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<H1><a name="Tcl"></a>37 SWIG and Tcl</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Tcl_nn2">Preliminaries</a>
<ul>
<li><a href="#Tcl_nn3">Getting the right header files</a>
<li><a href="#Tcl_nn4">Compiling a dynamic module</a>
<li><a href="#Tcl_nn5">Static linking</a>
<li><a href="#Tcl_nn6">Using your module</a>
<li><a href="#Tcl_nn7">Compilation of C++ extensions</a>
<li><a href="#Tcl_nn8">Compiling for 64-bit platforms</a>
<li><a href="#Tcl_nn9">Setting a package prefix</a>
<li><a href="#Tcl_nn10">Using namespaces</a>
</ul>
<li><a href="#Tcl_nn11">Building Tcl/Tk Extensions under Windows 95/NT</a>
<ul>
<li><a href="#Tcl_nn12">Running SWIG from Developer Studio</a>
<li><a href="#Tcl_nn13">Using NMAKE</a>
</ul>
<li><a href="#Tcl_nn14">A tour of basic C/C++ wrapping</a>
<ul>
<li><a href="#Tcl_nn15">Modules</a>
<li><a href="#Tcl_nn16">Functions</a>
<li><a href="#Tcl_nn17">Global variables</a>
<li><a href="#Tcl_nn18">Constants and enums</a>
<li><a href="#Tcl_nn19">Pointers</a>
<li><a href="#Tcl_nn20">Structures</a>
<li><a href="#Tcl_nn21">C++ classes</a>
<li><a href="#Tcl_nn22">C++ inheritance</a>
<li><a href="#Tcl_nn23">Pointers, references, values, and arrays</a>
<li><a href="#Tcl_nn24">C++ overloaded functions</a>
<li><a href="#Tcl_nn25">C++ operators</a>
<li><a href="#Tcl_nn26">C++ namespaces</a>
<li><a href="#Tcl_nn27">C++ templates</a>
<li><a href="#Tcl_nn28">C++ Smart Pointers</a>
</ul>
<li><a href="#Tcl_nn29">Further details on the Tcl class interface</a>
<ul>
<li><a href="#Tcl_nn30">Proxy classes</a>
<li><a href="#Tcl_nn31">Memory management</a>
</ul>
<li><a href="#Tcl_nn32">Input and output parameters</a>
<li><a href="#Tcl_nn33">Exception handling </a>
<li><a href="#Tcl_nn34">Typemaps</a>
<ul>
<li><a href="#Tcl_nn35">What is a typemap?</a>
<li><a href="#Tcl_nn36">Tcl typemaps</a>
<li><a href="#Tcl_nn37">Typemap variables</a>
<li><a href="#Tcl_nn38">Converting  a Tcl list to a char ** </a>
<li><a href="#Tcl_nn39">Returning values in arguments</a>
<li><a href="#Tcl_nn40">Useful functions</a>
<li><a href="#Tcl_nn41">Standard  typemaps</a>
<li><a href="#Tcl_nn42">Pointer handling</a>
</ul>
<li><a href="#Tcl_nn43">Turning a SWIG module into a Tcl Package.</a>
<li><a href="#Tcl_nn44">Building new kinds of Tcl interfaces (in Tcl)</a>
<ul>
<li><a href="#Tcl_nn45">Proxy classes</a>
</ul>
<li><a href="#Tcl_nn46">Tcl/Tk Stubs</a>
</ul>
</div>
<!-- INDEX -->



<p>
<b>Caution: This chapter is under repair!</b>
</p>

<p>
This chapter discusses SWIG's support of Tcl. SWIG currently requires
Tcl 8.0 or a later release.   Earlier releases of SWIG supported Tcl 7.x, but
this is no longer supported.
</p>

<H2><a name="Tcl_nn2"></a>37.1 Preliminaries</H2>


<p>
To build a Tcl module, run SWIG using the <tt>-tcl</tt> option :
</p>

<div class="code"><pre>
$ swig -tcl example.i
</pre></div>

<p>
If building a C++ extension, add the <tt>-c++</tt> option:
</p>

<div class="code"><pre>
$ swig -c++ -tcl example.i
</pre></div>

<p>
This creates a file <tt>example_wrap.c</tt> or
<tt>example_wrap.cxx</tt> that contains all of the code needed to
build a Tcl extension module.  To finish building the module, you 
need to compile this file and link it with the rest of your program.
</p>

<H3><a name="Tcl_nn3"></a>37.1.1 Getting the right header files</H3>


<p>
In order to compile the wrapper code, the compiler needs the <tt>tcl.h</tt> header file.
This file is usually contained in the directory
</p>

<div class="code"><pre>
/usr/local/include
</pre></div>

<p>
Be aware that some Tcl versions install this header file with a version number attached to it.  If
this is the case, you should probably make a symbolic link so that <tt>tcl.h</tt> points to the correct
header file.
</p>

<H3><a name="Tcl_nn4"></a>37.1.2 Compiling a dynamic module</H3>


<p>
The preferred approach to building an extension module is to compile it into
a shared object file or DLL.   To do this, you will need to compile your program
using commands like this (shown for Linux):
</p>

<div class="code"><pre>
$ swig -tcl example.i
$ gcc -c example.c
$ gcc -c example_wrap.c -I/usr/local/include
$ gcc -shared example.o example_wrap.o -o example.so
</pre></div>

<p>
The exact commands for doing this vary from platform to platform. 
SWIG tries to guess the right options when it is installed.  Therefore, 
you may want to start with one of the examples in the <tt>SWIG/Examples/tcl</tt> 
directory.   If that doesn't work, you will need to read the man-pages for
your compiler and linker to get the right set of options.  You might also
check the <a href="http://www.dabeaz.com/cgi-bin/wiki.pl">SWIG Wiki</a> for
additional information.
</p>

<p>
When linking the module, the name of the output file has to match the name
of the module.  If the name of your SWIG module is "<tt>example</tt>", the
name of the corresponding object file should be
"<tt>example.so</tt>".
The name of the module is specified using the <tt>%module</tt> directive or the 
<tt> -module</tt> command line option.
</p>

<H3><a name="Tcl_nn5"></a>37.1.3 Static linking</H3>


<p>
An alternative approach to dynamic linking is to rebuild the Tcl
interpreter with your extension module added to it.  In the past,
this approach was sometimes necessary due to limitations in dynamic loading
support on certain machines.  However, the situation has improved greatly
over the last few years and you should not consider this approach 
unless there is really no other option.
</p>

<p>
The usual procedure for adding a new module to Tcl involves writing a
special function <tt>Tcl_AppInit()</tt> and using it to initialize the interpreter and
your module.  With SWIG, the <tt>tclsh.i</tt> and <tt>wish.i</tt> library files
can be used to rebuild the <tt>tclsh</tt> and <tt>wish</tt> interpreters respectively.
For example:
</p>

<div class="code"><pre>
%module example

%inline %{
extern int fact(int);
extern int mod(int, int);
extern double My_variable;
%}

%include "tclsh.i"       // Include code for rebuilding tclsh

</pre></div>

<p>
The <tt>tclsh.i</tt> library file includes supporting code that
contains everything needed to rebuild tclsh. To rebuild the interpreter,
you simply do something like this:
</p>

<div class="code"><pre>
$ swig -tcl example.i
$ gcc example.c example_wrap.c \
        -Xlinker -export-dynamic \
        -DHAVE_CONFIG_H -I/usr/local/include/ \
	-L/usr/local/lib -ltcl -lm -ldl \
	-o mytclsh

</pre></div>

<p>
You will need to supply the same libraries that were used to build Tcl the first
time.  This may include system libraries such as <tt>-lsocket</tt>, <tt>-lnsl</tt>,
and <tt>-lpthread</tt>.  If this actually works, the new version of Tcl
should be identical to the default version except that your extension module will be
a built-in part of the interpreter.
</p>

<p>
<b>Comment:</b> In practice, you should probably try to avoid static
linking if possible. Some programmers may be inclined
to use static linking in the interest of getting better performance.
However, the performance gained by static linking tends to be rather
minimal in most situations (and quite frankly not worth the extra
hassle in the opinion of this author). 
</p>

<H3><a name="Tcl_nn6"></a>37.1.4 Using your module</H3>


<p>
To use your module, simply use the Tcl <tt>load</tt> command.  If
all goes well, you will be able to this:
</p>

<div class="code"><pre>
$ tclsh
% load ./example.so
% fact 4
24
%
</pre></div>

<p>
A common error received by first-time users is the following:
</p>

<div class="code">
<pre>
% load ./example.so
couldn't find procedure Example_Init
% 
</pre>
</div>

<p>
This error is almost always caused when the name of the shared object file doesn't
match the name of the module supplied using the SWIG <tt>%module</tt> directive.
Double-check the interface to make sure the module name and the shared object
file match.  Another possible cause of this error is forgetting to link the SWIG-generated
wrapper code with the rest of your application when creating the extension module.
</p>

<p>
Another common error is something similar to the following:
</p>

<div class="code">
<pre>
% load ./example.so
couldn't load file "./example.so": ./example.so: undefined symbol: fact
% 
</pre>
</div>

<p>
This error usually indicates that you forgot to include some object
files or libraries in the linking of the shared library file.  Make
sure you compile both the SWIG wrapper file and your original program
into a shared library file.  Make sure you pass all of the required libraries
to the linker.  
</p>

<p>
Sometimes unresolved symbols occur because a wrapper has been created
for a function that doesn't actually exist in a library.  This usually
occurs when a header file includes a declaration for a function that
was never actually implemented or it was removed from a library
without updating the header file.  To fix this, you can either edit
the SWIG input file to remove the offending declaration or you can use
the <tt>%ignore</tt> directive to ignore the declaration.
</p>

<p>
Finally, suppose that your extension module is linked with another library like this:
</p>

<div class="code">
<pre>
$ gcc -shared example.o example_wrap.o -L/home/beazley/projects/lib -lfoo \
      -o example.so
</pre>
</div>

<p>
If the <tt>foo</tt> library is compiled as a shared library, you might get the following
problem when you try to use your module:
</p>

<div class="code">
<pre>
% load ./example.so
couldn't load file "./example.so": libfoo.so: cannot open shared object file:
No such file or directory
%        
</pre>
</div>

<p>
This error is generated because the dynamic linker can't locate the
<tt>libfoo.so</tt> library.  When shared libraries are loaded, the
system normally only checks a few standard locations such as
<tt>/usr/lib</tt> and <tt>/usr/local/lib</tt>.   To fix this problem,
there are several things you can do.  First, you can recompile your extension
module with extra path information. For example, on Linux you can do this:
</p>

<div class="code">
<pre>
$ gcc -shared example.o example_wrap.o -L/home/beazley/projects/lib -lfoo \
      -Xlinker -rpath /home/beazley/projects/lib \
      -o example.so
</pre>
</div>

<p>
Alternatively, you can set the <tt>LD_LIBRARY_PATH</tt> environment variable to
include the directory with your shared libraries. 
If setting <tt>LD_LIBRARY_PATH</tt>, be aware that setting this variable can introduce
a noticeable performance impact on all other applications that you run.
To set it only for Tcl, you might want to do this instead:
</p>

<div class="code">
<pre>
$ env LD_LIBRARY_PATH=/home/beazley/projects/lib tclsh
</pre>
</div>

<p>
Finally, you can use a command such as <tt>ldconfig</tt> to add additional search paths
to the default system configuration (this requires root access and you will need to read
the man pages). 
</p>

<H3><a name="Tcl_nn7"></a>37.1.5 Compilation of C++ extensions</H3>


<p>
Compilation of C++ extensions has traditionally been a tricky problem.
Since the Tcl interpreter is written in C, you need to take steps to
make sure C++ is properly initialized and that modules are compiled
correctly.
</p>

<p>
On most machines, C++ extension modules should be linked using the C++
compiler.  For example:
</p>

<div class="code"><pre>
% swig -c++ -tcl example.i
% g++ -c example.cxx
% g++ -c example_wrap.cxx -I/usr/local/include
% g++ -shared example.o example_wrap.o -o example.so
</pre></div>

<p>
In addition to this, you may need to include additional library
files to make it work.  For example, if you are using the Sun C++ compiler on
Solaris, you often need to add an extra library <tt>-lCrun</tt> like this:
</p>

<div class="code"><pre>
% swig -c++ -tcl example.i
% CC -c example.cxx
% CC -c example_wrap.cxx -I/usr/local/include
% CC -G example.o example_wrap.o -L/opt/SUNWspro/lib -o example.so -lCrun
</pre></div>

<p>
Of course, the extra libraries to use are completely non-portable---you will 
probably need to do some experimentation.
</p>

<p>
Sometimes people have suggested that it is necessary to relink the
Tcl interpreter using the C++ compiler to make C++ extension modules work.
In the experience of this author, this has never actually appeared to be
necessary.   Relinking the interpreter with C++ really only includes the 
special run-time libraries described above---as long as you link your extension 
modules with these libraries, it should not be necessary to rebuild Tcl.
</p>

<p>
If you aren't entirely sure about the linking of a C++ extension, you
might look at an existing C++ program.  On many Unix machines, the
<tt>ldd</tt> command will list library dependencies.  This should give
you some clues about what you might have to include when you link your
extension module. For example:
</p>

<div class="code">
<pre>
$ ldd swig
        libstdc++-libc6.1-1.so.2 =&gt; /usr/lib/libstdc++-libc6.1-1.so.2 (0x40019000)
        libm.so.6 =&gt; /lib/libm.so.6 (0x4005b000)
        libc.so.6 =&gt; /lib/libc.so.6 (0x40077000)
        /lib/ld-linux.so.2 =&gt; /lib/ld-linux.so.2 (0x40000000)
$
</pre>
</div>

<p>
As a final complication, a major weakness of C++ is that it does not
define any sort of standard for binary linking of libraries.  This
means that C++ code compiled by different compilers will not link
together properly as libraries nor is the memory layout of classes and
data structures implemented in any kind of portable manner.  In a
monolithic C++ program, this problem may be unnoticed.  However, in Tcl, it
is possible for different extension modules to be compiled with
different C++ compilers.  As long as these modules are self-contained,
this probably won't matter.  However, if these modules start sharing data,
you will need to take steps to avoid segmentation faults and other
erratic program behavior.   If working with lots of software components, you
might want to investigate using a more formal standard such as COM.
</p>

<H3><a name="Tcl_nn8"></a>37.1.6 Compiling for 64-bit platforms</H3>


<p>
On platforms that support 64-bit applications (Solaris, Irix, etc.),
special care is required when building extension modules.  On these
machines, 64-bit applications are compiled and linked using a different
set of compiler/linker options.  In addition, it is not generally possible to mix 
32-bit and 64-bit code together in the same application.
</p>

<p>
To utilize 64-bits, the Tcl executable will need to be recompiled
as a 64-bit application.  In addition, all libraries, wrapper code,
and every other part of your application will need to be compiled for
64-bits.  If you plan to use other third-party extension modules, they
will also have to be recompiled as 64-bit extensions.
</p>

<p>
If you are wrapping commercial software for which you have no source
code, you will be forced to use the same linking standard as used by
that software.  This may prevent the use of 64-bit extensions.  It may
also introduce problems on platforms that support more than one
linking standard (e.g., -o32 and -n32 on Irix).
</p>

<H3><a name="Tcl_nn9"></a>37.1.7 Setting a package prefix</H3>


<p>
To avoid namespace problems, you can instruct SWIG to append a package
prefix to all of your functions and variables. This is done using the
-prefix option as follows :
</p>

<div class="code"><pre>
swig -tcl -prefix Foo example.i
</pre></div>

<p>
If you have a function "<tt>bar</tt>" in the SWIG file, the prefix
option will append the prefix to the name when creating a command and
call it "<tt>Foo_bar</tt>".
</p>

<H3><a name="Tcl_nn10"></a>37.1.8 Using namespaces</H3>


<p>
Alternatively, you can have SWIG install your module into a Tcl
namespace by specifying the <tt>-namespace</tt> option :
</p>

<div class="code"><pre>
swig -tcl -namespace example.i
</pre></div>

<p>
By default, the name of the namespace will be the same as the module
name, but you can override it using the <tt>-prefix</tt> option.
</p>

<p>
When the<tt> -namespace</tt> option is used, objects in the module
are always accessed with the namespace name such as <tt>Foo::bar</tt>.
</p>

<H2><a name="Tcl_nn11"></a>37.2 Building Tcl/Tk Extensions under Windows 95/NT</H2>


<p>
Building a SWIG extension to Tcl/Tk under Windows 95/NT is roughly
similar to the process used with Unix.  Normally, you will want to
produce a DLL that can be loaded into tclsh or wish.  This section
covers the process of using SWIG with Microsoft Visual C++.
although the procedure may be similar with other compilers.
</p>

<H3><a name="Tcl_nn12"></a>37.2.1 Running SWIG from Developer Studio</H3>


<p>
If you are developing your application within Microsoft developer
studio, SWIG can be invoked as a custom build option.  The process
roughly follows these steps :
</p>

<ul>
<li>Open up a new workspace and use the AppWizard to select a DLL project.

<li>Add both the SWIG interface file (the .i file), any supporting C
files, and the name of the wrapper file that will be created by SWIG
(ie. <tt>example_wrap.c</tt>).  Note : If using C++, choose a
different suffix for the wrapper file such as
<tt>example_wrap.cxx</tt>. Don't worry if the wrapper file doesn't
exist yet--Developer studio will keep a reference to it around.

<li>Select the SWIG interface file and go to the settings menu.  Under
settings, select the "Custom Build" option.

<li>Enter "SWIG" in the description field.

<li>Enter "<tt>swig -tcl -o $(ProjDir)\$(InputName)_wrap.c
$(InputPath)</tt>" in the "Build command(s) field"

<li>Enter "<tt>$(ProjDir)\$(InputName)_wrap.c</tt>" in the "Output files(s) field".

<li>Next, select the settings for the entire project and go to
"C++:Preprocessor". Add the include directories for your Tcl
installation under "Additional include directories".

<li>Finally, select the settings for the entire project and go to
"Link Options".  Add the Tcl library file to your link libraries.  For
example "<tt>tcl80.lib</tt>".  Also, set the name of the output file
to match the name of your Tcl module (ie. example.dll).

<li>Build your project.
</ul>

<p>
Now, assuming all went well, SWIG will be automatically invoked when
you build your project.  Any changes made to the interface file will
result in SWIG being automatically invoked to produce a new version of
the wrapper file.  To run your new Tcl extension, simply run
<tt>tclsh</tt> or <tt>wish</tt> and use the <tt>load</tt> command.
For example :
</p>

<div class="code"><pre>
MSDOS &gt; tclsh80
% load example.dll
% fact 4
24
%
</pre></div>

<H3><a name="Tcl_nn13"></a>37.2.2 Using NMAKE</H3>


<p>
Alternatively, SWIG extensions can be built by writing a Makefile for
NMAKE.  To do this, make sure the environment variables for MSVC++ are
available and the MSVC++ tools are in your path.  Now, just write a
short Makefile like this :
</p>

<div class="code"><pre>
# Makefile for building various SWIG generated extensions

SRCS          = example.c
IFILE         = example
INTERFACE     = $(IFILE).i
WRAPFILE      = $(IFILE)_wrap.c

# Location of the Visual C++ tools (32 bit assumed)

TOOLS         = c:\msdev
TARGET        = example.dll
CC            = $(TOOLS)\bin\cl.exe
LINK          = $(TOOLS)\bin\link.exe
INCLUDE32     = -I$(TOOLS)\include
MACHINE       = IX86

# C Library needed to build a DLL

DLLIBC        = msvcrt.lib oldnames.lib  

# Windows libraries that are apparently needed
WINLIB        = kernel32.lib advapi32.lib user32.lib gdi32.lib comdlg32.lib 
winspool.lib

# Libraries common to all DLLs
LIBS          = $(DLLIBC) $(WINLIB) 

# Linker options
LOPT      = -debug:full -debugtype:cv /NODEFAULTLIB /RELEASE /NOLOGO /
MACHINE:$(MACHINE) -entry:_DllMainCRTStartup@12 -dll

# C compiler flags

CFLAGS    = /Z7 /Od /c /nologo
TCL_INCLUDES  = -Id:\tcl8.0a2\generic -Id:\tcl8.0a2\win
TCLLIB        = d:\tcl8.0a2\win\tcl80.lib

tcl::
	..\..\swig -tcl -o $(WRAPFILE) $(INTERFACE)
	$(CC) $(CFLAGS) $(TCL_INCLUDES) $(SRCS) $(WRAPFILE)
	set LIB=$(TOOLS)\lib
	$(LINK) $(LOPT) -out:example.dll $(LIBS) $(TCLLIB) example.obj example_wrap.obj

</pre></div>

<p>
To build the extension, run NMAKE (you may need to run vcvars32
first).  This is a pretty minimal Makefile, but hopefully its enough
to get you started.  With a little practice, you'll be making lots of
Tcl extensions.
</p>

<H2><a name="Tcl_nn14"></a>37.3 A tour of basic C/C++ wrapping</H2>


<p>
By default, SWIG tries to build a very natural Tcl interface to your
C/C++ code.  Functions are wrapped as functions, classes are wrapped
in an interface that mimics the style of Tk widgets and [incr Tcl]
classes.  This section briefly covers the essential aspects of this
wrapping.
</p>

<H3><a name="Tcl_nn15"></a>37.3.1 Modules</H3>


<p>
The SWIG <tt>%module</tt> directive specifies the name of the Tcl
module. If you specify `<tt>%module example</tt>', then everything is
compiled into an extension module <tt>example.so</tt>. When choosing a
module name, make sure you don't use the same name as a built-in
Tcl command.
</p>

<p>
One pitfall to watch out for is module names involving numbers.  If
you specify a module name like <tt>%module md5</tt>, you'll find that the
load command no longer seems to work:
</p>

<div class="code">
<pre>
% load ./md5.so
couldn't find procedure Md_Init
</pre>
</div>

<p>
To fix this, supply an extra argument to <tt>load</tt> like this:
</p>

<div class="code">
<pre>
% load ./md5.so md5
</pre>
</div>

<H3><a name="Tcl_nn16"></a>37.3.2 Functions</H3>


<p>
Global functions are wrapped as new Tcl built-in commands.  For example,
</p>

<div class="code"><pre>
%module example
int fact(int n);
</pre></div>

<p>
creates a built-in function <tt>fact</tt> that works exactly
like you think it does:
</p>

<div class="code"><pre>
% load ./example.so
% fact 4
24
% set x [fact 6]
%
</pre></div>

<H3><a name="Tcl_nn17"></a>37.3.3 Global variables</H3>


<p>
C/C++ global variables are wrapped by Tcl global variables.  For example:
</p>

<div class="code"><pre>
// SWIG interface file with global variables
%module example
...
%inline %{
extern double density;
%}
...
</pre></div>

<p>
Now look at the Tcl interface:
</p>

<div class="code"><pre>
% puts $density          # Output value of C global variable
1.0
% set density 0.95       # Change value
</pre></div>

<p>
If you make an error in variable assignment, you will get an
error message.  For example:
</p>

<div class="code"><pre>
% set density "hello"
can't set "density": Type error. expected a double.
%
</pre></div>

<p>
If a variable is declared as <tt>const</tt>, it is wrapped as a
read-only variable.  Attempts to modify its value will result in an
error.
</p>

<p>
To make ordinary variables read-only, you can use the <tt>%immutable</tt> directive. For example:
</p>

<div class="code">
<pre>
%{
extern char *path;
%}
%immutable;
extern char *path;
%mutable;
</pre>
</div>

<p>
The <tt>%immutable</tt> directive stays in effect until it is explicitly disabled or cleared using
<tt>%mutable</tt>.
See the <a href="SWIG.html#SWIG_readonly_variables">Creating read-only variables</a> section for further details.
</p>

<p>
If you just want to make a specific variable immutable, supply a declaration name.  For example:
</p>

<div class="code">
<pre>
%{
extern char *path;
%}
%immutable path;
...
extern char *path;      // Read-only (due to %immutable)
</pre>
</div>

<H3><a name="Tcl_nn18"></a>37.3.4 Constants and enums</H3>


<p>
C/C++ constants are installed as global Tcl variables containing the
appropriate value.  To create a constant, use <tt>#define</tt>, <tt>enum</tt>, or the
<tt>%constant</tt> directive.  For example:
</p>

<div class="code">
<pre>
#define PI 3.14159
#define VERSION "1.0"

enum Beverage { ALE, LAGER, STOUT, PILSNER };

%constant int FOO = 42;
%constant const char *path = "/usr/local";
</pre>
</div>

<p>
For enums, make sure that the definition of the enumeration actually appears in a header
file or in the wrapper file somehow---if you just stick an enum in a SWIG interface without
also telling the C compiler about it, the wrapper code won't compile.
</p>

<p>
Note:  declarations declared as <tt>const</tt> are wrapped as read-only variables and
will be accessed using the <tt>cvar</tt> object described in the previous section.  They
are not wrapped as constants.   For further discussion about this, see the <a href="SWIG.html#SWIG">SWIG Basics</a> chapter.
</p>

<p>
Constants are not guaranteed to remain constant in Tcl---the value
of the constant could be accidentally reassigned.You will just have to be careful.
</p>

<p>
A peculiarity of installing constants as variables is that it is necessary to use the Tcl <tt>global</tt> statement to
access constants in procedure bodies.  For example:
</p>

<div class="code">
<pre>
proc blah {} {
   global FOO
   bar $FOO
}
</pre>
</div>

<p>
If a program relies on a lot of constants, this can be extremely annoying.  To fix the problem, consider using the
following typemap rule:
</p>

<div class="code">
<pre>
%apply int CONSTANT { int x };
#define FOO 42
...
void bar(int x);
</pre>
</div>

<p>
When applied to an input argument, the <tt>CONSTANT</tt> rule allows a constant to be passed to a function using
its actual value or a symbolic identifier name.  For example:
</p>

<div class="code">
<pre>
proc blah {} {
   bar FOO
}
</pre>
</div>

<p>
When an identifier name is given, it is used to perform an implicit hash-table lookup of the value during argument 
conversion.  This allows the <tt>global</tt> statement to be omitted.
</p>

<H3><a name="Tcl_nn19"></a>37.3.5 Pointers</H3>


<p>
C/C++ pointers are fully supported by SWIG.  Furthermore, SWIG has no problem working with
incomplete type information.  Here is a rather simple interface:
</p>

<div class="code">
<pre>
%module example

FILE *fopen(const char *filename, const char *mode);
int fputs(const char *, FILE *);
int fclose(FILE *);
</pre>
</div>

<p>
When wrapped, you will be able to use the functions in a natural way from Tcl. 
For example:
</p>

<div class="code">
<pre>
% load ./example.so
% set f [fopen junk w]
% fputs "Hello World\n" $f
% fclose $f
</pre>
</div>

<p>
If this makes you uneasy, rest assured that there is no
deep magic involved.  Underneath the covers, pointers to C/C++ objects are
simply represented as opaque values--normally an encoded character
string like this:
</p>

<div class="code"><pre>
% puts $f
_c0671108_p_FILE
% 
</pre></div>

<p>
This pointer value can be freely passed around to different C functions that
expect to receive an object of type <tt>FILE *</tt>.  The only thing you can't do is 
dereference the pointer from Tcl.
</p>

<p>
The NULL pointer is represented by the string <tt>NULL</tt>.
</p>

<p>
As much as you might be inclined to modify a pointer value directly
from Tcl, don't.  The hexadecimal encoding is not necessarily the
same as the logical memory address of the underlying object.  Instead
it is the raw byte encoding of the pointer value.  The encoding will
vary depending on the native byte-ordering of the platform (i.e.,
big-endian vs. little-endian).  Similarly, don't try to manually cast
a pointer to a new type by simply replacing the type-string.  This
may not work like you expect and it is particularly dangerous when
casting C++ objects. If you need to cast a pointer or
change its value, consider writing some helper functions instead.  For
example:
</p>

<div class="code">
<pre>
%inline %{
/* C-style cast */
Bar *FooToBar(Foo *f) {
   return (Bar *) f;
}

/* C++-style cast */
Foo *BarToFoo(Bar *b) {
   return dynamic_cast&lt;Foo*&gt;(b);
}

Foo *IncrFoo(Foo *f, int i) {
    return f+i;
}
%}
</pre>
</div>

<p>
Also, if working with C++, you should always try
to use the new C++ style casts.  For example, in the above code, the
C-style cast may return a bogus result whereas as the C++-style cast will return
<tt>None</tt> if the conversion can't be performed.
</p>

<H3><a name="Tcl_nn20"></a>37.3.6 Structures</H3>


<p>
If you wrap a C structure, it is wrapped by a Tcl interface that somewhat resembles a Tk widget.
This provides a very natural interface.  For example,
</p>

<div class="code"><pre>
struct Vector {
	double x,y,z;
};

</pre></div>

<p>
is used as follows:
</p>

<div class="code"><pre>
% Vector v
% v configure -x 3.5 -y 7.2
% puts "[v cget -x] [v cget -y] [v cget -z]"
3.5 7.2 0.0
% 
</pre></div>

<p>
Similar access is provided for unions and the data members of C++ classes.
</p>

<p>
In the above example, <tt>v</tt> is a name that's used for the object.  However,
underneath the covers, there's a pointer to a raw C structure.  This can be obtained
by looking at the <tt>-this</tt> attribute.  For example:
</p>

<div class="code">
<pre>
% puts [v cget -this]
_88e31408_p_Vector
</pre>
</div>

<p>
Further details about the relationship between the Tcl and the underlying C structure
are covered a little later.
</p>

<p>
<tt>const</tt> members of a structure are read-only. Data members
can also be forced to be read-only using the <tt>%immutable</tt> directive. For example:
</p>

<div class="code">
<pre>
struct Foo {
   ...
   %immutable;
   int x;        /* Read-only members */
   char *name;
   %mutable;
   ...
};
</pre>
</div>

<p>
When <tt>char *</tt> members of a structure are wrapped, the contents are assumed to be
dynamically allocated using <tt>malloc</tt> or <tt>new</tt> (depending on whether or not
SWIG is run with the -c++ option).   When the structure member is set, the old contents will be 
released and a new value created.   If this is not the behavior you want, you will have to use
a typemap (described later).
</p>

<p>
If a structure contains arrays, access to those arrays is managed through pointers.  For
example, consider this:
</p>

<div class="code">
<pre>
struct Bar {
    int  x[16];
};
</pre>
</div>

<p>
If accessed in Tcl, you will see behavior like this:
</p>

<div class="code">
<pre>
% Bar b
% puts [b cget -x]
_801861a4_p_int
% 
</pre>
</div>

<p>
This pointer can be passed around to functions that expect to receive
an <tt>int *</tt> (just like C).   You can also set the value of an array member using
another pointer.  For example:
</p>

<div class="code">
<pre>
% Bar c
% c configure -x [b cget -x]   # Copy contents of b.x to c.x
</pre>
</div>

<p>
For array assignment, SWIG copies the entire contents of the array starting with the data pointed
to by <tt>b.x</tt>.   In this example, 16 integers would be copied.  Like C, SWIG makes
no assumptions about bounds checking---if you pass a bad pointer, you may get a segmentation
fault or access violation.
</p>

<p>
When a member of a structure is itself a structure, it is handled as a
pointer.  For example, suppose you have two structures like this:
</p>

<div class="code">
<pre>
struct Foo {
   int a;
};

struct Bar {
   Foo f;
};
</pre>
</div>

<p>
Now, suppose that you access the <tt>f</tt> attribute of <tt>Bar</tt> like this:
</p>

<div class="code">
<pre>
% Bar b
% set x [b cget -f]
</pre>
</div>

<p>
In this case, <tt>x</tt> is a pointer that points to the <tt>Foo</tt> that is inside <tt>b</tt>.
This is the same value as generated by this C code:
</p>

<div class="code">
<pre>
Bar b;
Foo *x = &amp;b-&gt;f;       /* Points inside b */
</pre>
</div>

<p>
However, one peculiarity of accessing a substructure like this is that the returned
value does work quite like you might expect.  For example:
</p>

<div class="code">
<pre>
% Bar b
% set x [b cget -f]
% x cget -a
invalid command name "x"
</pre>
</div>

<p>
This is because the returned value was not created in a normal way from the interpreter (x is 
not a command object). To make it function normally, just
evaluate the variable like this:
</p>

<div class="code">
<pre>
% Bar b
% set x [b cget -f]
% $x cget -a
0
%
</pre>
</div>

<p>
In this example, <tt>x</tt> points inside the original structure.  This means that modifications
work just like you would expect.  For example:
</p>

<div class="code">
<pre>

% Bar b
% set x [b cget -f]
% $x configure -a 3            # Modifies contents of f (inside b)
% [b cget -f] -configure -a 3  # Same thing
</pre>
</div>

<p>
In many of these structure examples, a simple name like "v" or "b" has been given
to wrapped structures.  If necessary, this name can be passed to functions
that expect to receive an object.  For example, if you have a function like this,
</p>

<div class="code">
<pre>
void blah(Foo *f);
</pre>
</div>

<p>
you can call the function in Tcl as follows:
</p>

<div class="code">
<pre>
% Foo x            # Create a Foo object 
% blah x           # Pass the object to a function
</pre>
</div>

<p>
It is also possible to call the function using the raw pointer value. For
instance:
</p>

<div class="code">
<pre>
% blah [x cget -this]   # Pass object to a function
</pre>
</div>

<p>
It is also possible to create and use objects using variables.  For example:
</p>

<div class="code">
<pre>
% set b [Bar]            # Create a Bar
% $b cget -f             # Member access
% puts $b
_108fea88_p_Bar
%
</pre>
</div>

<p>
Finally, to destroy objects created from Tcl, you can either let the object
name go out of scope or you can explicitly delete the object.  For example:
</p>

<div class="code">
<pre>
% Foo f                 # Create object f
% rename f ""
</pre>
</div>

<p>
or
</p>

<div class="code">
<pre>
% Foo f                 # Create object f
% f -delete
</pre>
</div>

<p>
Note: Tcl only destroys the underlying object if it has ownership.  See the
memory management section that appears shortly.
</p>

<H3><a name="Tcl_nn21"></a>37.3.7 C++ classes</H3>


<p>
C++ classes are wrapped as an extension of structure wrapping. For example, if you have this class,
</p>

<div class="code"><pre>
class List {
public:
  List();
  ~List();
  int  search(char *item);
  void insert(char *item);
  void remove(char *item);
  char *get(int n);
  int  length;
};
</pre></div>

<p>
you can use it in Tcl like this:
</p>

<div class="code"><pre>
% List x
% x insert Ale
% x insert Stout
% x insert Lager
% x get 1
Stout
% puts [l cget -length]
3
%
</pre></div>

<p>
Class data members are accessed in the same manner as C structures.  
</p>

<p>
Static class members are accessed as global functions or variables.
To illustrate, suppose you have a class like this:
</p>

<div class="code">
<pre>
class Spam {
public:
   static void foo();
   static int bar;

};
</pre>
</div>

<p>
In Tcl, the static member is accessed as follows:
</p>

<div class="code">
<pre>
% Spam_foo        # Spam::foo()
% puts $Spam_bar  # Spam::bar
</pre>
</div>

<H3><a name="Tcl_nn22"></a>37.3.8 C++ inheritance</H3>


<p>
SWIG is fully aware of issues related to C++ inheritance.  Therefore, if you have
classes like this
</p>

<div class="code">
<pre>
class Foo {
...
};

class Bar : public Foo {
...
};
</pre>
</div>

<p>
An object of type <tt>Bar</tt> can be used where a <tt>Foo</tt> is expected.  For
example, if you have this function:
</p>

<div class="code">
<pre>
void spam(Foo *f);
</pre>
</div>

<p>
then the function <tt>spam()</tt> accepts a <tt>Foo *</tt> or a pointer to any class derived from <tt>Foo</tt>.
For instance:
</p>

<div class="code">
<pre>
% Foo f      # Create a Foo
% Bar b      # Create a Bar
% spam f     # OK
% spam b     # OK
</pre>
</div>

<p>
It is safe to use multiple inheritance with SWIG.
</p>

<H3><a name="Tcl_nn23"></a>37.3.9 Pointers, references, values, and arrays</H3>


<p>
In C++, there are many different ways a function might receive
and manipulate objects.  For example:
</p>

<div class="code">
<pre>
void spam1(Foo *x);      // Pass by pointer
void spam2(Foo &amp;x);      // Pass by reference
void spam3(Foo x);       // Pass by value
void spam4(Foo x[]);     // Array of objects
</pre>
</div>

<p>
In Tcl, there is no detailed distinction like this.
Because of this, SWIG unifies all of these types
together in the wrapper code.  For instance, if you actually had the
above functions, it is perfectly legal to do this:
</p>

<div class="code">
<pre>
% Foo f             # Create a Foo
% spam1 f           # Ok. Pointer
% spam2 f           # Ok. Reference
% spam3 f           # Ok. Value.
% spam4 f           # Ok. Array (1 element)
</pre>
</div>

<p>
Similar behavior occurs for return values.  For example, if you had
functions like this,
</p>

<div class="code">
<pre>
Foo *spam5();
Foo &amp;spam6();
Foo  spam7();
</pre>
</div>

<p>
then all three functions will return a pointer to some <tt>Foo</tt> object.
Since the third function (spam7) returns a value, newly allocated memory is used 
to hold the result and a pointer is returned (Tcl will release this memory 
when the return value is garbage collected).
</p>

<H3><a name="Tcl_nn24"></a>37.3.10 C++ overloaded functions</H3>


<p>
C++ overloaded functions, methods, and constructors are mostly supported by SWIG.  For example,
if you have two functions like this:
</p>

<div class="code">
<pre>
void foo(int);
void foo(char *c);
</pre>
</div>

<p>
You can use them in Tcl in a straightforward manner:
</p>

<div class="code">
<pre>
% foo 3            # foo(int)
% foo Hello        # foo(char *c)
</pre>
</div>

<p>
Similarly, if you have a class like this,
</p>

<div class="code">
<pre>
class Foo {
public:
    Foo();
    Foo(const Foo &amp;);
    ...
};
</pre>
</div>

<p>
you can write Tcl code like this:
</p>

<div class="code">
<pre>
% Foo f                # Create a Foo
% Foo g f              # Copy f
</pre>
</div>

<p>
Overloading support is not quite as flexible as in C++. Sometimes there are methods that SWIG
can't disambiguate. For example:
</p>

<div class="code">
<pre>
void spam(int);
void spam(short);
</pre>
</div>

<p>
or
</p>

<div class="code">
<pre>
void foo(Bar *b);
void foo(Bar &amp;b);
</pre>
</div>

<p>
If declarations such as these appear, you will get a warning message like this:
</p>

<div class="code">
<pre>
example.i:12: Warning 509: Overloaded method spam(short) effectively ignored,
example.i:11: Warning 509: as it is shadowed by spam(int).
</pre>
</div>

<p>
To fix this, you either need to ignore or rename one of the methods.  For example:
</p>

<div class="code">
<pre>
%rename(spam_short) spam(short);
...
void spam(int);    
void spam(short);   // Accessed as spam_short
</pre>
</div>

<p>
or
</p>

<div class="code">
<pre>
%ignore spam(short);
...
void spam(int);    
void spam(short);   // Ignored
</pre>
</div>

<p>
SWIG resolves overloaded functions and methods using a disambiguation scheme that ranks and sorts
declarations according to a set of type-precedence rules.    The order in which declarations appear
in the input does not matter except in situations where ambiguity arises--in this case, the
first declaration takes precedence.
</p>

<p>
Please refer to the "SWIG and C++" chapter for more information about overloading. 
</p>

<H3><a name="Tcl_nn25"></a>37.3.11 C++ operators</H3>


<p>
Certain C++ overloaded operators can be handled automatically by SWIG.  For example,
consider a class like this:
</p>

<div class="code">
<pre>
class Complex {
private:
  double rpart, ipart;
public:
  Complex(double r = 0, double i = 0) : rpart(r), ipart(i) { }
  Complex(const Complex &amp;c) : rpart(c.rpart), ipart(c.ipart) { }
  Complex &amp;operator=(const Complex &amp;c);
  Complex operator+(const Complex &amp;c) const;
  Complex operator-(const Complex &amp;c) const;
  Complex operator*(const Complex &amp;c) const;
  Complex operator-() const;
  
  double re() const { return rpart; }
  double im() const { return ipart; }
};
</pre>
</div>

<p>
When wrapped, it works like this:
</p>

<div class="code">
<pre>
% Complex c 3 4
% Complex d 7 8
% set e [c + d]
% $e re
10.0
% $e im
12.0
</pre>
</div>

<p>
It should be stressed that operators in SWIG have no relationship to operators
in Tcl.  In fact, the only thing that's happening here is that an operator like
<tt>operator +</tt> has been renamed to a method <tt>+</tt>.    Therefore, the
statement <tt>[c + d]</tt> is really just invoking the <tt>+</tt> method on <tt>c</tt>.
When more than operator is defined (with different arguments), the standard
method overloading facilities are used.  Here is a rather odd looking example:
</p>

<div class="code">
<pre>
% Complex c 3 4
% Complex d 7 8
% set e [c - d]       # operator-(const Complex &amp;)
% puts "[$e re] [$e im]"
10.0 12.0
% set f [c -]         # operator-()
% puts "[$f re] [$f im]"
-3.0 -4.0
%
</pre>
</div>

<p>
One restriction with operator overloading support is that SWIG is not
able to fully handle operators that aren't defined as part of the class.
For example, if you had code like this
</p>

<div class="code">
<pre>
class Complex {
...
friend Complex operator+(double, const Complex &amp;c);
...
};
</pre>
</div>

<p>
then SWIG doesn't know what to do with the friend function--in fact,
it simply ignores it and issues a warning.   You can still wrap the operator,
but you may have to encapsulate it in a special function.  For example:
</p>

<div class="code">
<pre>
%rename(Complex_add_dc) operator+(double, const Complex &amp;);
...
Complex operator+(double, const Complex &amp;c);
</pre>
</div>

<p>
There are ways to make this operator appear as part of the class using the <tt>%extend</tt> directive.
Keep reading.
</p>

<H3><a name="Tcl_nn26"></a>37.3.12 C++ namespaces</H3>


<p>
SWIG is aware of C++ namespaces, but namespace names do not appear in
the module nor do namespaces result in a module that is broken up into
submodules or packages.  For example, if you have a file like this,
</p>

<div class="code">
<pre>
%module example

namespace foo {
   int fact(int n);
   struct Vector {
       double x,y,z;
   };
};
</pre>
</div>

<p>
it works in Tcl as follows:
</p>

<div class="code">
<pre>
% load ./example.so
% fact 3
6
% Vector v
% v configure -x 3.4
</pre>
</div>

<p>
If your program has more than one namespace, name conflicts (if any) can be resolved using <tt>%rename</tt>
For example:
</p>

<div class="code">
<pre>
%rename(Bar_spam) Bar::spam;

namespace Foo {
    int spam();
}

namespace Bar {
    int spam();
}
</pre>
</div>

<p>
If you have more than one namespace and your want to keep their
symbols separate, consider wrapping them as separate SWIG modules.
For example, make the module name the same as the namespace and create
extension modules for each namespace separately.  If your program
utilizes thousands of small deeply nested namespaces each with
identical symbol names, well, then you get what you deserve.
</p>

<H3><a name="Tcl_nn27"></a>37.3.13 C++ templates</H3>


<p>
C++ templates don't present a huge problem for SWIG.  However, in order
to create wrappers, you have to tell SWIG to create wrappers for a particular
template instantiation.  To do this, you use the <tt>%template</tt> directive.
For example:
</p>

<div class="code">
<pre>
%module example
%{
#include "pair.h"
%}

template&lt;class T1, class T2&gt;
struct pair {
   typedef T1 first_type;
   typedef T2 second_type;
   T1 first;
   T2 second;
   pair();
   pair(const T1&amp;, const T2&amp;);
  ~pair();
};

%template(pairii) pair&lt;int,int&gt;;
</pre>
</div>

<p>
In Tcl:
</p>

<div class="code">
<pre>
% pairii p 3 4
% p cget -first
3
% p cget -second
4
</pre>
</div>

<p>
Obviously, there is more to template wrapping than shown in this example.
More details can be found in the <a href="SWIGPlus.html#SWIGPlus">SWIG and C++</a> chapter.   Some more complicated
examples will appear later.
</p>

<H3><a name="Tcl_nn28"></a>37.3.14 C++ Smart Pointers</H3>


<p>
In certain C++ programs, it is common to use classes that have been wrapped by
so-called "smart pointers."   Generally, this involves the use of a template class
that implements <tt>operator-&gt;()</tt> like this:
</p>

<div class="code">
<pre>
template&lt;class T&gt; class SmartPtr {
   ...
   T *operator-&gt;();
   ...
}
</pre>
</div>

<p>
Then, if you have a class like this,
</p>

<div class="code">
<pre>
class Foo {
public:
     int x;
     int bar();
};
</pre>
</div>

<p>
A smart pointer would be used in C++ as follows:
</p>

<div class="code">
<pre>
SmartPtr&lt;Foo&gt; p = CreateFoo();   // Created somehow (not shown)
...
p-&gt;x = 3;                        // Foo::x
int y = p-&gt;bar();                // Foo::bar
</pre>
</div>

<p>
To wrap this in Tcl, simply tell SWIG about the <tt>SmartPtr</tt> class and the low-level
<tt>Foo</tt> object.  Make sure you instantiate <tt>SmartPtr</tt> using <tt>%template</tt> if necessary.
For example:
</p>

<div class="code">
<pre>
%module example
...
%template(SmartPtrFoo) SmartPtr&lt;Foo&gt;;
...
</pre>
</div>

<p>
Now, in Tcl, everything should just "work":
</p>

<div class="code">
<pre>
% set p [CreateFoo]                  # Create a smart-pointer somehow
% $p configure -x 3                  # Foo::x
% $p bar                             # Foo::bar
</pre>
</div>

<p>
If you ever need to access the underlying pointer returned by <tt>operator-&gt;()</tt> itself,
simply use the <tt>__deref__()</tt> method.  For example:
</p>

<div class="code">
<pre>
% set f [$p __deref__]    # Returns underlying Foo *
</pre>
</div>

<H2><a name="Tcl_nn29"></a>37.4 Further details on the Tcl class interface</H2>


<p>
In the previous section, a high-level view of Tcl wrapping was
presented.  A key component of this wrapping is that structures and
classes are wrapped by Tcl class-like objects. This provides a very
natural Tcl interface and allows SWIG to support a number of
advanced features such as operator overloading.   However, a number
of low-level details were omitted.  This section provides a brief overview
of how the proxy classes work.
</p>

<H3><a name="Tcl_nn30"></a>37.4.1 Proxy classes</H3>


<p>
In the <a href="SWIG.html#SWIG">"SWIG basics"</a> and <a href="SWIGPlus.html#SWIGPlus">"SWIG and C++"</a> chapters,
details of low-level structure and class wrapping are described.  To summarize those chapters, if you
have a class like this
</p>

<div class="code">
<pre>
class Foo {
public:
     int x;
     int spam(int);
     ...
</pre>
</div>

<p>
then SWIG transforms it into a set of low-level procedural wrappers. For example:
</p>

<div class="code">
<pre>
Foo *new_Foo() {
    return new Foo();
}
void delete_Foo(Foo *f) {
    delete f;
}
int Foo_x_get(Foo *f) {
    return f-&gt;x;
}
void Foo_x_set(Foo *f, int value) {
    f-&gt;x = value;
}
int Foo_spam(Foo *f, int arg1) {
    return f-&gt;spam(arg1);
}
</pre>
</div>

<p>
These wrappers are actually found in the Tcl extension module.  For example, you can certainly do this:
</p>

<div class="code">
<pre>
% load ./example.so
% set f [new_Foo]
% Foo_x_get $f
0
% Foo_spam $f 3
1
%
</pre>
</div>

<p>
However, in addition to this, the classname <tt>Foo</tt> is used as an object constructor
function.   This allows objects to be encapsulated objects that look a lot like Tk widgets
as shown in the last section.
</p>

<H3><a name="Tcl_nn31"></a>37.4.2 Memory management</H3>


<p>
Associated with each wrapped object, is an ownership flag <tt>thisown</tt>   The value of this
flag determines who is responsible for deleting the underlying C++ object.  If set to 1,
the Tcl interpreter destroys the C++ object when the proxy class is 
garbage collected.   If set to 0 (or if the attribute is missing), then the destruction
of the proxy class has no effect on the C++ object.
</p>

<p>
When an object is created by a constructor or returned by value, Tcl automatically takes
ownership of the result.  For example:
</p>

<div class="code">
<pre>
class Foo {
public:
    Foo();
    Foo bar();
};
</pre>
</div>

<p>
In Tcl:
</p>

<div class="code">
<pre>
% Foo f
% f cget -thisown
1
% set g [f bar]
% $g cget -thisown
1
</pre>
</div>

<p>
On the other hand, when pointers are returned to Tcl, there is often no way to know where
they came from.  Therefore, the ownership is set to zero.  For example:
</p>

<div class="code">
<pre>
class Foo {
public:
    ...
    Foo *spam();
    ...
};
</pre>
</div>

<br>

<div class="code">
<pre>
% Foo f
% set s [f spam]
% $s cget -thisown
0
% 
</pre>
</div>

<p>
This behavior is especially important for classes that act as
containers.  For example, if a method returns a pointer to an object
that is contained inside another object, you definitely don't want
Tcl to assume ownership and destroy it!
</p>

<p>
Related to containers, ownership issues can arise whenever an object is assigned to a member
or global variable.  For example, consider this interface:
</p>

<div class="code">
<pre>
%module example

struct Foo {
    int  value;
    Foo  *next;
};

Foo *head = 0;
</pre>
</div>

<p>
When wrapped in Tcl, careful observation will reveal that ownership changes whenever an object
is assigned to a global variable.  For example:
</p>

<div class="code">
<pre>
% Foo f
% f cget -thisown
1
% set head f
% f cget -thisown
0
</pre>
</div>

<p>
In this case, C is now holding a reference to the object---you probably don't want Tcl to destroy it.
Similarly, this occurs for members.  For example:
</p>

<div class="code">
<pre>
% Foo f
% Foo g
% f cget -thisown
1
% g cget -thisown
1
% f configure -next g
% g cget -thisown 
0
%
</pre>
</div>

<p>
For the most part, memory management issues remain hidden.  However,
there are occasionally situations where you might have to manually
change the ownership of an object.  For instance, consider code like this:
</p>

<div class="code">
<pre>
class Node {
   Object *value;
public:
   void set_value(Object *v) { value = v; }
   ...
};
</pre>
</div>

<p>
Now, consider the following Tcl code:
</p>

<div class="code">
<pre>
% Object v                 # Create an object
% Node n                   # Create a node
% n setvalue v             # Set value
% v cget -thisown
1
% 
</pre>
</div>

<p>
In this case, the object <tt>n</tt> is holding a reference to
<tt>v</tt> internally.  However, SWIG has no way to know that this
has occurred.  Therefore, Tcl still thinks that it has ownersh…
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