "push_back" (C_string "baz") ;;
>     - : Example.c_obj = C_void
>     # (invoke x) "[]" (C_int 1) ;;
>     - : Example.c_obj = C_string "bar"
>     # (invoke x) "set" (C_list [ C_int 1 ; C_string "spam" ]) ;;
>     - : Example.c_obj = C_void
>     # (invoke x) "[]" (C_int 1) ;;
>     - : Example.c_obj = C_string "spam"
>     #

#### 16.2.3.2 C++ Class Example

Here's a simple example using Trolltech's Qt Library:

qt.i

    %module qt
    %{
    #include <qapplication.h>
    #include <qpushbutton.h>
    %}
    class QApplication {
    public:
            QApplication( int argc, char **argv );
            void setMainWidget( QWidget *widget );
            void exec();
    };

    class QPushButton {
    public:
            QPushButton( char *str, QWidget *w );
            void resize( int x, int y );
            void show();
    };

#### 16.2.3.3 Compiling the example

>     bash-2.05a$ QTPATH=/your/qt/path
>     bash-2.05a$ swig -ocaml -c++ -I$QTPATH/include  qt.i
>     bash-2.05a$ mv qt_wrap.cxx qt_wrap.c
>     bash-2.05a$ ocamlc -c -ccopt -xc++ -ccopt -g -g -ccopt -I$QTPATH/include qt_wrap.c
>     bash-2.05a$ ocamlc -c qt.mli
>     bash-2.05a$ ocamlc -c qt.ml
>     bash-2.05a$ ocamlmktop -custom qt_wrap.o qt.cmo -o qt_top -cclib -L$QTPATH/lib -cclib -lqt

#### 16.2.3.4 Sample Session

>     bash-2.05a$ ./qt_top
>             Objective Caml version 3.06

>     # open Qt ;;
>     # let a = new_QApplication (C_list [ C_int 0; C_int 0 ]) ;;
>     val a : Qt.c_obj = C_obj <fun>
>     # let hello = new_QPushButton (C_list [ C_string "hi" ; C_int 0 ]) ;;
>     val hello : Qt.c_obj = C_obj <fun>
>     # (invoke hello) "resize" (C_list [ C_int 100 ; C_int 30 ]) ;;
>     - : Qt.c_obj = C_void
>     # (invoke hello) "show" C_void ;;
>     - : Qt.c_obj = C_void
>     # (invoke a) "exec" C_void ;;

Or with the camlp4 module:

>     bash-2.05a$ ./qt_top
>             Objective Caml version 3.06

>     # #load "camlp4o.cma" ;;
>             Camlp4 Parsing version 3.06

>     # #load "./swig.cmo" ;;
>     # open Qt ;;
>     # let a = new_QApplication '(0,0) ;;
>     val a : Qt.c_obj = C_obj <fun>
>     # let hello = new_QPushButton '("hi",0) ;;
>     val hello : Qt.c_obj = C_obj <fun>
>     # hello -> "resize" (100,30) ;;
>     - : Qt.c_obj = C_void
>     # hello -> "show" () ;;
>     - : Qt.c_obj = C_void
>     # a -> "exec" () ;;

In either case, assuming you have a working installation of QT, you will see a window containing the string "hi" in a button.

### 16.2.4 Director Classes

Director classes are classes which allow Ocaml code to override the public methods of a C++ object. This facility allows the user to use C++ libraries that require a derived class to provide application specific functionality in the context of an application or utility framework.

You can turn on director classes by using an optional module argument like this:

    %module(directors="1")

    ...

    // Turn on the director class for a specific class like this:
    %feature("director")
    class foo {
      ...
    };

Because the Ocaml language module treats C++ method calls as calls to a certain function, all you need to do is to define the function that will handle the method calls in terms of the public methods of the object, and any other relevant information. The function `new_derived_object` uses a stub class to call your methods in place of the ones provided by the underlying implemenation. The object you receive is the underlying object, so you are free to call any methods you want from within your derived method. Note that calls to the underlying object do not invoke Ocaml code. You need to handle that yourself.

`new_derived_object` receives your function, the function that creates the underlying object, and any constructor arguments, and provides an object that you can use in any usual way. When C++ code calls one of the object's methods, the object invokes the Ocaml function as if it had been invoked from Ocaml, allowing any method definitions to override the C++ ones.

In this example, I'll examine the objective caml code involved in providing an overloaded class. This example is contained in Examples/ocaml/shapes.

example_prog.ml

    open Example

    ...

    let triangle_class pts ob meth args =
      match meth with
          "cover" ->
            (match args with
                 C_list [ x_arg ; y_arg ] ->
                   C_bool (point_in_triangle
                             pts (get_float x_arg) (get_float y_arg))
               | _ -> raise (Failure "cover needs two double arguments."))
        | _ -> (invoke ob) meth args ;;

    let triangle =
      new_derived_object
        new_shape
        (triangle_class ((0.0,0.0),(0.5,1.0),(1.0,0.0)))
        C_void ;;

    let _ = _draw_shape_coverage (C_list [ triangle ; C_int 60 ; C_int 20 ]) ;;

This is the meat of what you need to do. The actual "class" definition containing the overloaded method is defined in the function triangle_class. This is a lot like the class definitions emitted by SWIG, if you look at example.ml, which is generated when SWIG consumes example.i. Basically, you are given the arguments as a c_obj and the method name as a string, and you must intercept the method you are interested in and provide whatever return value you need. Bear in mind that the underlying C++ code needs the right return type, or an exception will be thrown. This exception will generally be Failure, or NotObject. That having been said, your method will be called whenever the underlying method that it depends on will be called, but only when the __up pointer is not set. Therefore, if you need to rely on other methods from your own object, that you also have overrides for, you'll need to call them yourself, rather than relying on the normal method interface, because the underlying code won't call you back a second time. This mechanism prevents infinite recursion on method calls, and allows the cathall case at the end of the example code to succeed in calling a method of the underlying object.

In the example, the draw_shape_coverage function plots the indicated number of points as either covered (`x`) or uncovered (`` ) between 0 and 1 on the X and Y axes. Your shape implementation can provide any coverage map it likes, as long as it responds to the "cover" method call with a boolean return (the underlying method returns bool). This might allow a tricky shape implementation, such as a boolean combination, to be expressed in a more effortless style in ocaml, while leaving the "engine" part of the program in C++.

The definition of the actual object triangle can be described this way:

    let triangle =
      new_derived_object
        new_shape
        (triangle_class ((0.0,0.0),(0.5,1.0),(1.0,0.0)))
        C_void

The first argument to `new_derived_object`, new_shape is the method which returns a shape instance. This function will be invoked with the third argument will be appended to the argument list [ C_void ]. In the example, the actual argument list is sent as (C_list [ C_void ; C_void ]). The augmented constructor for a director class needs the first argument to determine whether it is being constructed as a derived object, or as an object of the indicated type only (in this case `shape`). The Second argument is a closure that will be added to the final C_obj.

The actual object passed to the self parameter of the director object will be a C_director_core, containing a c_obj option ref and a c_obj. The c_obj provided is the same object that will be returned from new_derived object, that is, the object exposing the overridden methods. The other part is an option ref that will have its value extracted before becoming the `ob` parameter of your class closure. This ref will contain `None` if the C++ object underlying is ever destroyed, and will consequently trigger an exception when any method is called on the object after that point (the actual raise is from an inner function used by new_derived_object, and throws NotObject).

### 16.2.5 Exceptions

Catching exceptions is now supported using SWIG's %exception feature. A simple but not too useful example is provided by the throw_exception testcase in Examples/test-suite. You can provide your own exceptions, too.