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- [section boost/python/with_custodian_and_ward.hpp]
- [section Introduction]
- This header provides facilities for establishing a lifetime dependency between two of a function's Python argument or result objects. The ward object will not be destroyed until after the custodian as long as the custodian object supports [@http://www.python.org/doc/current/lib/module-weakref.html weak references] (Boost.Python extension classes all support weak references). If the custodian object does not support weak references and is not `None`, an appropriate exception will be thrown. The two class templates `with_custodian_and_ward` and `with_custodian_and_ward_postcall` differ in the point at which they take effect.
- In order to reduce the chance of inadvertently creating dangling pointers, the default is to do lifetime binding before the underlying C++ object is invoked. However, before invocation the result object is not available, so `with_custodian_and_ward_postcall` is provided to bind lifetimes after invocation. Also, if a C++ exception is thrown after `with_custodian_and_ward<>::precall` but before the underlying C++ object actually stores a pointer, the lifetime of the custodian and ward objects will be artificially bound together, so one might choose `with_custodian_and_ward_postcall` instead, depending on the semantics of the function being wrapped.
- Please note that this is not the appropriate tool to use when wrapping functions which transfer ownership of a raw pointer across the function-call boundary. Please see the FAQ if you want to do that.
- [endsect]
- [section Class `with_custodian_and_ward`]
- [table
- [[Parameter][Requirements][Description][Default]]
- [[custodian][ A positive compile-time constant of `type std::size_t`. ][ The 1-based index of the parameter which is the dependency in the lifetime relationship to be established. If used to wrap a member function, parameter 1 is the target object (`*this`). Note that if the target Python object type doesn't support weak references, a Python TypeError exception will be raised when the C++ object being wrapped is called. ][]]
- [[ward][ A positive compile-time constant of type `std::size_t`. ][ The 1-based index of the parameter which is the dependent in the lifetime relationship to be established. If used to wrap a member function, parameter 1 is the target object (`*this`). ][]]
- [[Base][ A model of [link concepts.callpolicies `CallPolicies`]][ Used for policy [link concepts.callpolicies.callpolicies_composition composition]. ][default_call_policies]]
- ]
- ``
- namespace boost { namespace python
- {
- template <std::size_t custodian, std::size_t ward, class Base = default_call_policies>
- struct with_custodian_and_ward : Base
- {
- static bool precall(PyObject* args);
- };
- }}``
- [endsect]
- [section Class `with_custodian_and_ward` static functions]
- ``bool precall(PyObject* args);``
- [variablelist
- [[Requires][`PyTuple_Check(args) != 0`]]
- [[Effects][Makes the lifetime of the argument indicated by ward dependent on the lifetime of the argument indicated by custodian. ]]
- [[Returns][false and PyErr_Occurred() != 0 upon failure, true otherwise.]]
- ]
- [endsect]
- [section Class `with_custodian_and_ward_postcall`]
- [table
- [[Parameter][Requirements][Description][Default]]
- [[custodian][ A positive compile-time constant of type `std::size_t`. ][ The index of the parameter which is the dependency in the lifetime relationship to be established. Zero indicates the result object; 1 indicates the first argument. If used to wrap a member function, parameter 1 is the target object (`*this`). Note that if the target Python object type doesn't support weak references, a Python TypeError exception will be raised when the C++ object being wrapped is called. ][]]
- [[ward][ A positive compile-time constant of type `std::size_t`. ][ The index of the parameter which is the dependent in the lifetime relationship to be established. Zero indicates the result object; 1 indicates the first argument. If used to wrap a member function, parameter 1 is the target object (`*this`). ][]]
- [[Base][ A model of [link concepts.callpolicies `CallPolicies`]][ Used for policy [link concepts.callpolicies.callpolicies_composition composition]. ][default_call_policies]]
- ]
- ``
- namespace boost { namespace python
- {
- template <std::size_t custodian, std::size_t ward, class Base = default_call_policies>
- struct with_custodian_and_ward_postcall : Base
- {
- static PyObject* postcall(PyObject* args, PyObject* result);
- };
- }}
- ``
- [endsect]
- [section Class `with_custodian_and_ward_postcall` static functions]
- ``PyObject *postcall(PyObject* args, PyObject* result);``
- [variablelist
- [[Requires][`PyTuple_Check(args) != 0`, `result != 0`]]
- [[Effects][Makes the lifetime of the object indicated by ward dependent on the lifetime of the object indicated by custodian. ]]
- [[Returns][`0` and `PyErr_Occurred() != 0` upon failure, `true` otherwise. ]]
- ]
- [endsect]
- [section Example]
- The following example shows how `with_custodian_and_ward_postcall` is used by the library to implement `return_internal_reference`
- ``
- template <std::size_t owner_arg = 1, class Base = default_call_policies>
- struct return_internal_reference
- : with_custodian_and_ward_postcall<0, owner_arg, Base>
- {
- typedef reference_existing_object result_converter;
- };
- ``
- [endsect]
- [endsect]
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