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# Boost.Python

## Header <boost/python/with_custodian_and_ward.hpp>

## Contents

Introduction
Classes
  Class Template `with_custodian_and_ward`
    Class Template `with_custodian_and_ward` synopsis
    Class `with_custodian_and_ward` static functions
  Class Template `with_custodian_and_ward_postcall`
    Class Template `with_custodian_and_ward_postcall` synopsis
    Class `with_custodian_and_ward_postcall` static functions
Example

## Introduction

This header provides faciliites 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 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.

## Classes

### Class template `with_custodian_and_ward`

`with_custodian_and_ward` template parameters
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 CallPolicies | Used for policy composition. | `default_call_policies`

#### Class template `with_custodian_and_ward` synopsis

    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);
       };
    }}

#### Class `with_custodian_and_ward` static functions

    bool precall(PyObject* args);

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.

### Class template `with_custodian_and_ward_postcall`

`with_custodian_and_ward_postcall` template parameters
Parameter | Requirements | Description | Default
--- | --- | --- | ---
`custodian` | A 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 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 CallPolicies | Used for policy composition. | `default_call_policies`

#### Class template `with_custodian_and_ward_postcall` synopsis

    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);
       };
    }}

#### Class `with_custodian_and_ward_postcall` static functions

    PyObject* postcall(PyObject* args, PyObject* result);

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.

## 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;
    };

Revised 13 November, 2002

© Copyright Dave Abrahams 2002.