# 11 Preprocessing

- File inclusion
- File imports
- Conditional Compilation
- Macro Expansion
- SWIG Macros
- Variadic Macros
- Preprocessing and delimiters
  - Preprocessing and %{ ... %} & " ... " delimiters
  - Preprocessing and { ... } delimiters
- Preprocessor and Typemaps
- Viewing preprocessor output
- The #error and #warning directives
- Trigraphs
- Digraphs

SWIG includes its own enhanced version of the C preprocessor. The preprocessor supports the standard preprocessor directives and macro expansion rules. However, a number of modifications and enhancements have been made. This chapter describes some of these modifications.

## 11.1 File inclusion

To include another file into a SWIG interface, use the `%include` directive like this:

    %include "cpointer.i"

Unlike, `#include`, `%include` includes each file once (and will not reload the file on subsequent `%include` declarations). Therefore, it is not necessary to use include-guards in SWIG interfaces.

By default, the `#include` is ignored unless you run SWIG with the `-includeall` option. The reason for ignoring traditional includes is that you often don't want SWIG to try and wrap everything included in standard header system headers and auxiliary files.

## 11.2 File imports

SWIG provides another file inclusion directive with the `%import` directive. For example:

    %import "foo.i"

The purpose of `%import` is to collect certain information from another SWIG interface file or a header file without actually generating any wrapper code. Such information generally includes type declarations (e.g., `typedef`) as well as C++ classes that might be used as base-classes for class declarations in the interface. The use of `%import` is also important when SWIG is used to generate extensions as a collection of related modules. This is an advanced topic and is described in later in the Working with Modules chapter.

The `-importall` directive tells SWIG to follow all `#include` statements as imports. This might be useful if you want to extract type definitions from system header files without generating any wrappers.

## 11.3 Conditional Compilation

SWIG fully supports the use of `#if`, `#ifdef`, `#ifndef`, `#else`, `#endif` to conditionally include parts of an interface.

SWIG's preprocessor conditionals support the standard C/C++ preprocessor integer expressions. As a SWIG-specific extension, string equality and inequality tests are also supported, for example:

    #if defined __cplusplus && (#__VA_ARGS__ != "" || #TYPE == "void")

The following symbols are predefined by SWIG when it is parsing the interface:

    SWIG                            Always defined when SWIG is processing a file
    SWIGIMPORTED                    Defined when SWIG is importing a file with %import
    SWIG_VERSION                    Hexadecimal (binary-coded decimal) number containing SWIG version,
                                    such as 0x010311 (corresponding to SWIG-1.3.11).

    SWIGCSHARP                      Defined when using C#
    SWIGD                           Defined when using D
    SWIGGO                          Defined when using Go
    SWIGGUILE                       Defined when using Guile
    SWIGJAVA                        Defined when using Java
    SWIGJAVASCRIPT                  Defined when using Javascript
    SWIG_JAVASCRIPT_JSC             Defined when using Javascript with -jsc
    SWIG_JAVASCRIPT_V8              Defined when using Javascript with -v8 or -node
    SWIG_JAVASCRIPT_NAPI            Defined when using Javascript with -napi
    SWIGLUA                         Defined when using Lua
    SWIGOCAML                       Defined when using OCaml
    SWIGOCTAVE                      Defined when using Octave
    SWIGPERL                        Defined when using Perl
    SWIGPHP                         Defined when using PHP (any version)
    SWIGPHP7                        Defined when using PHP 7 or later (with a compatible C API)
    SWIGPYTHON                      Defined when using Python
    SWIGR                           Defined when using R
    SWIGRUBY                        Defined when using Ruby
    SWIGSCILAB                      Defined when using Scilab
    SWIGTCL                         Defined when using Tcl
    SWIGXML                         Defined when using XML

SWIG also defines `SWIG_VERSION` and a target language macro in the generated wrapper file (since SWIG 4.1.0 - in older versions these were defined for some target languages but this wasn't consistent). Best practice is to use SWIG-time conditional checks because that results in smaller generated wrapper sources.

In addition, SWIG defines the following set of standard C/C++ macros:

    __LINE__                        Current line number
    __FILE__                        Current file name
    __STDC__                        Defined to indicate ISO C/C++
    __cplusplus                     Defined when -c++ option used, value controlled by -std=c++NN
    __STDC_VERSION__                May be defined when -c++ option is not used, value controlled by -std=cNN

Since SWIG 4.2.0, `__STDC__` is defined to `1` to match the behaviour of ISO C/C++ compilers. Before this SWIG defined it to have an empty value.

Since SWIG 4.2.0, `__cplusplus` is defined to `199711L` (the value for C++98) by default. Before this SWIG always defined it to have the value `__cplusplus`.

Since SWIG 4.2.0, SWIG supports command line options `-std=cNN` and `-std=c++NN` to specify the C/C++ standards version. The only effect of these options is to set appropriate values for `__STDC_VERSION__` and `__cplusplus` respectively, which is useful if you're wrapping headers which have preprocessor checks based on their values.

If your code requires these macros to be set to a version of the standard that is not a final official version, or one that SWIG is not yet aware of, you can simply redefine the appropriate macro to an alternative value at the top of your interface file, for example:

    #undef __cplusplus
    #define __cplusplus 202211L

The following are language specific symbols that might be defined:

    SWIG_D_VERSION                  Unsigned integer target version when using D
    SWIGGO_CGO                      Defined when using Go for cgo
    SWIGGO_GCCGO                    Defined when using Go for gccgo
    SWIGGO_INTGO_SIZE               Size of the Go type int when using Go (32 or 64)
    SWIGPYTHON_BUILTIN              Defined when using Python with -builtin
    SWIGPYTHON_NOGIL                Defined when using Python with -nogil
    SWIG_RUBY_AUTORENAME            Defined when using Ruby with -autorename

Interface files can look at these symbols as necessary to change the way in which an interface is generated or to mix SWIG directives with C code.

## 11.4 Macro Expansion

Traditional preprocessor macros can be used in SWIG interfaces. Be aware that the `#define` statement is also used to try and detect constants. Therefore, if you have something like this in your file,

    #ifndef FOO_H 1
    #define FOO_H 1
    ...
    #endif

you may get some extra constants such as `FOO_H` showing up in the scripting interface.

More complex macros can be defined in the standard way. For example:

    #define EXTERN extern
    #ifdef __STDC__
    #define ISOC_(args)   (args)
    #else
    #define ISOC_(args) ()
    #endif

The following operators can appear in macro definitions:

- `#x`
  Converts macro argument `x` to a string surrounded by double quotes ("x").
- `x ## y`
  Concatenates x and y together to form `xy`.
- ``x``
  If `x` is a string surrounded by double quotes, do nothing. Otherwise, turn into a string like `#x`. This is a non-standard SWIG extension.

## 11.5 SWIG Macros

SWIG provides an enhanced macro capability with the `%define` and `%enddef` directives. For example:

    %define ARRAYHELPER(type, name)
    %inline %{
    type *new_ ## name (int nitems) {
      return (type *) malloc(sizeof(type)*nitems);
    }
    void delete_ ## name(type *t) {
      free(t);
    }
    type name ## _get(type *t, int index) {
      return t[index];
    }
    void name ## _set(type *t, int index, type val) {
      t[index] = val;
    }
    %}
    %enddef

    ARRAYHELPER(int, IntArray)
    ARRAYHELPER(double, DoubleArray)

The primary purpose of `%define` is to define large macros of code. Unlike normal C preprocessor macros, it is not necessary to terminate each line with a continuation character (\)--the macro definition extends to the first occurrence of `%enddef`. Furthermore, when such macros are expanded, they are reparsed through the C preprocessor. Thus, SWIG macros can contain all other preprocessor directives except for nested `%define` statements.

The SWIG macro capability is a very quick and easy way to generate large amounts of code. In fact, many of SWIG's advanced features and libraries are built using this mechanism (such as C++ template support).

## 11.6 Variadic Macros

SWIG-1.3.12 and newer releases support variadic preprocessor macros which were standardised by C99 and C++11. For example:

    #define DEBUGF(fmt, ...)   fprintf(stderr, fmt, __VA_ARGS__)

When used, any extra arguments to `...` are placed into the special variable `__VA_ARGS__`. This also works with special SWIG macros defined using `%define`.

The variable arguments can be empty. However, this often results in an extra comma (`,`) and syntax error in the resulting expansion. For example:

    DEBUGF("hello");   --> fprintf(stderr, "hello", );

C++20 and C23 added `__VA_OPT__()` as a solution to this, which SWIG 4.3.0 added support for. `__VA_OPT__()` expands to its argument if the variable arguments contain any tokens, and to nothing otherwise. It can be used to solve the problem above like so:

    #define DEBUGF(fmt, ...)   fprintf(stderr, fmt __VA_OPT__(,) __VA_ARGS__)

An early non-standardised solution to this problem which gave a special meaning to the token sequence `, ## __VA_ARGS__` is supported by several C and C++ compilers, and also by SWIG 4.3.0 and later (it was documented as supported by earlier SWIG versions, but didn't actually work in at least SWIG 2.x and 3.x). Using this feature you can get rid of the extra comma like this:

    #define DEBUGF(fmt, ...)   fprintf(stderr, fmt, ##__VA_ARGS__)

SWIG also supports GNU-style variadic macros, which specify a name for the variable arguments instead of using `__VA_ARGS__`. For example:

    #define DEBUGF(fmt, args...)  fprintf(stdout, fmt, args)

SWIG supports `__VA_OPT__()` in combination with GNU-style variadic macros (following the lead of GCC and clang which also support this, albeit with a warning by default).

## 11.7 Preprocessing and delimiters

The preprocessor handles { }, " " and %{ %} delimiters differently.

### 11.7.1 Preprocessing and %{ ... %} & " ... " delimiters

The SWIG preprocessor does not process any text enclosed in a code block `%{` ... `%}` or in double quotes `"` ... `"`. Therefore, if you write code like this,

    %{
    #ifdef NEED_BLAH
    int blah() {
      ...
    }
    #endif
    %}

the contents of the `%{ ... %}` block are copied without modification to the output (including all preprocessor directives).

### 11.7.2 Preprocessing and { ... } delimiters

SWIG always runs the preprocessor on text appearing inside `{ ... }`. However, sometimes it is desirable to make a preprocessor directive pass through to the output file. For example:

    %extend Foo {
      void bar() {
        #ifdef DEBUG
          printf("I'm in bar\n");
        #endif
      }
    }

By default, SWIG will interpret the `#ifdef DEBUG` statement. However, if you really wanted that code to actually go into the wrapper file, prefix the preprocessor directives with `%` like this:

    %extend Foo {
      void bar() {
        %#ifdef DEBUG
          printf("I'm in bar\n");
        %#endif
      }
    }

SWIG will strip the extra `%` and leave the preprocessor directive in the code.

## 11.8 Preprocessor and Typemaps

Typemaps support a special attribute called `noblock` where the { ... } delimiters can be used, but the delimiters are not actually generated into the code. The effect is then similar to using "" or %{ %} delimiters but the code is run through the preprocessor. For example:

    #define SWIG_macro(CAST) (CAST)$input
    %typemap(in) Int {$1= SWIG_macro(int);}

might generate

      {
        arg1=(int)jarg1;
      }

whereas

    #define SWIG_macro(CAST) (CAST)$input
    %typemap(in, noblock=1) Int {$1= SWIG_macro(int);}

might generate

      arg1=(int)jarg1;

and

    #define SWIG_macro(CAST) (CAST)$input
    %typemap(in) Int %{$1=SWIG_macro(int);%}

would generate

      arg1=SWIG_macro(int);

## 11.9 Viewing preprocessor output

Like many compilers, SWIG supports a `-E` command line option to display the output from the preprocessor. When the `-E` option is used, SWIG will not generate any wrappers. Instead the results after the preprocessor has run are displayed. This might be useful as an aid to debugging and viewing the results of macro expansions.

## 11.10 The #error and #warning directives

SWIG supports the standard `#warning` and `#error` preprocessor directives. The `#warning` directive will cause SWIG to issue a warning then continue processing. It was standardised in C++23 and C23, and has been widely supported as an extension by most C and C++ compilers for a long time. The `#error` directive will cause SWIG to exit with a fatal error. Example usage:

    #error "This is a fatal error message"
    #warning "This is a warning message"

The `#error` behaviour can be made to work like `#warning` if the `-cpperraswarn` commandline option is used. Alternatively, the `#pragma` directive can be used to the same effect, for example:

      /* Modified behaviour: #error does not cause SWIG to exit with error */
      #pragma SWIG cpperraswarn=1
      /* Normal behaviour: #error does cause SWIG to exit with error */
      #pragma SWIG cpperraswarn=0

## 11.11 Trigraphs

SWIG's preprocessor does not implement trigraphs (such as `??!` being mapped to `|`). They are very rarely used deliberately but these character sequences sometimes occur in code where they aren't intended as trigraphs. Compilers typically don't enable trigraph support by default, and they've been removed in C++17 and C23.

## 11.12 Digraphs

SWIG's preprocessor does not currently implement digraphs (such as `<%` being an alternative way to write the token `{`). These are standard in C++ and C95, but they're intended to support working with code on systems with very restricted character sets which are really rare these days so digraphs just don't seem to be used in practice.