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<title>SWIG:Examples:tcl:constants</title>
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<tt>SWIG/Examples/tcl/constants/</tt>
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<H2>Wrapping C Constants</H2>
<tt>$Header: /cvs/projects/SWIG/Examples/tcl/constants/index.html,v 1.1 2000/06/17 21:41:00 beazley Exp $</tt><br>
<p>
When SWIG encounters C preprocessor macros and C declarations that look like constants,
it creates Tcl variables with an identical value. Click <a href="example.i">here</a>
to see a SWIG interface with some constant declarations in it.
<h2>Accessing Constants from Tcl</h2>
Click <a href="example.tcl">here</a> to see a script that prints out the values
of the constants contained in the above file.
<h2>Key points</h2>
<ul>
<li>The values of preprocessor macros are converted into Tcl read-only variables.
<li>Types are inferred by syntax (e.g., "3" is an integer and "3.5" is a float).
<li>Character constants such as 'x' are converted into strings.
<li>C string literals such as "Hello World" are converted into strings.
<li>Macros that are not fully defined are simply ignored. For example:
<blockquote>
<pre>
#define EXTERN extern
</pre>
</blockquote>
is ignored because SWIG has no idea what type of variable this would be.
<p>
<li>Expressions are allowed provided that all of their components are defined. Otherwise, the constant is ignored.
<li>Certain C declarations involving 'const' are also turned into Tcl constants.
<li>The constants that appear in a SWIG interface file do not have to appear in any sort
of matching C source file since the creation of a constant does not require linkage
to a stored value (i.e., a value held in a C global variable or memory location).
<li>Since constants are turned into Tcl variables, you have to use the global
statement when accessing from a procedure. For example:
<blockquote>
<pre>
proc foo {} {
global ICONST # Some C constant
puts $ICONST
}
</pre>
</blockquote>
</ul>
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