ceph/src/boost/libs/python/doc/reference/implicit.qbk

   1 [section boost/python/implicit.hpp]
   2 [section Introduction]
   3 `implicitly_convertible` allows Boost.Python to implicitly take advantage of a C++ implicit or explicit conversion when matching Python objects to C++ argument types.
   4 [endsect]
   5 [section Function template `implicit_convertible`]
   6 ``
   7 template <class Source, class Target>
   8 void implicitly_convertible();
   9 ``
  10 [table
  11 [[Parameter][Description]]
  12 [[Source][The source type of the implicit conversion]]
  13 [[Target][The target type of the implicit conversion]]
  14 ]
  15 [variablelist
  16 [[Requires][The declaration `Target t(s);`, where s is of type Source, is valid.]]
  17 [[Effects][registers an rvalue `from_python` converter to Target which can succeed for any `PyObject* p` iff there exists any registered converter which can produce Source rvalues]]
  18 [[Rationale][C++ users expect to be able to take advantage of the same sort of interoperability in Python as they do in C++.]]
  19 ]
  20 [endsect]
  21 [section Example]
  22 In C++:
  23 ``
  24 #include <boost/python/class.hpp>
  25 #include <boost/python/implicit.hpp>
  26 #include <boost/python/module.hpp>
  27
  28 using namespace boost::python;
  29
  30 struct X
  31 {
  32     X(int x) : v(x) {}
  33     operator int() const { return v; }
  34     int v;
  35 };
  36
  37 int x_value(X const& x)
  38 {
  39     return x.v;
  40 }
  41
  42 X make_x(int n) { return X(n); }
  43
  44 BOOST_PYTHON_MODULE(implicit_ext)
  45 {
  46     def("x_value", x_value);
  47     def("make_x", make_x);
  48
  49     class_<X>("X",
  50         init<int>())
  51         ;
  52
  53     implicitly_convertible<X,int>();
  54     implicitly_convertible<int,X>();
  55 }
  56 ``
  57 In Python:
  58 ``
  59 >>> from implicit_ext import *
  60 >>> x_value(X(42))
  61 42
  62 >>> x_value(42)
  63 42
  64 >>> x = make_x(X(42))
  65 >>> x_value(x)
  66 42
  67 ``
  68 [endsect]
  69 [endsect]