...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Models of the CallPolicies concept are used to specialize the behavior of Python callable objects generated by Boost.Python to wrapped C++ objects like function and member function pointers, providing three behaviors:
precall
- Python argument tuple management before the
wrapped object is invokedresult_converter
- C++ return value handlingpostcall
- Python argument tuple and result management
after the wrapped object is invokedextract_return_type
- metafunction for extracting the return type from a given signature type sequenceBase
which defaults to default_call_policies
.
By convention, the precall
function of the Base
is invoked after the precall
function supplied by the
outer template, and the postcall
function of the
Base
is invoked before the postcall
function of the outer template. If a result_converter
is
supplied by the outer template, it replaces any
result_converter
supplied by the Base
. For an
example, see return_internal_reference
.
In the table below, x
denotes an object whose type
P
is a model of CallPolicies, a
denotes a PyObject*
pointing to a Python argument tuple
object, and r
denotes a PyObject*
referring to a "preliminary" result object.
Expression | Type | Result/Semantics |
x.precall(a) |
convertible to bool |
returns false and PyErr_Occurred() != 0
upon failure, true otherwise. |
P::result_converter |
A model of ResultConverterGenerator. | An MPL unary Metafunction Class used produce the "preliminary" result object. |
x.postcall(a, r) |
convertible to PyObject* |
0 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. |
P::extract_return_type |
A model of Metafunction. | An MPL unary Metafunction used extract the return type from a given signature. By default it is derived from mpl::front. |
Revised 13 November, 2002
© Copyright Dave Abrahams 2002.
Permission to copy, use, modify, sell and distribute this software is granted provided this copyright notice appears in all copies. This software is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.