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95 lines
3.2 KiB
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95 lines
3.2 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN"
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"http://www.w3.org/TR/REC-html40/strict.dtd">
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<title>
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Wrapping enums
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</title>
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<div>
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<h1>
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<img width="277" height="86" id="_x0000_i1025" align="center"
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src="c++boost.gif" alt= "c++boost.gif (8819 bytes)">Wrapping enums
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</h1>
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<p>Because there is in general no way to deduce that a value of arbitrary type T
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is an enumeration constant, py_cpp cannot automatically convert enum values to
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and from Python. To handle this case, you need to decide how you want the enum
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to show up in Python (since Python doesn't have enums). Once you have done that,
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you can write some simple <code>from_python()</code> and
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<code>to_python()</code> functions.
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<p>If you are satisfied with a Python int as a way to represent your enum
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values, py_cpp provides a shorthand for these functions. You just need to
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instantiate <code>python::enum_as_int_converters<EnumType></code> where
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<code>EnumType</code> is your enumerated type. There are two convenient ways to do this:
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<ol>
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<li><blockquote>
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<pre>
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// drop into namespace python and explicitly instantiate
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BOOST_PYTHON_BEGIN_CONVERSION_NAMESPACE // this is a gcc 2.95.2 bug workaround
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template class enum_as_int_converters<extclass_demo::EnumOwner::enum_type>;
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BOOST_PYTHON_END_CONVERSION_NAMESPACE
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</pre>
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</blockquote>
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<li><blockquote><pre>
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// instantiate as base class in any namespace
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struct EnumTypeConverters
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: python::py_enum_as_int_converters<EnumType>
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{
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};
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</blockquote></pre>
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</ol>
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<p>Either of the above is equivalent to the following declarations:
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<blockquote><pre>
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BOOST_PYTHON_BEGIN_CONVERSION_NAMESPACE // this is a gcc 2.95.2 bug workaround
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MyEnumType from_python(PyObject* x, python::type<MyEnumType>)
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{
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return static_cast<MyEnum>(
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from_python(x, python::type<long>()));
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}
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MyEnumType from_python(PyObject* x, python::type<const MyEnumType&>)
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{
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return static_cast<MyEnum>(
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from_python(x, python::type<long>()));
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}
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PyObject* to_python(MyEnumType x)
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{
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return to_python(static_cast<long>(x));
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}
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BOOST_PYTHON_END_CONVERSION_NAMESPACE
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</pre></blockquote>
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<p>This technique defines the conversions of
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<code>MyEnumType</code> in terms of the conversions for the built-in
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<code>long</code> type.
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You may also want to add a bunch of lines like this to your module
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initialization:
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<blockquote><pre>
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mymodule.add(python::to_python(enum_value_1), "enum_value_1");
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mymodule.add(python::to_python(enum_value_2), "enum_value_2");
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...
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</pre></blockquote>
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You can also add these to an extension class definition, if your enum happens to
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be local to a class and you want the analogous interface in Python:
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<blockquote><pre>
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my_class.add(python::to_python(enum_value_1), "enum_value_1");
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my_class.add(python::to_python(enum_value_2), "enum_value_2");
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...
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</pre></blockquote>
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<p>
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© Copyright David Abrahams 2000. Permission to copy, use, modify,
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sell and distribute this document is granted provided this copyright
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notice appears in all copies. This document is provided "as is" without
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express or implied warranty, and with no claim as to its suitability
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for any purpose.
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<p>
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Updated: Nov 12, 2000
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</div>
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