11 Customization Features

In many cases, it is desirable to change the default wrapping of particular declarations in an interface. For example, you might want to provide hooks for catching C++ exceptions, add assertions, or provide hints to the underlying code generator. This chapter describes some of these customization techniques. First, a discussion of exception handling is presented. Then, a more general-purpose customization mechanism known as "features" is described.

11.1 Exception handling with %exception

The %exception directive allows you to define a general purpose exception handler. For example, you can specify the following:

%exception {
    try {
        $action
    }
    catch (RangeError) {
        ... handle error ...
    }
}

How the exception is handled depends on the target language, for example, Python:

%exception {
    try {
        $action
    }
    catch (RangeError) {
        PyErr_SetString(PyExc_IndexError,"index out-of-bounds");
        SWIG_fail;
    }
}

When defined, the code enclosed in braces is inserted directly into the low-level wrapper functions. The special variable $action is one of a few %exception special variable supported and gets replaced with the actual operation to be performed (a function call, method invocation, attribute access, etc.). An exception handler remains in effect until it is explicitly deleted. This is done by using either %exception or %noexception with no code. For example:

%exception;   // Deletes any previously defined handler

Compatibility note: Previous versions of SWIG used a special directive %except for exception handling. That directive is deprecated--%exception provides the same functionality, but is substantially more flexible.

11.1.1 Handling exceptions in C code

C has no formal exception handling mechanism so there are several approaches that might be used. A somewhat common technique is to simply set a special error code. For example:

/* File : except.c */

static char error_message[256];
static int error_status = 0;

void throw_exception(char *msg) {
	strncpy(error_message,msg,256);
	error_status = 1;
}

void clear_exception() {
	error_status = 0;
}
char *check_exception() {
	if (error_status) return error_message;
	else return NULL;
}

To use these functions, functions simply call throw_exception() to indicate an error occurred. For example :

double inv(double x) {
	if (x != 0) return 1.0/x;
	else {
		throw_exception("Division by zero");
		return 0;
	}
}

To catch the exception, you can write a simple exception handler such as the following (shown for Perl5) :

%exception {
    char *err;
    clear_exception();
    $action
    if ((err = check_exception())) {
       croak(err);
    }
}

In this case, when an error occurs, it is translated into a Perl error. Each target language has its own approach to creating a runtime error/exception in and for Perl it is the croak method shown above.

11.1.2 Exception handling with longjmp()

Exception handling can also be added to C code using the <setjmp.h> library. Here is a minimalistic implementation that relies on the C preprocessor :

/* File : except.c
   Just the declaration of a few global variables we're going to use */

#include <setjmp.h>
jmp_buf exception_buffer;
int exception_status;

/* File : except.h */
#include <setjmp.h>
extern jmp_buf exception_buffer;
extern int exception_status;

#define try if ((exception_status = setjmp(exception_buffer)) == 0)
#define catch(val) else if (exception_status == val)
#define throw(val) longjmp(exception_buffer,val)
#define finally else

/* Exception codes */

#define RangeError     1
#define DivisionByZero 2
#define OutOfMemory    3

Now, within a C program, you can do the following :

double inv(double x) {
	if (x) return 1.0/x;
	else throw(DivisionByZero);
}

Finally, to create a SWIG exception handler, write the following :

%{
#include "except.h"
%}

%exception {
	try {
		$action
	} catch(RangeError) {
		croak("Range Error");
	} catch(DivisionByZero) {
		croak("Division by zero");
	} catch(OutOfMemory) {
		croak("Out of memory");
	} finally {
		croak("Unknown exception");
	}
}

Note: This implementation is only intended to illustrate the general idea. To make it work better, you'll need to modify it to handle nested try declarations.

11.1.3 Handling C++ exceptions

Handling C++ exceptions is also straightforward. For example:

%exception {
	try {
		$action
	} catch(RangeError) {
		croak("Range Error");
	} catch(DivisionByZero) {
		croak("Division by zero");
	} catch(OutOfMemory) {
		croak("Out of memory");
	} catch(...) {
		croak("Unknown exception");
	}
}

The exception types need to be declared as classes elsewhere, possibly in a header file :

class RangeError {};
class DivisionByZero {};
class OutOfMemory {};

11.1.4 Exception handlers for variables

By default all variables will ignore %exception, so it is effectively turned off for all variables wrappers. This applies to global variables, member variables and static member variables. The approach is certainly a logical one when wrapping variables in C. However, in C++, it is quite possible for an exception to be thrown while the variable is being assigned. To ensure %exception is used when wrapping variables, it needs to be 'turned on' using the %allowexception feature. Note that %allowexception is just a macro for %feature("allowexcept"), that is, it is a feature called "allowexcept". Any variable which has this feature attached to it, will then use the %exception feature, but of course, only if there is a %exception attached to the variable in the first place. The %allowexception feature works like any other feature and so can be used globally or for selective variables.

%allowexception;                // turn on globally
%allowexception Klass::MyVar;   // turn on for a specific variable

%noallowexception Klass::MyVar; // turn off for a specific variable
%noallowexception;              // turn off globally

11.1.5 Defining different exception handlers

By default, the %exception directive creates an exception handler that is used for all wrapper functions that follow it. Unless there is a well-defined (and simple) error handling mechanism in place, defining one universal exception handler may be unwieldy and result in excessive code bloat since the handler is inlined into each wrapper function.

To fix this, you can be more selective about how you use the %exception directive. One approach is to only place it around critical pieces of code. For example:

%exception {
	... your exception handler ...
}
/* Define critical operations that can throw exceptions here */

%exception;

/* Define non-critical operations that don't throw exceptions */

More precise control over exception handling can be obtained by attaching an exception handler to specific declaration name. For example:

%exception allocate {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}

In this case, the exception handler is only attached to declarations named "allocate". This would include both global and member functions. The names supplied to %exception follow the same rules as for %rename described in the section on Ambiguity resolution and renaming. For example, if you wanted to define an exception handler for a specific class, you might write this:

%exception Object::allocate {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}

When a class prefix is supplied, the exception handler is applied to the corresponding declaration in the specified class as well as for identically named functions appearing in derived classes.

%exception can even be used to pinpoint a precise declaration when overloading is used. For example:

%exception Object::allocate(int) {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}

Attaching exceptions to specific declarations is a good way to reduce code bloat. It can also be a useful way to attach exceptions to specific parts of a header file. For example:

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

// Define a few exception handlers for specific declarations
%exception Object::allocate(int) {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}

%exception Object::getitem {
    try {
       $action
    }
    catch (RangeError) {
       croak("Index out of range");
    }
}
...
// Read a raw header file
%include "someheader.h"

Compatibility note: The %exception directive replaces the functionality provided by the deprecated "except" typemap. The typemap would allow exceptions to be thrown in the target language based on the return type of a function and was intended to be a mechanism for pinpointing specific declarations. However, it never really worked that well and the new %exception directive is much better.

11.1.6 Special variables for %exception

The %exception directive supports a few special variables which are placeholders for code substitution. The following table shows the available special variables and details what the special variables are replaced with.

$action The actual operation to be performed (a function call, method invocation, variable access, etc.)
$symname The symbol name used internally by SWIG
$overname The extra mangling used in the symbol name for overloaded method. Expands to nothing if the wrapped method is not overloaded.
$wrapname The language specific wrapper name (usually a C function name exported from the shared object/dll)
$decl The fully qualified C/C++ declaration of the method being wrapped without the return type
$fulldecl The fully qualified C/C++ declaration of the method being wrapped including the return type

The special variables are often used in situations where method calls are logged. Exactly which form of the method call needs logging is up to individual requirements, but the example code below shows all the possible expansions, plus how an exception message could be tailored to show the C++ method declaration:

%exception Special::something {
  log("symname: $symname");
  log("overname: $overname");
  log("wrapname: $wrapname");
  log("decl: $decl");
  log("fulldecl: $fulldecl");
  try {
    $action
  } 
  catch (MemoryError) {
      croak("Out of memory in $decl");
  }
}
void log(const char *message);
struct Special {
  void something(const char *c);
  void something(int i);
};

Below shows the expansions for the 1st of the overloaded something wrapper methods for Perl:

  log("symname: Special_something");
  log("overname: __SWIG_0");
  log("wrapname: _wrap_Special_something__SWIG_0");
  log("decl: Special::something(char const *)");
  log("fulldecl: void Special::something(char const *)");
  try {
    (arg1)->something((char const *)arg2);
  } 
  catch (MemoryError) {
    croak("Out of memory in Special::something(char const *)");
  }

11.1.7 Using The SWIG exception library

The exception.i library file provides support for creating language independent exceptions in your interfaces. To use it, simply put an "%include exception.i" in your interface file. This creates a function SWIG_exception() that can be used to raise common scripting language exceptions in a portable manner. For example :

// Language independent exception handler
%include exception.i       

%exception {
    try {
        $action
    } catch(RangeError) {
        SWIG_exception(SWIG_ValueError, "Range Error");
    } catch(DivisionByZero) {
        SWIG_exception(SWIG_DivisionByZero, "Division by zero");
    } catch(OutOfMemory) {
        SWIG_exception(SWIG_MemoryError, "Out of memory");
    } catch(...) {
        SWIG_exception(SWIG_RuntimeError,"Unknown exception");
    }
}

As arguments, SWIG_exception() takes an error type code (an integer) and an error message string. The currently supported error types are :

SWIG_UnknownError
SWIG_IOError
SWIG_RuntimeError
SWIG_IndexError
SWIG_TypeError
SWIG_DivisionByZero
SWIG_OverflowError
SWIG_SyntaxError
SWIG_ValueError
SWIG_SystemError
SWIG_AttributeError
SWIG_MemoryError
SWIG_NullReferenceError

Since the SWIG_exception() function is defined at the C-level it can be used elsewhere in SWIG. This includes typemaps and helper functions.

11.2 Object ownership and %newobject

A common problem in some applications is managing proper ownership of objects. For example, consider a function like this:

Foo *blah() {
   Foo *f = new Foo();
   return f;
}

If you wrap the function blah(), SWIG has no idea that the return value is a newly allocated object. As a result, the resulting extension module may produce a memory leak (SWIG is conservative and will never delete objects unless it knows for certain that the returned object was newly created).

To fix this, you can provide an extra hint to the code generator using the %newobject directive. For example:

%newobject blah;
Foo *blah();

%newobject works exactly like %rename and %exception. In other words, you can attach it to class members and parameterized declarations as before. For example:

%newobject ::blah();                   // Only applies to global blah
%newobject Object::blah(int,double);   // Only blah(int,double) in Object
%newobject *::copy;                    // Copy method in all classes
...

When %newobject is supplied, many language modules will arrange to take ownership of the return value. This allows the value to be automatically garbage-collected when it is no longer in use. However, this depends entirely on the target language (a language module may also choose to ignore the %newobject directive).

Closely related to %newobject is a special typemap. The "newfree" typemap can be used to deallocate a newly allocated return value. It is only available on methods for which %newobject has been applied and is commonly used to clean-up string results. For example:

%typemap(newfree) char * "free($1);";
...
%newobject strdup;
...
char *strdup(const char *s);

In this case, the result of the function is a string in the target language. Since this string is a copy of the original result, the data returned by strdup() is no longer needed. The "newfree" typemap in the example simply releases this memory.

As a complement to the %newobject, from SWIG 1.3.28, you can use the %delobject directive. For example, if you have two methods, one to create objects and one to destroy them, you can use:

%newobject create_foo;
%delobject destroy_foo;
...
Foo *create_foo();
void destroy_foo(Foo *foo);

or in a member method as:

%delobject Foo::destroy;

class Foo {
public:
  void destroy() { delete this;}

private:
  ~Foo();
};

%delobject instructs SWIG that the first argument passed to the method will be destroyed, and therefore, the target language should not attempt to deallocate it twice. This is similar to use the DISOWN typemap in the first method argument, and in fact, it also depends on the target language on implementing the 'disown' mechanism properly.

Compatibility note: Previous versions of SWIG had a special %new directive. However, unlike %newobject, it only applied to the next declaration. For example:

%new char *strdup(const char *s);

For now this is still supported but is deprecated.

How to shoot yourself in the foot: The %newobject directive is not a declaration modifier like the old %new directive. Don't write code like this:

%newobject
char *strdup(const char *s);

The results might not be what you expect.

11.3 Features and the %feature directive

Both %exception and %newobject are examples of a more general purpose customization mechanism known as "features." A feature is simply a user-definable property that is attached to specific declarations. Features are attached using the %feature directive. For example:

%feature("except") Object::allocate {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}

%feature("new","1") *::copy;

In fact, the %exception and %newobject directives are really nothing more than macros involving %feature:

#define %exception %feature("except")
#define %newobject %feature("new","1")

The name matching rules outlined in the Ambiguity resolution and renaming section applies to all %feature directives. In fact the %rename directive is just a special form of %feature. The matching rules mean that features are very flexible and can be applied with pinpoint accuracy to specific declarations if needed. Additionally, if no declaration name is given, a global feature is said to be defined. This feature is then attached to every declaration that follows. This is how global exception handlers are defined. For example:

/* Define a global exception handler */
%feature("except") {
   try {
     $action
   }
   ...
}

... bunch of declarations ...

The %feature directive can be used with different syntax. The following are all equivalent:

%feature("except") Object::method { $action };
%feature("except") Object::method %{ $action %};
%feature("except") Object::method " $action ";
%feature("except","$action") Object::method;

The syntax in the first variation will generate the { } delimiters used whereas the other variations will not.

11.3.1 Feature attributes

The %feature directive also accepts XML style attributes in the same way that typemaps do. Any number of attributes can be specified. The following is the generic syntax for features:

%feature("name","value", attribute1="AttributeValue1") symbol;
%feature("name", attribute1="AttributeValue1") symbol {value};
%feature("name", attribute1="AttributeValue1") symbol %{value%};
%feature("name", attribute1="AttributeValue1") symbol "value";

More than one attribute can be specified using a comma separated list. The Java module is an example that uses attributes in %feature("except"). The throws attribute specifies the name of a Java class to add to a proxy method's throws clause. In the following example, MyExceptionClass is the name of the Java class for adding to the throws clause.

%feature("except", throws="MyExceptionClass") Object::method { 
   try {
     $action
   } catch (...) {
     ... code to throw a MyExceptionClass Java exception ...
   }
};

Further details can be obtained from the Java exception handling section.

11.3.2 Feature flags

Feature flags are used to enable or disable a particular feature. Feature flags are a common but simple usage of %feature and the feature value should be either 1 to enable or 0 to disable the feature.

%feature("featurename")          // enables feature
%feature("featurename", "1")     // enables feature
%feature("featurename", "x")     // enables feature
%feature("featurename", "0")     // disables feature
%feature("featurename", "")      // clears feature

Actually any value other than zero will enable the feature. Note that if the value is omitted completely, the default value becomes 1, thereby enabling the feature. A feature is cleared by specifying no value, see Clearing features. The %immutable directive described in the Creating read-only variables section, is just a macro for %feature("immutable"), and can be used to demonstrates feature flags:

                                // features are disabled by default
int red;                        // mutable

%feature("immutable");          // global enable
int orange;                     // immutable

%feature("immutable","0");      // global disable
int yellow;                     // mutable

%feature("immutable","1");      // another form of global enable
int green;                      // immutable

%feature("immutable","");       // clears the global feature
int blue;                       // mutable

Note that features are disabled by default and must be explicitly enabled either globally or by specifying a targeted declaration. The above intersperses SWIG directives with C code. Of course you can target features explicitly, so the above could also be rewritten as:

%feature("immutable","1") orange;
%feature("immutable","1") green;
int red;                        // mutable
int orange;                     // immutable
int yellow;                     // mutable
int green;                      // immutable
int blue;                       // mutable

The above approach allows for the C declarations to be separated from the SWIG directives for when the C declarations are parsed from a C header file. The logic above can of course be inverted and rewritten as:

%feature("immutable","1");
%feature("immutable","0") red;
%feature("immutable","0") yellow;
%feature("immutable","0") blue;
int red;                        // mutable
int orange;                     // immutable
int yellow;                     // mutable
int green;                      // immutable
int blue;                       // mutable

As hinted above for %immutable, most feature flags can also be specified via alternative syntax. The alternative syntax is just a macro in the swig.swg Library file. The following shows the alternative syntax for the imaginary featurename feature:

%featurename       // equivalent to %feature("featurename", "1") ie enables feature
%nofeaturename     // equivalent to %feature("featurename", "0") ie disables feature
%clearfeaturename  // equivalent to %feature("featurename", "")  ie clears feature

The concept of clearing features is discussed next.

11.3.3 Clearing features

A feature stays in effect until it is explicitly cleared. A feature is cleared by supplying a %feature directive with no value. For example %feature("name",""). A cleared feature means that any feature exactly matching any previously defined feature is no longer used in the name matching rules. So if a feature is cleared, it might mean that another name matching rule will apply. To clarify, let's consider the except feature again (%exception):

// Define global exception handler
%feature("except") {
    try {
        $action
    } catch (...) {
        croak("Unknown C++ exception");
    }
}

// Define exception handler for all clone methods to log the method calls
%feature("except") *::clone() {
    try {
        logger.info("$action");
        $action
    } catch (...) {
        croak("Unknown C++ exception");
    }
}

... initial set of class declarations with clone methods ...

// clear the previously defined feature
%feature("except","") *::clone();

... final set of class declarations with clone methods ...

In the above scenario, the initial set of clone methods will log all method invocations from the target language. This specific feature is cleared for the final set of clone methods. However, these clone methods will still have an exception handler (without logging) as the next best feature match for them is the global exception handler.

Note that clearing a feature is not always the same as disabling it. Clearing the feature above with %feature("except","") *::clone() is not the same as specifying %feature("except","0") *::clone(). The former will disable the feature for clone methods - the feature is still a better match than the global feature. If on the other hand, no global exception handler had been defined at all, then clearing the feature would be the same as disabling it as no other feature would have matched.

Note that the feature must match exactly for it to be cleared by any previously defined feature. For example the following attempt to clear the initial feature will not work:

%feature("except") clone() { logger.info("$action"); $action }
%feature("except","") *::clone();

but this will:

%feature("except") clone() { logger.info("$action"); $action }
%feature("except","") clone();

SWIG provides macros for disabling and clearing features. Many of these can be found in the swig.swg library file. The typical pattern is to define three macros; one to define the feature itself, one to disable the feature and one to clear the feature. The three macros below show this for the "except" feature:

#define %exception      %feature("except")
#define %noexception    %feature("except","0")
#define %clearexception %feature("except","")

11.3.4 Features and default arguments

SWIG treats methods with default arguments as separate overloaded methods as detailed in the default arguments section. Any %feature targeting a method with default arguments will apply to all the extra overloaded methods that SWIG generates if the default arguments are specified in the feature. If the default arguments are not specified in the feature, then the feature will match that exact wrapper method only and not the extra overloaded methods that SWIG generates. For example:

%feature("except") void hello(int i=0, double d=0.0) { ... }
void hello(int i=0, double d=0.0);

will apply the feature to all three wrapper methods, that is:

void hello(int i, double d);
void hello(int i);
void hello();

If the default arguments are not specified in the feature:

%feature("except") void hello(int i, double d) { ... }
void hello(int i=0, double d=0.0);

then the feature will only apply to this wrapper method:

void hello(int i, double d);

and not these wrapper methods:

void hello(int i);
void hello();

If compactdefaultargs are being used, then the difference between specifying or not specifying default arguments in a feature is not applicable as just one wrapper is generated.

Compatibility note: The different behaviour of features specified with or without default arguments was introduced in SWIG-1.3.23 when the approach to wrapping methods with default arguments was changed.

11.3.5 Feature example

As has been shown earlier, the intended use for the %feature directive is as a highly flexible customization mechanism that can be used to annotate declarations with additional information for use by specific target language modules. Another example is in the Python module. You might use %feature to rewrite proxy/shadow class code as follows:

%module example
%rename(bar_id) bar(int,double);

// Rewrite bar() to allow some nice overloading

%feature("shadow") Foo::bar(int) %{
def bar(*args):
    if len(args) == 3:
         return apply(examplec.Foo_bar_id,args)
    return apply(examplec.Foo_bar,args)
%}
    
class Foo {
public:
    int bar(int x);
    int bar(int x, double y);
}

Further details of %feature usage is described in the documentation for specific language modules.