SWIG and D

# 24 SWIG and D

- Introduction
- Command line invocation
- Typemaps
  - C# <-> D name comparison
  - ctype, imtype, dtype
  - in, out, directorin, directorout
  - din, dout, ddirectorin, ddirectorout
  - typecheck typemaps
  - Code injection typemaps
  - Special variable macros
- Other D code control features
  - D begin
  - D and %feature
- Pragmas
- D Exceptions
- D Directors
- Other features
  - Extended namespace support (nspace)
  - Native pointer support
  - Operator overloading
  - Running the test-suite
- D Typemap examples
- Work in progress and planned features

## 24.1 Introduction

From the D Programming Language web site: D is a systems programming language. Its focus is on combining the power and high performance of C and C++ with the programmer productivity of modern languages like Ruby and Python. [...] The D language is statically typed and compiles directly to machine code. As such, it is not very surprising that D is able to directly interface with C libraries. Why would a SWIG module for D be needed then in the first place?

Well, besides the obvious downside that the C header files have to be manually converted to D modules for this to work, there is one major inconvenience with this approach: D code usually is on a higher abstraction level than C, and many of the features that make D interesting are simply not available when dealing with C libraries, requiring you e.g. to manually convert strings between pointers to `\0`-terminated char arrays and D char arrays, making the algorithms from the D2 standard library unusable with C arrays and data structures, and so on.

While these issues can be worked around relatively easy by hand-coding a thin wrapper layer around the C library in question, there is another issue where writing wrapper code per hand is not feasible: C++ libraries. Support for C++ was added in D2 via `extern(C++)` but this support is still very limited, and a custom wrapper layer is still required in many cases.

To help addressing these issues, the SWIG C# module has been forked to support D. Is has evolved quite a lot since then, but there are still many similarities, so if you do not find what you are looking for on this page, it might be worth having a look at the chapter on C# (and also on Java, since the C# module was in turn forked from it).

## 24.2 Command line invocation

To activate the D module, pass the `-d` option to SWIG at the command line. The same standard command line options as with any other language module are available, plus the following D specific ones:

`-d2`

  Prior to SWIG 4.2.0, SWIG generated wrappers for D1/Tango by default and `-d2` could be used to generate D2/Phobos wrappers instead. SWIG 4.2.0 dropped support for D1, and D2 wrappers are now produced by default. This option is still recognised to allow existing build systems calling SWIG to work, but is now a no-op.

`-splitproxy`

  By default, SWIG generates two D modules: the proxy module, named like the source module (either specified via the `%module` directive or via the `module` command line option), which contains all the proxy classes, functions, enums, etc., and the intermediary module (named like the proxy module, but suffixed with `_im`), which contains all the `extern(C)` function declarations and other private parts only used internally by the proxy module.

  If the split proxy mode is enabled by passing this option at the command line, all proxy classes and enums are emitted to their own D module instead. The main proxy module only contains free functions and constants in this case.

`-package <pkg>`

  By default, the proxy D modules and the intermediary D module are written to the root package. Using this option, you can specify another target package instead.

`-wrapperlibrary <wl>`

  The code SWIG generates to dynamically load the C/C++ wrapper layer looks for a library called `$module_wrap` by default. With this option, you can override the name of the file the wrapper code loads at runtime (the `lib` prefix and the suffix for shared libraries are appended automatically, depending on the OS).

  This might especially be useful if you want to invoke SWIG several times on separate modules, but compile the resulting code into a single shared library.

## 24.3 Typemaps

### 24.3.1 C# <-> D name comparison

If you already know the SWIG C# module, you might find the following name comparison table useful:

     ctype                  <->  ctype
     imtype                 <->  imtype
     cstype                 <->  dtype
     csin                   <->  din
     csout                  <->  dout
     csdirectorin           <->  ddirectorin
     csdirectorout          <->  ddirectorout
     csinterfaces           <->  dinterfaces
     csinterfaces_derived   <->  dinterfaces_derived
     csbase                 <->  dbase
     csclassmodifiers       <->  dclassmodifiers
     cscode                 <->  dcode
     csimports              <->  dimports
     csbody                 <->  dbody
     csfinalize             <->  ddestructor
     csdisposing            <->  ddispose
     csdisposing_derived    <->  ddispose_derived

### 24.3.2 ctype, imtype, dtype

Mapping of types between the C/C++ library, the C/C++ library wrapper exposing the C functions, the D wrapper module importing these functions and the D proxy code.

The `ctype` typemap is used to determine the types to use in the C wrapper functions. The types from the `imtype` typemap are used in the extern(C) declarations of these functions in the intermediary D module. The `dtype` typemap contains the D types used in the D proxy module/class.

### 24.3.3 in, out, directorin, directorout

Used for converting between the types for C/C++ and D when generating the code for the wrapper functions (on the C++ side).

The code from the `in` typemap is used to convert arguments to the C wrapper function to the type used in the wrapped code (`ctype`->original C++ type), the `out` typemap is utilized to convert values from the wrapped code to wrapper function return types (original C++ type->`ctype`).

The `directorin` typemap is used to convert parameters to the type used in the D director callback function, its return value is processed by `directorout` (see below).

### 24.3.4 din, dout, ddirectorin, ddirectorout

Typemaps for code generation in D proxy and type wrapper classes.

The `din` typemap is used for converting function parameter types from the type used in the proxy module or class to the type used in the intermediary D module (the $dinput macro is replaced). To inject further parameter processing code before or after the call to the intermediary layer, the `pre`, `post` and `terminator` attributes can be used (please refer to the C# date marshalling example for more information on these).

The `dout` typemap is used for converting function return values from the return type used in the intermediary D module to the type returned by the proxy function. The `$excode` special variable in `dout` typemaps is replaced by the `excode` typemap attribute code if the method can throw any exceptions from unmanaged code, otherwise by nothing (the `$imcall` and `$owner` macros are replaced).

The code from the `ddirectorin` and `ddirectorout` typemaps is used for conversion in director callback functions. Arguments are converted to the type used in the proxy class method they are calling by using the code from `ddirectorin`, the proxy class method return value is converted to the type the C++ code expects via the `ddirectorout` typemap (the `$dcall` and `$winput` macros are replaced).

The full chain of type conversions when a director callback is invoked looks like this:

          type              CPPClass::method(type a)
            ↑                       ↓
       <directorout>          <directorin>
            ↑                       ↓
          ctype             methodCallback(ctype a)           C++
     :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
          imtype            methodCallback(imtype a)           D
            ↑                       ↓
      <ddirectorout>          <ddirectorin>
            ↑                       ↓
          dtype             DClass.method(dtype a)

### 24.3.5 typecheck typemaps

Because, unlike many scripting languages supported by SWIG, D does not need any dynamic dispatch helper to access an overloaded function, the purpose of these is merely to issue a warning for overloaded C++ functions that cannot be overloaded in D (as more than one C++ type maps to a single D type).

### 24.3.6 Code injection typemaps

These typemaps are used for generating the skeleton of proxy classes for C++ types.

By overriding `dbase`, `dinterfaces` or `dinterfaces_derived`, the inheritance chain of the generated proxy class for a type can be modified. `dclassmodifiers` allows you to add any custom modifiers around the class keyword.

Using `dcode` and `dimports`, you can specify additional D code which will be emitted into the class body respectively the imports section of the D module the class is written to.

`dconstructor`, `ddestructor`, `ddispose` and `ddispose_derived` are used to generate the class constructor, destructor and `dispose()` method, respectively. The auxiliary code for handling the pointer to the C++ object is stored in `dbody` and `dbody_derived`. You can override them for specific types.

Code can also be injected into the D proxy class using `%proxycode`.

### 24.3.7 Special variable macros

The standard SWIG special variables are available for use within typemaps as described in the Typemaps documentation, for example `$1`, `$input`, `$result` etc.

When generating D wrappers, a few additional macros are available:

`$dclassname` (C#: `$csclassname`)

  This special variable works similar to `$n_type` in that it returns the name of a type - it expands to the D proxy class name of the type being wrapped. If the type does not have an associated proxy class, it expands to the type wrapper class name, for example, `SWIGTYPE_p_p_SomeCppClass` is generated when wrapping `SomeCppClass **`.

  There are two other variants available, `$&dclassname` and `$*dclassname`. The former adds a level of indirection, while the latter removes one. For instance, when wrapping `Foo **`, `$*dclassname` would be replaced by the proxy class name corresponding to `Foo *`.

`$dclazzname` (C#: `$csclazzname`)

  This special variable expands the fully qualified C++ class into the package name, if used by the `nspace` feature, and the proxy class name, mangled for use as a function name. For example, `Namespace1::Namespace2::Klass` is expanded into `Namespace1_Namespace2_Klass_`.

  This special variable might be useful for calling certain functions in the wrapper layer (e.g. upcast wrappers) which are mangled like this.

`$null`

  In code inserted into the generated C/C++ wrapper functions, this variable is replaced by either `0` or nothing at all, depending on whether the function has a return value or not. It can be used to bail out early e.g. in case of errors (`return $null;`).

`$dinput` (C#: `$csinput`)

  This variable is used in `din` typemaps and is replaced by the expression which is to be passed to C/C++.

  For example, this input

      %typemap(din) SomeClass * "SomeClass.getCPointer($dinput)"

      %inline %{
        class SomeClass {};
        void foo(SomeClass *arg);
      %}

  leads to the following D proxy code being generated:

      void foo(SomeClass arg) {
        example_im.foo(SomeClass.getCPointer(arg));
      }

`$imcall` and `$owner` (C#: `$imcall`)

  These variables are used in `dout` typemaps. `$imcall` contains the call to the intermediary module which provides the value to be used, and `$owner` signals if the caller is responsible for managing the object lifetime (that is, if the called method is a constructor or has been marked via `%newobject`).

  Consider the following example:

      %typemap(dout) SomeClass * {
        return new SomeClass($imcall, $owner);
      }

      %inline %{
        class SomeClass;
        SomeClass *foo();

        %newobject bar();
        SomeClass *bar();
      %}

  The code generated for `foo()` and `bar()` looks like this:

      SomeClass foo() {
        return new SomeClass(example_im.foo(), false);
      }

      SomeClass bar() {
        return new SomeClass(example_im.bar(), true);
      }

`$dcall` and `$winput` (C#: `$cscall`, `$iminput`)

  These variables are used in the director-specific typemaps `ddirectorin` and `ddirectorout`. They are more or less the reverse of the `$imcall` and `$dinput` macros: `$dcall` contains the invocation of the D proxy method of which the return value is to be passed back to C++, `$winput` contains the parameter value from C++.

`$excode`

  This variable is used in `dout` and `dconstructor` typemaps and is filled with the contents of the `excode` typemap attribute if an exception could be thrown from the C++ side. See the C# documentation for details.

`$dbaseclass`

  Currently for internal use only, it contains the D name of the C++ base class (if any) inside proxy classes.

`$directorconnect`

  This macro is only valid inside the `dconstructor` typemap and contains the value of the `dconstructor` typemap attribute if the currently wrapped class has directors enabled.

  This is how the default `dconstructor` typemap looks like (you usually do not want to specify a custom one):

      %typemap(dconstructor, excode=SWIGEXCODE,
               directorconnect="\n  swigDirectorConnect();") SWIGTYPE {
        this($imcall, true);$excode$directorconnect
      }

`$imfuncname`

  This special variable expands to the name of the function in the intermediary class that will be used in $imcall. Like, $imcall, this special variable is only expanded in the "dout" typemap.

`$importtype(SomeDType)`

  This macro is used in the `dimports` typemap if a dependency on another D type generated by SWIG is added by a custom typemap.

  Consider the following code snippet:

      %typemap(dinterfaces) SomeClass "AnInterface, AnotherInterface"

  This causes SWIG to add `AnInterface` and `AnotherInterface` to the base class list of `SomeClass`:

      class SomeClass : AnInterface, AnotherInterface {
        ...
      }

  For this to work, `AnInterface` and `AnotherInterface` have to be in scope. If SWIG is not in split proxy mode, this is already the case, but if it is, they have to be added to the import list via the `dimports` typemap. Additionally, the import statement depends on the package SWIG is configured to emit the modules to.

  The `$importtype` macro helps you to elegantly solve this problem:

      %typemap(dimports) RemoteMpe %{
      $importtype(AnInterface)
      $importtype(AnotherInterface)
      %}

  If SWIG is in split proxy mode, it expands to an `import` statement for the specified type, to nothing if not.

`$module`

  Expands to the name of the main proxy D module.

`$imdmodule`

  Contains the fully qualified name of the intermediary D module.

## 24.4 Other D code control features

### 24.4.1 D begin

It is possible to add a common comment at the start of every generated D file. The `%module` directive supports the `dbegin` option for this. The provided text is generated at the very beginning of each generated D file. As it is generated before the D module statement, is only really useful for adding in a common comment into all generated D files. For example, copyright text for each file:

    %module(dbegin="/* Common comment. Copyright (C) 2000 Mr Nobody. */\n") nobodymodule

### 24.4.2 D and %feature

The D module defines a number of directives which modify the SWIG features set globally or for a specific declaration:

`%dmanifestconst` and `%dconstvalue(value)`

  Out of the box, SWIG generates accessor methods for C `#defines` and C++ constants. The `%dmanifestconst` directive enables wrapping these constants as D manifest constants (`enum` in D2).

  For this to work, the C/C++ code for the constant value must directly compile as D code, though. If this is not the case, you can manually override the expression written to the D proxy module using the `%dconstvalue` directive, passing the new value as parameter.

  For `enum`s, again `%dconstvalue` can be used to override the value of an enum item if the initializer should not compile in D.

`%dmethodmodifiers`

  This directive can be used to override the modifiers for a proxy function. For instance, you could make a `public` C++ member function `private` in D like this:

      %dmethodmodifiers A::foo "private";

      %inline %{
      struct A {
        void foo();
      };
      %}

## 24.5 Pragmas

There are a few SWIG pragmas specific to the D module, which you can use to influence the D code SWIG generates:

`%pragma(d) imdmodulecode`

  The passed text (D code) is copied verbatim to the intermediary D module. For example, it can be (and is, internally) used to emit additional private helper code for the use by proxy typemaps.

`%pragma(d) imdmoduleimports`

  Additional code to be emitted to the imports section of the intermediary D module (the $importtype macro can be used here). You probably want to use this in conjunction with the `imdmodulecode` pragma.

`%pragma(d) proxydmodulecode`

  Just like `proxydmodulecode`, the argument is copied to the proxy D module (if SWIG is in split proxy mode and/or the `nspace` feature is used, it is emitted to the main proxy D module only).

`%pragma(d) globalproxyimports`

  The D module currently does not support specifying dependencies on external modules (e.g. from the standard library) for the D typemaps. To add the import statements to the proxy modules (resp. to all proxy modules if in split proxy mode), you can use the `globalproxyimports` directive.

  For example:

      %typemap(din) char[] "($dinput ? tango.stdc.stringz.toStringz($dinput) : null)"
      %pragma(d) globalproxyimports = "static import tango.stdc.stringz;";

`%pragma(d) wrapperloadercode`

  The D code for loading the wrapper library (it is copied to the intermediary D module). The `$wrapperloaderbindcode` variable is replaced by the list of commands for binding the functions from the wrapper library to the symbols in the intermediary D module.

  Each time this pragma is specified, the previous value is overwritten.

`%pragma(d) wrapperloaderbindcommand`

  The D command to use for binding the wrapper functions from the C/C++ library to the symbols in the intermediary D module. The `$function` variable contains the name of the D function in the wrap module, the `$symbol` variable is replaced by the name of the symbol in the library.

  Each time this pragma is specified, the previous value is overwritten.

## 24.6 D Exceptions

Out of the box, C++ exceptions are fundamentally incompatible to their equivalent in the D world and cannot simply be propagated to a calling D method. There is, however, an easy way to solve this problem: Just catch the exception in the C/C++ wrapper layer, pass the contents to D, and make the wrapper code rethrow the exception in the D world.

The implementation details of this are a bit crude, but the SWIG D module automatically takes care of this, as long as it is able to detect that an exception could potentially be thrown (e.g. because the C++ method has a `throw(...)` exception specification).

As this feature is implemented in exactly the same way it is for C#, please see the C# documentation for a more detailed explanation.

## 24.7 D Directors

When the directors feature is activated, SWIG generates extra code on both the C++ and the D side to enable cross-language polymorphism. Essentially, this means that if you subclass a proxy class in D, C++ code can access any overridden virtual methods just as if you created a derived class in C++.

There is no D specific documentation yet, but the way the feature is implemented is very similar to how it is done in Java and C#.

## 24.8 Other features

### 24.8.1 Extended namespace support (nspace)

By default, SWIG flattens all C++ namespaces into a single target language namespace, but as for Java and C#, the `nspace` feature is supported for D. If it is active, C++ namespaces are mapped to D packages/modules. Note, however, that like for the other languages, free variables and functions are not supported yet; currently, they are all allows written to the main proxy D module.

### 24.8.2 Native pointer support

Contrary to many of the scripting languages supported by SWIG, D fully supports C-style pointers. The D module thus includes a custom mechanism to wrap C pointers directly as D pointers where applicable, that is, if the type that is pointed to is represented the same in C and D (on the bit-level), dubbed a primitive type below.

Central to this custom pointer handling scheme are two typemap attributes: the `cprimitive` attribute on the `dtype` typemap and the `nativepointer` attribute on all the typemaps which influence the D side of the code (`dtype`, `din`, `dout`, ...). When a D typemap is looked up, the following happens behind the scenes:

First, the matching typemap is determined by the usual typemap lookup rules. Then, it is checked if the result has the `nativepointer` attribute set. If it is present, it means that its value should replace the typemap value if and only if the actual type the typemap is looked up for is a primitive type, a pointer to a primitive type (through an arbitrary level of indirections), or a function pointer with only primitive types in its signature.

To determine if a type should be considered primitive, the `cprimitive` attribute on its `dtype` attribute is used. For example, the `dtype` typemap for `float` has `cprimitive="1"`, so the code from the `nativepointer` attribute is taken into account e.g. for `float **` or the function pointer `float (*)(float *)`.

### 24.8.3 Operator overloading

The D module comes with basic operator overloading support. There are, however, a few limitations arising from conceptual differences between C++ and D:

The first key difference is that C++ supports free functions as operators (along with argument-dependent lookup), while D requires operators to be member functions of the class they are operating on. SWIG can only automatically generate wrapping code for member function operators; if you want to use operators defined as free functions in D, you need to handle them manually.

Another set of differences between C++ and D concerns individual operators. For example, there are quite a few operators which are overloadable in C++, but not in D, for example `&&` and `||`, but also `!`, and postfix increment/decrement operators (see the D documentation for details).

There are also some cases where the operators can be translated to D, but the differences in the implementation details are big enough that a rather involved scheme would be required for automatic wrapping them, which has not been implemented yet. This affects, for example, the array subscript operator, `[]`, in combination with assignments - while `operator []` in C++ simply returns a reference which is then written to, D resorts to a separate `opIndexAssign` method -, or implicit casting (which was introduced in D2 via `alias this`). Despite the lack of automatic support, manually handling these cases should be perfectly possible.

### 24.8.4 Running the test-suite

As with any other language, the SWIG test-suite can be built for D using the `*-d-test-suite` targets of the top-level Makefile.

Note: If you want to use GDC on Linux or another platform which requires you to link `libdl` for dynamically loading the shared library, you might have to add `-ldl` manually to the `d_compile` target in `Examples/Makefile`, because GDC does not currently honor the `pragma(lib, ...)` statement.

## 24.9 D Typemap examples

There are no D-specific typemap examples yet. However, with the above name comparison table, you should be able to get an idea what can be done by looking at the corresponding C# section.

## 24.10 Work in progress and planned features

There are a couple of features which are not implemented yet, but would be very useful and might be added in the near future:

- Static linking: Currently, the C wrapper code is compiled into a dynamic library, out of which the symbol addresses are looked up at runtime by the D part. If statically linking the different languages into one binary was supported, a tool-chain capable of performing IPO at link time could inline the wrapping code, effectively reducing the overhead for simple calls to zero.
- C array handling: Many data structures in some C/C++ libraries contain array containing of a pointer to the first element and the element count. Currently, one must manually writing wrapper code to be able to access these from D. It should be possible to add a set of SWIG macros to semi-automatically generate conversion code.

Some generated code might also be a bit rough around the edges, particularly in the following areas:

- Memory management: Although the currently generated wrapper code works fine with regard to the GC for the test-suite, there might be issues coming up in real-world multi-threaded usage.
- D2 support: Originally, the module has been developed for the use with D1, D2/Phobos support has been added in later. The basic features should work equally well for both, but there could be issues concerning const-correctness etc.