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git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk@10005 626c5289-ae23-0410-ae9c-e8d60b6d4f22
67 lines
1.9 KiB
HTML
67 lines
1.9 KiB
HTML
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<title>SWIG:Examples:python:constants</title>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/python/constants/</tt>
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<hr>
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<H2>Wrapping C Constants</H2>
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<p>
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When SWIG encounters C preprocessor macros and C declarations that look like constants,
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it creates Python variables with an identical value. Click <a href="example.i">here</a>
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to see a SWIG interface with some constant declarations in it.
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<h2>Accessing Constants from Python</h2>
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Click <a href="example.py">here</a> to see a script that prints out the values
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of the constants contained in the above file.
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<h2>Key points</h2>
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<ul>
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<li>The values of preprocessor macros are converted into Python constants.
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<li>Types are inferred by syntax (e.g., "3" is an integer and "3.5" is a float).
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<li>Character constants such as 'x' are converted into Python strings.
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<li>C string literals such as "Hello World" are converted into Python strings.
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<li>Macros that are not fully defined are simply ignored. For example:
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<blockquote>
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<pre>
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#define EXTERN extern
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</pre>
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</blockquote>
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is ignored because SWIG has no idea what type of variable this would be.
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<p>
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<li>Expressions are allowed provided that all of their components are defined. Otherwise, the constant is ignored.
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<li>Certain C declarations involving 'const' are also turned into Python constants.
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<li>The Python variables that SWIG creates are not protected from modification. For example,
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even if you had this:
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<blockquote>
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<pre>
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#define FOO 73
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</pre>
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</blockquote>
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a user could come along in a script and type
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<blockquote>
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<pre>
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example.FOO = 13
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</pre>
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</blockquote>
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Unfortunately, there's no easy way to prevent this.
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<p>
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<li>The constants that appear in a SWIG interface file do not have to appear in any sort
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of matching C source file since the creation of a constant does not require linkage
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to a stored value (i.e., a value held in a C global variable or memory location).
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</ul>
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<hr>
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</body>
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</html>
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