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POBoost.Python - How to return by reference?
 

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  1. POBoost.Python - How to return by reference?
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    <p>I'm using Boost.Python to create Python modules from C++ classes. And I ran into a problem with references.</p> <p>Condider the following case where I have a class Foo with overloaded get methods that can either return by value or reference.</p> <p>Specifying that the return by value should be used was easy once I typedefed a signature. But I think it should be possible return a reference as well by using a <code>return_value_policy</code>. However, using what seemed appropriate (<a href="http://www.boost.org/doc/libs/1_40_0/libs/python/doc/v2/reference_existing_object.html" rel="noreferrer">doc</a>); <code>return_value_policy&lt;reference_existing_object&gt;</code> did not seem to work. </p> <p>Have I misunderstood what it does?</p> <pre><code>struct Foo { Foo(float x) { _x = x; } float&amp; get() { return _x; } float get() const { return _x; } private: float _x; }; // Wrapper code BOOST_PYTHON_MODULE(my_module) { using namespace boost::python; typedef float (Foo::*get_by_value)() const; typedef float&amp; (Foo::*get_by_ref)(); class_&lt;Foo&gt;("Foo", init&lt;float&gt;()) .def("get", get_by_value(&amp;Foo::get)) .def("get_ref", get_by_ref(&amp;Foo::get), return_value_policy&lt;reference_existing_object&gt;())//Doesn't work ; } </code></pre> <p>Note: I know it could be dangerous to reference existing object without life-time managing.</p> <p><strong>Update:</strong><br> It looks like it works for objects but not basic data types.<br> Take this revised example:</p> <pre><code>struct Foo { Foo(float x) { _x = x; } float&amp; get() { return _x; } float get() const { return _x; } void set( float f ){ _x = f;} Foo&amp; self(){return *this;} private: float _x; }; // Wrapper code using namespace boost::python; BOOST_PYTHON_MODULE(my_module) { typedef float (Foo::*get_by_value)() const; class_&lt;Foo&gt;("Foo", init&lt;float&gt;()) .def("get", get_by_value(&amp;Foo::get)) .def("get_self", &amp;Foo::self, return_value_policy&lt;reference_existing_object&gt;()) .def("set", &amp;Foo::set); ; } </code></pre> <p>Which in a test gave the expected result:</p> <pre><code>&gt;&gt;&gt; foo1 = Foo(123) &gt;&gt;&gt; foo1.get() 123.0 &gt;&gt;&gt; foo2 = foo1.get_self() &gt;&gt;&gt; foo2.set(1) &gt;&gt;&gt; foo1.get() 1.0 &gt;&gt;&gt; id(foo1) == id(foo2) False </code></pre>
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