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<title>Customization Features</title>
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<body bgcolor="#ffffff">
<H1><a name="Customization"></a>11 Customization Features</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Customization_exception">Exception handling with %exception</a>
<ul>
<li><a href="#Customization_nn3">Handling exceptions in C code</a>
<li><a href="#Customization_nn4">Exception handling with longjmp()</a>
<li><a href="#Customization_nn5">Handling C++ exceptions</a>
<li><a href="#Customization_allowexcept">Exception handlers for variables</a>
<li><a href="#Customization_nn6">Defining different exception handlers</a>
<li><a href="#Customization_exception_special_variables">Special variables for %exception</a>
<li><a href="#Customization_nn7">Using The SWIG exception library</a>
</ul>
<li><a href="#Customization_ownership">Object ownership and %newobject</a>
<li><a href="#Customization_features">Features and the %feature directive</a>
<ul>
<li><a href="#Customization_feature_attributes">Feature attributes</a>
<li><a href="#Customization_feature_flags">Feature flags</a>
<li><a href="#Customization_clearing_features">Clearing features</a>
<li><a href="#Customization_features_default_args">Features and default arguments</a>
<li><a href="#Customization_features_example">Feature example</a>
</ul>
</ul>
</div>
<!-- INDEX -->



<p>
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.
</p>

<H2><a name="Customization_exception"></a>11.1 Exception handling with %exception</H2>


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

<div class="code"><pre>
%exception {
    try {
        $action
    }
    catch (RangeError) {
        ... handle error ...
    }
}
</pre></div>

<p>
How the exception is handled depends on the target language, for example, Python:
</p>

<div class="code"><pre>
%exception {
    try {
        $action
    }
    catch (RangeError) {
        PyErr_SetString(PyExc_IndexError,"index out-of-bounds");
        SWIG_fail;
    }
}
</pre></div>

<p>
When defined, the code enclosed in braces is inserted directly into the low-level wrapper
functions.  The special variable <tt>$action</tt> is one of a few
<a href="Customization.html#Customization_exception_special_variables">%exception special variables</a>
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 <tt>%exception</tt> 
or <tt>%noexception</tt> with no code. For example:
</p>

<div class="code"><pre>
%exception;   // Deletes any previously defined handler
</pre></div>

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

<H3><a name="Customization_nn3"></a>11.1.1 Handling exceptions in C code</H3>


<p>
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:
</p>

<div class="code"><pre>
/* 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;
}

</pre></div>

<p>
To use these functions, functions simply call
<tt>throw_exception()</tt> to indicate an error occurred. For example
:</p>

<div class="code"><pre>
double inv(double x) {
	if (x != 0) return 1.0/x;
	else {
		throw_exception("Division by zero");
		return 0;
	}
}

</pre></div>

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

<div class="code"><pre>
%exception {
    char *err;
    clear_exception();
    $action
    if ((err = check_exception())) {
       croak(err);
    }
}
</pre></div>

<p>
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 <tt>croak</tt> method shown above.
</p>

<H3><a name="Customization_nn4"></a>11.1.2 Exception handling with longjmp()</H3>


<p>
Exception handling can also be added to C code using the
<tt>&lt;setjmp.h&gt;</tt> library.  Here is a minimalistic implementation that
relies on the C preprocessor :
</p>

<div class="code"><pre>
/* File : except.c
   Just the declaration of a few global variables we're going to use */

#include &lt;setjmp.h&gt;
jmp_buf exception_buffer;
int exception_status;

/* File : except.h */
#include &lt;setjmp.h&gt;
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

</pre></div>

<p>
Now, within a C program, you can do the following :</p>

<div class="code"><pre>
double inv(double x) {
	if (x) return 1.0/x;
	else throw(DivisionByZero);
}

</pre></div>

<p>
Finally, to create a SWIG exception handler, write the following :</p>

<div class="code"><pre>
%{
#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");
	}
}
</pre></div>

<p>
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 <tt>try</tt> declarations.
</p>

<H3><a name="Customization_nn5"></a>11.1.3 Handling C++ exceptions</H3>


<p>
Handling C++ exceptions is also straightforward.  For example: 
</p>

<div class="code"><pre>
%exception {
	try {
		$action
	} catch(RangeError) {
		croak("Range Error");
	} catch(DivisionByZero) {
		croak("Division by zero");
	} catch(OutOfMemory) {
		croak("Out of memory");
	} catch(...) {
		croak("Unknown exception");
	}
}

</pre></div>

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

<div class="code"><pre>
class RangeError {};
class DivisionByZero {};
class OutOfMemory {};
</pre>
</div>

<H3><a name="Customization_allowexcept"></a>11.1.4 Exception handlers for variables</H3>


<p>
By default all variables will ignore <tt>%exception</tt>, 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 <tt>%exception</tt> is used when wrapping variables, it needs to be 'turned on' using the <tt>%allowexception</tt> feature.
Note that <tt>%allowexception</tt> is just a macro for <tt>%feature("allowexcept")</tt>, that is, it is a feature called "allowexcept".
Any variable which has this feature attached to it, will then use the <tt>%exception</tt> feature, but of course,
only if there is a <tt>%exception</tt> attached to the variable in the first place.
The <tt>%allowexception</tt> feature works like any other feature and so can be used globally or for selective variables.
</p>

<div class="code">
<pre>
%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
</pre>
</div>

<H3><a name="Customization_nn6"></a>11.1.5 Defining different exception handlers</H3>


<p>
By default, the <tt>%exception</tt> 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.
</p>

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

<div class="code"><pre>
%exception {
	... your exception handler ...
}
/* Define critical operations that can throw exceptions here */

%exception;

/* Define non-critical operations that don't throw exceptions */
</pre></div>

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

<div class="code">
<pre>
%exception allocate {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}
</pre>
</div>

<p>
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 <tt>%exception</tt> follow the same
rules as for <tt>%rename</tt> described in the section on 
<a href="SWIGPlus.html#SWIGPlus_ambiguity_resolution_renaming">Ambiguity resolution and renaming</a>.
For example, if you wanted to define
an exception handler for a specific class, you might write this:
</p>

<div class="code">
<pre>
%exception Object::allocate {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}
</pre>
</div>

<p>
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.  
</p>

<p>
<tt>%exception</tt> can even be used to pinpoint a precise declaration when overloading is used. For example:
</p>

<div class="code">
<pre>
%exception Object::allocate(int) {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}
</pre>
</div>

<p>
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:
</p>

<div class="code">
<pre>
%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"
</pre>
</div>

<p>
<b>Compatibility note:</b> The <tt>%exception</tt> 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.
</p>

<H3><a name="Customization_exception_special_variables"></a>11.1.6 Special variables for %exception</H3>


<p>
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.
</p>

<table summary="Special variables for %exception">

<tr>
<td>$action</td>
<td>The actual operation to be performed (a function call, method invocation, variable access, etc.)</td>
</tr>

<tr>
<td>$symname</td>
<td>The symbol name used internally by SWIG</td>
</tr>

<tr>
<td>$overname</td>
<td>The extra mangling used in the symbol name for overloaded method. Expands to nothing if the wrapped method is not overloaded.</td>
</tr>

<tr>
<td>$wrapname</td>
<td>The language specific wrapper name (usually a C function name exported from the shared object/dll)</td>
</tr>

<tr>
<td>$decl</td>
<td>The fully qualified C/C++ declaration of the method being wrapped without the return type</td>
</tr>

<tr>
<td>$fulldecl</td>
<td>The fully qualified C/C++ declaration of the method being wrapped including the return type</td>
</tr>

</table>

<p>
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:
</p>

<div class="code"><pre>
%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);
};
</pre></div>

<p>
Below shows the expansions for the 1st of the overloaded <tt>something</tt> wrapper methods for Perl:
</p>

<div class="code"><pre>
  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)-&gt;something((char const *)arg2);
  } 
  catch (MemoryError) {
    croak("Out of memory in Special::something(char const *)");
  }
</pre></div>


<H3><a name="Customization_nn7"></a>11.1.7 Using The SWIG exception library</H3>


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

<div class="code"><pre>
// 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");
    }
}

</pre></div>

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

<div class="diagram"><pre>
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
</pre></div>

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

<H2><a name="Customization_ownership"></a>11.2 Object ownership and %newobject</H2>


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

<div class="code">
<pre>
Foo *blah() {
   Foo *f = new Foo();
   return f;
}
</pre>
</div>

<p>
If you wrap the function <tt>blah()</tt>, 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).
</p>

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

<div class="code">
<pre>
%newobject blah;
Foo *blah();
</pre>
</div>

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

<div class="code">
<pre>
%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
...
</pre>
</div>

<p>
When <tt>%newobject</tt> 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 <tt>%newobject</tt> directive).
</p>

<p>
Closely related to <tt>%newobject</tt> 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 <tt>%newobject</tt> has been applied and is commonly used to clean-up string
results.  For example:
</p>

<div class="code">
<pre>
%typemap(newfree) char * "free($1);";
...
%newobject strdup;
...
char *strdup(const char *s);
</pre>
</div>

<p>
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 <tt>strdup()</tt> is no longer needed.  
The "newfree" typemap in the example simply releases this memory.
</p>

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

<div class="code">
<pre>
%newobject create_foo;
%delobject destroy_foo;
...
Foo *create_foo();
void destroy_foo(Foo *foo);
</pre>
</div>

<p> or in a member method as: </p>
<div class="code">
<pre>
%delobject Foo::destroy;

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

private:
  ~Foo();
};
</pre>
</div>

<p>
<tt>%delobject</tt> 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.
</p>

<p>
The use of <tt>%newobject</tt> is also integrated with reference counting and is covered in the
<a href="SWIGPlus.html#SWIGPlus_ref_unref">C++ reference counted objects</a> section.
</p>

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

<div class="code">
<pre>
%new char *strdup(const char *s);
</pre>
</div>

<p>
For now this is still supported but is deprecated.  
</p>

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

<div class="code">
<pre>
%newobject
char *strdup(const char *s);
</pre>
</div>
<p>
The results might not be what you expect.
</p>

<H2><a name="Customization_features"></a>11.3 Features and the %feature directive</H2>


<p>
Both <tt>%exception</tt> and <tt>%newobject</tt> 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 <tt>%feature</tt> directive. For example:
</p>

<div class="code">
<pre>
%feature("except") Object::allocate {
    try {
        $action
    } 
    catch (MemoryError) {
        croak("Out of memory");
    }
}

%feature("new","1") *::copy;
</pre>
</div>

<p>
In fact, the <tt>%exception</tt> and <tt>%newobject</tt> directives are really nothing more than macros 
involving <tt>%feature</tt>:
</p>

<div class="code">
<pre>
#define %exception %feature("except")
#define %newobject %feature("new","1")
</pre>
</div>

<p>
The name matching rules outlined in the <a href="SWIGPlus.html#SWIGPlus_ambiguity_resolution_renaming">Ambiguity resolution and renaming</a>
section applies to all <tt>%feature</tt> directives.
In fact the <tt>%rename</tt> directive is just a special form of <tt>%feature</tt>. 
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 <em>every</em> declaration that follows.  This is how global exception handlers
are defined.  For example:
</p>

<div class="code">
<pre>
/* Define a global exception handler */
%feature("except") {
   try {
     $action
   }
   ...
}

... bunch of declarations ...
</pre>
</div>

<p>
The <tt>%feature</tt> directive can be used with different syntax.
The following are all equivalent:
</p>

<div class="code">
<pre>
%feature("except") Object::method { $action };
%feature("except") Object::method %{ $action %};
%feature("except") Object::method " $action ";
%feature("except","$action") Object::method;
</pre>
</div>

<p>
The syntax in the first variation will generate the <tt>{ }</tt> delimiters used whereas the other variations will not.
</p>

<H3><a name="Customization_feature_attributes"></a>11.3.1 Feature attributes</H3>


<p>
The <tt>%feature</tt> 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:
</p>

<div class="code">
<pre>
%feature("name","value", attribute1="AttributeValue1") symbol;
%feature("name", attribute1="AttributeValue1") symbol {value};
%feature("name", attribute1="AttributeValue1") symbol %{value%};
%feature("name", attribute1="AttributeValue1") symbol "value";
</pre>
</div>

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

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

<p>
Further details can be obtained from the <a href="Java.html#Java_exception_handling">Java exception handling</a> section.
</p>

<H3><a name="Customization_feature_flags"></a>11.3.2 Feature flags</H3>


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

<div class="code">
<pre>
%feature("featurename")          // enables feature
%feature("featurename", "1")     // enables feature
%feature("featurename", "x")     // enables feature
%feature("featurename", "0")     // disables feature
%feature("featurename", "")      // clears feature
</pre>
</div>

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

<div class="code">
<pre>
                                // 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
</pre>
</div>

<p>
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:
</p>

<div class="code">
<pre>
%feature("immutable","1") orange;
%feature("immutable","1") green;
int red;                        // mutable
int orange;                     // immutable
int yellow;                     // mutable
int green;                      // immutable
int blue;                       // mutable
</pre>
</div>

<p>
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:
</p>

<div class="code">
<pre>
%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
</pre>
</div>

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

<div class="code">
<pre>
%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
</pre>
</div>

<p>
The concept of clearing features is discussed next.
</p>

<H3><a name="Customization_clearing_features"></a>11.3.3 Clearing features</H3>


<p>
A feature stays in effect until it is explicitly cleared.  A feature is cleared by
supplying a <tt>%feature</tt> directive with no value.  For example <tt>%feature("name","")</tt>.
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 <tt>except</tt> feature again (<tt>%exception</tt>):
</p>

<div class="code">
<pre>
// 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 ...
</pre>
</div>

<p>
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.
</p>

<p>
Note that clearing a feature is not always the same as disabling it.
Clearing the feature above with <tt>%feature("except","") *::clone()</tt> is not the same as specifying
<tt>%feature("except","0") *::clone()</tt>. 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.
</p>

<p>
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:
</p>

<div class="code">
<pre>
%feature("except") clone() { logger.info("$action"); $action }
%feature("except","") *::clone();
</pre>
</div>

<p>
but this will:
</p>

<div class="code">
<pre>
%feature("except") clone() { logger.info("$action"); $action }
%feature("except","") clone();
</pre>
</div>

<p>
SWIG provides macros for disabling and clearing features. Many of these can be found in the <tt>swig.swg</tt> 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:
</p>

<div class="code">
<pre>
#define %exception      %feature("except")
#define %noexception    %feature("except","0")
#define %clearexception %feature("except","")
</pre>
</div>

<H3><a name="Customization_features_default_args"></a>11.3.4 Features and default arguments</H3>


<p>
SWIG treats methods with default arguments as separate overloaded methods as detailed
in the <a href="SWIGPlus.html#SWIGPlus_default_args">default arguments</a> section.
Any <tt>%feature</tt> 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:
</p>

<div class="code">
<pre>
%feature("except") void hello(int i=0, double d=0.0) { ... }
void hello(int i=0, double d=0.0);
</pre>
</div>

<p>
will apply the feature to all three wrapper methods, that is:
</p>

<div class="code">
<pre>
void hello(int i, double d);
void hello(int i);
void hello();
</pre>
</div>

<p>
If the default arguments are not specified in the feature:
</p>

<div class="code">
<pre>
%feature("except") void hello(int i, double d) { ... }
void hello(int i=0, double d=0.0);
</pre>
</div>

<p>
then the feature will only apply to this wrapper method:
</p>

<div class="code">
<pre>
void hello(int i, double d);
</pre>
</div>

<p>
and not these wrapper methods:
</p>

<div class="code">
<pre>
void hello(int i);
void hello();
</pre>
</div>

<p>
If <a href="SWIGPlus.html#SWIGPlus_default_args">compactdefaultargs</a> 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.
</p>

<p>
<b>Compatibility note:</b> 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.
</p>

<H3><a name="Customization_features_example"></a>11.3.5 Feature example</H3>


<p>
As has been shown earlier, the intended use for the <tt>%feature</tt> 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 <tt>%feature</tt> to rewrite proxy/shadow class code as follows:
</p>

<div class="code">
<pre>
%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);
}
</pre>
</div>

<p>
Further details of <tt>%feature</tt> usage is described in the documentation for specific language modules.
</p>

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