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

   1 [chapter Concepts
   2     [quickbook 1.7]
   3 ]
   4
   5 [section CallPolicies]
   6 [section Introduction]
   7
   8 Models of the CallPolicies concept are used to specialize the behavior of Python callable objects
   9 generated by Boost.Python to wrapped C++ objects like function and member function pointers,
  10 providing three behaviors:
  11
  12 # `precall` - Python argument tuple management before the wrapped object is invoked
  13 # `result_converter` - C++ return value handling
  14 # `postcall` - Python argument tuple and result management after the wrapped object is invoked
  15 # `extract_return_type` - metafunction for extracting the return type from a given signature type sequence
  16
  17 [endsect]
  18 [section CallPolicies Composition]
  19
  20 In order to allow the use of multiple models of CallPolicies in the same callable object,
  21 Boost.Python's CallPolicies class templates provide a chaining interface which allows them to be
  22 recursively composed. This interface takes the form of an optional template parameter, `Base`, which
  23 defaults to `default_call_policies`. By convention, the `precall` function of the `Base` is invoked after
  24 the `precall` function supplied by the `outer` template, and the `postcall` function of the `Base` is invoked
  25 before the `postcall` function of the `outer` template. If a `result_converter` is supplied by the `outer`
  26 template, it replaces any `result_converter` supplied by the `Base`. For an example, see
  27 `return_internal_reference`.
  28
  29 [endsect]
  30 [section Concept Requirements]
  31 [table
  32  [[Expression][Type][Result/Semantics]]
  33  [[`x.precall(a)`][convertible to `bool`]
  34   [returns `false` and `PyErr_Occurred() != 0` upon failure, `true` otherwise.]]
  35  [[`P::result_converter`][A model of `ResultConverterGenerator`.]
  36   [An MPL unary Metafunction Class used produce the "preliminary" result object.]]
  37  [[`x.postcall(a, r)`][convertible to `PyObject*`]
  38   [`0` and `PyErr_Occurred() != 0` upon failure. Must "conserve references" even in the event of an exception. In other words, if `r` is not returned, its reference count must be decremented; if another existing object is returned, its reference count must be incremented.]]
  39  [[`P::extract_return_type`][A model of Metafunction.]
  40   [An MPL unary Metafunction used extract the return type from a given signature. By default it is derived from `mpl::front`.]]
  41 ]
  42 [endsect]
  43 [endsect]
  44 [section Dereferenceable]
  45 [section Introduction]
  46 Instances of a `Dereferenceable` type can be used like a pointer to access an lvalue.
  47 [endsect]
  48 [section Concept Requirements]
  49 In the table below, `T` is a model of Dereferenceable, and `x` denotes an object of type `T`. In addition, all pointers are `Dereferenceable`.
  50 [table
  51  [[Expression][Result][Operational Semantics]]
  52  [[`get_pointer(x)`][convertible to `pointee<T>::type*`]
  53   [`&*x`, or a null pointer ]]
  54 ]
  55 [endsect]
  56 [endsect]
  57 [section Extractor]
  58 [section Introduction]
  59 An Extractor is a class which Boost.Python can use to extract C++ objects from Python objects, and is typically used by facilities that define `from_python` conversions for "traditional" Python extension types.
  60 [endsect]
  61 [section Concept Requirements]
  62 In the table below, `X` denotes a model of `Extractor` and `a` denotes an instance of a Python object type.
  63 [table
  64  [[Expression][Type][Semantics]]
  65  [[`X::execute(a)`][non-void]
  66   [Returns the C++ object being extracted. The execute function must not be overloaded.]]
  67  [[`&a.ob_type`][`PyTypeObject**`]
  68   [Points to the `ob_type` field of an object which is layout-compatible with `PyObject`]]
  69 ]
  70 [endsect]
  71 [section Notes]
  72 Informally, an Extractor's execute member must be a non-overloaded static function whose single argument is a Python object type. Acceptable Python object types include those publicly (and unambiguously) derived from PyObject, and POD types which are layout-compatible with PyObject.
  73 [endsect]
  74 [endsect]
  75 [section HolderGenerator]
  76 [section Introduction]
  77 A HolderGenerator is a unary metafunction class which returns types suitable for holding instances of its argument in a wrapped C++ class instance.
  78 [endsect]
  79 [section Concept Requirements]
  80 In the table below, `G` denotes an type which models `HolderGenerator`, and `X` denotes a class type.
  81 [table
  82  [[Expression][Requirements]]
  83  [[`G::apply<X>::type`][A concrete subclass of `instance_holder` which can hold objects of type `X`. ]]
  84 ]
  85 [endsect]
  86 [endsect]
  87 [section ResultConverter]
  88 [section Introduction]
  89 A ResultConverter for a type `T` is a type whose instances can be used to convert C++ return values of type `T` `to_python`. A ResultConverterGenerator is an MPL unary metafunction class which, given the return type of a C++ function, returns a ResultConverter for that type. ResultConverters in Boost.Python generally inspect library's registry of converters to find a suitable converter, but converters which don't use the registry are also possible.
  90 [endsect]
  91 [section ResultConverter Concept Requirements]
  92 In the table below, `C` denotes a ResultConverter type for a type `R`, `c` denotes an object of type `C`, and `r` denotes an object of type `R`.
  93 [table
  94  [[Expression][Type][Semantics]]
  95  [[`C c`][]
  96   [Constructs a `c` object.]]
  97  [[`c.convertible()`][convertible to `bool`]
  98   [`false` iff no conversion from any `R` value to a Python object is possible.]]
  99  [[`c(r)`][convertible to `PyObject*`]
 100   [A pointer to a Python object corresponding to `r`, or `0` iff `r` could not be converted `to_python`, in which case `PyErr_Occurred` should return non-zero.]]
 101  [[`c.get_pytype()`][`PyTypeObject const *`]
 102   [A pointer to a Python Type object corresponding to result of the conversion, or `0`. Used for documentation generation. If `0` is returned the generated type in the documentation will be object.]]
 103 ]
 104 [endsect]
 105 [section ResultConverterGenerator Concept Requirements]
 106 In the table below, `G` denotes a ResultConverterGenerator type and `R` denotes a possible C++ function return type.
 107 [table
 108  [[Expression][Requirements]]
 109  [[`G::apply<R>::type`][A ResultConverter type for `R`.]]
 110 ]
 111 [endsect]
 112 [endsect]
 113 [section ObjectWrapper]
 114 [section Introduction]
 115 This page defines two concepts used to describe classes which manage a Python objects, and which are intended to support usage with a Python-like syntax.
 116 [endsect]
 117 [section ObjectWrapper Concept Requirements]
 118 Models of the ObjectWrapper concept have [link object_wrappers.boost_python_object_hpp.class_object object] as a publicly-accessible base class, and are used to supply special construction behavior and/or additional convenient functionality through (often templated) member functions. Except when the return type R is itself an [link concepts.objectwrapper.typewrapper_concept_requirements TypeWrapper], a member function invocation of the form ``x.some_function(a1, a2,...an)`` always has semantics equivalent to:
 119 ``extract<R>(x.attr("some_function")(object(a1), object(a2),...object(an)))()`` (see [link concepts.objectwrapper.caveat caveat] below).
 120 [endsect]
 121 [section TypeWrapper Concept Requirements]
 122 TypeWrapper is a refinement of [link concepts.objectwrapper.objectwrapper_concept_requiremen ObjectWrapper] which is associated with a particular Python type `X`. For a given TypeWrapper `T`, a valid constructor expression ``T(a1, a2,...an)`` builds a new T object managing the result of invoking X with arguments corresponding to ``object(a1), object(a2),...object(an)``.
 123 When used as arguments to wrapped C++ functions, or as the template parameter to [link to_from_python_type_conversion.boost_python_extract_hpp.class_template_extract extract<>], only instances of the associated Python type will be considered a match.
 124 [endsect]
 125 [section Caveat]
 126 The upshot of the special member function invocation rules when the return type is a TypeWrapper is that it is possible for the returned object to manage a Python object of an inappropriate type. This is not usually a serious problem; the worst-case result is that errors will be detected at runtime a little later than they might otherwise be. For an example of how this can occur, note that the [link object_wrappers.boost_python_dict_hpp.class_dict dict] member function `items` returns an object of type [link object_wrappers.boost_python_list_hpp.class_list list]. Now suppose the user defines this `dict` subclass in Python:
 127 ``
 128   >>> class mydict(dict):
 129   ...     def items(self):
 130   ...         return tuple(dict.items(self)) # return a tuple
 131 ``
 132 Since an instance of `mydict` is also an instance of `dict`, when used as an argument to a wrapped C++ function, [link object_wrappers.boost_python_dict_hpp.class_dict boost::python::dict] can accept objects of Python type `mydict`. Invoking `items()` on this object can result in an instance of [link object_wrappers.boost_python_list_hpp.class_list boost::python::list] which actually holds a Python `tuple`. Subsequent attempts to use `list` methods (e.g. `append`, or any other mutating operation) on this object will raise the same exception that would occur if you tried to do it from Python.
 133 [endsect]
 134 [endsect]