提升 Python:使用 return_internal_reference 将参数的生存期与返回值绑定

我开始学习使用boost python，并有一个新手问题。

我想写一个函数，可以将其参数的生命周期与其结果联系起来，这样当我调用r = func(a)时，如果我仍然有r的引用，参数a永远不会被破坏。该文档建议对此类请求使用return_internal_reference调用策略。但这是否要求r像顾名思义的那样成为a的内部参考？

在下面的（过度简化的）示例中，假设我想将输入数组a的生存期与生成的 lambda 函数联系起来，该函数不是输入a的内部引用。

#include <functional>
#include <boost/python.hpp>
#include <boost/python/return_internal_reference.hpp>
using namespace std;
using namespace boost::python;
function<float(int)> func(const float* a) {
  return [=](int n) { return a[n]; };
}
BOOST_PYTHON_MODULE(test) {
  def("func", func, return_internal_reference<1>());
}

我希望能够在python中执行以下操作：

f = func(a)   # 'a' can be a temporary variable, say returned by another function
f(5)          # but 'a' should not be destroyed at this step, 
              # because its lifetime is tied to 'f'

当我尝试编译上面的代码时，我遇到了下面列出的一堵错误墙，但是如果我删除return_internal_reference<1>()调用策略，代码就会成功编译。

我很确定我用错了这个呼叫策略，但不确定如何使其正确。任何指针都将受到高度赞赏。多谢！

$ g++ -std=c++11 -shared Test.cc -I/opt/local/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -L/opt/local/lib -lboost_python-mt -lpython2.7 -o test.so
In file included from /opt/local/include/boost/preprocessor/iteration/detail/iter/forward1.hpp:52:0,
                 from /opt/local/include/boost/python/detail/invoke.hpp:63,
                 from /opt/local/include/boost/python/detail/caller.hpp:16,
                 from /opt/local/include/boost/python/object/function_handle.hpp:8,
                 from /opt/local/include/boost/python/converter/arg_to_python.hpp:19,
                 from /opt/local/include/boost/python/call.hpp:15,
                 from /opt/local/include/boost/python/object_core.hpp:14,
                 from /opt/local/include/boost/python/args.hpp:25,
                 from /opt/local/include/boost/python.hpp:11,
                 from Test.cc:2:
/opt/local/include/boost/python/detail/invoke.hpp: In instantiation of 'PyObject* boost::python::detail::invoke(boost::python::detail::invoke_tag_<false, false>, const RC&, F&, AC0&) [with RC = boost::python::detail::reference_existing_object_requires_a_pointer_or_reference_return_type<std::function<float(int)> >; F = std::function<float(int)> (*)(const float*); AC0 = boost::python::arg_from_python<const float*>; PyObject = _object]':
/opt/local/include/boost/python/detail/caller.hpp:223:13:   required from 'PyObject* boost::python::detail::caller_arity<1u>::impl<F, Policies, Sig>::operator()(PyObject*, PyObject*) [with F = std::function<float(int)> (*)(const float*); Policies = boost::python::return_internal_reference<>; Sig = boost::mpl::vector2<std::function<float(int)>, const float*>; PyObject = _object]'
/opt/local/include/boost/python/object/py_function.hpp:38:33:   required from 'PyObject* boost::python::objects::caller_py_function_impl<Caller>::operator()(PyObject*, PyObject*) [with Caller = boost::python::detail::caller<std::function<float(int)> (*)(const float*), boost::python::return_internal_reference<>, boost::mpl::vector2<std::function<float(int)>, const float*> >; PyObject = _object]'
Test.cc:14:1:   required from here
/opt/local/include/boost/python/detail/invoke.hpp:75:82: error: no match for call to '(const boost::python::detail::reference_existing_object_requires_a_pointer_or_reference_return_type<std::function<float(int)> >) (std::function<float(int)>)'
     return rc(f( BOOST_PP_ENUM_BINARY_PARAMS_Z(1, N, ac, () BOOST_PP_INTERCEPT) ));
                                                                                  ^
In file included from /opt/local/include/boost/python/object/function_handle.hpp:8:0,
                 from /opt/local/include/boost/python/converter/arg_to_python.hpp:19,
                 from /opt/local/include/boost/python/call.hpp:15,
                 from /opt/local/include/boost/python/object_core.hpp:14,
                 from /opt/local/include/boost/python/args.hpp:25,
                 from /opt/local/include/boost/python.hpp:11,
                 from Test.cc:2:
/opt/local/include/boost/python/detail/caller.hpp: In instantiation of 'static const PyTypeObject* boost::python::detail::converter_target_type<ResultConverter>::get_pytype() [with ResultConverter = boost::python::detail::reference_existing_object_requires_a_pointer_or_reference_return_type<std::function<float(int)> >; PyTypeObject = _typeobject]':
/opt/local/include/boost/python/detail/caller.hpp:240:19:   required from 'static boost::python::detail::py_func_sig_info boost::python::detail::caller_arity<1u>::impl<F, Policies, Sig>::signature() [with F = std::function<float(int)> (*)(const float*); Policies = boost::python::return_internal_reference<>; Sig = boost::mpl::vector2<std::function<float(int)>, const float*>]'
/opt/local/include/boost/python/object/py_function.hpp:48:35:   required from 'boost::python::detail::py_func_sig_info boost::python::objects::caller_py_function_impl<Caller>::signature() const [with Caller = boost::python::detail::caller<std::function<float(int)> (*)(const float*), boost::python::return_internal_reference<>, boost::mpl::vector2<std::function<float(int)>, const float*> >]'
Test.cc:14:1:   required from here
/opt/local/include/boost/python/detail/caller.hpp:102:109: error: 'struct boost::python::detail::reference_existing_object_requires_a_pointer_or_reference_return_type<std::function<float(int)> >' has no member named 'get_pytype'
         return create_result_converter((PyObject*)0, (ResultConverter *)0, (ResultConverter *)0).get_pytype();
                                                                                                             ^

考虑使用 with_custodian_and_ward_postcall CallPolicy。此策略允许按值返回返回类型，同时仍延长另一个对象的生存期，使其至少与返回对象的生存期一样长。

BOOST_PYTHON_MODULE(test) {
  def("func", func, with_custodian_and_ward_postcall<0, 1>());
}

如return_internal_reference文档中所述，返回的对象引用了现有的内部对象：

return_internal_reference [...] 允许安全地返回对内部持有的对象的指针和引用 [...]

文档还简要提到了它的使用 with_custodian_and_ward_postcall . 总之，return_internal_reference对所公开的函数有两个值得注意的影响：

返回的 Python 对象既没有引用的 C++ 对象的显式所有权，也没有共享所有权。
返回的 Python 对象是一个保管人，并且将延长 ward 对象的生存期（由owner_arg指示），至少与保管人一样长。

由于返回的 Python 对象引用的现有对象既没有显式所有权也没有共享所有权，因此 Boost.Python 执行类型检查以防止创建悬空引用。在示例代码中，func()按值返回函子，从而导致编译器错误，提示返回类型必须是指针或引用：

结构提升：:p年：:d尾部：：reference_existing_object_requires_a_pointer_or_reference_return_type

若要显式控制返回对象的生存期，应考虑使用 ResultConverterGenerator 的return_value_policy和模型。例如，如果func()通过new()创建的指针返回一个函子，并希望将对象的所有权传递给 Python，同时仍然保持保管人和病房关系，那么可以将策略与策略组合链接起来：

BOOST_PYTHON_MODULE(test) {
  def("func", func, 
    return_value_policy<manage_new_object,
      with_custodian_and_ward_postcall<0, 1> >());
}

下面是一个完整的最小示例，基于原始代码，具有详细输出，用于演示with_custodian_and_ward_postcall行为：

#include <boost/python.hpp>
#include <iostream>
/// @brief Mockup class with verbose construction and destruction.
class foo
{
public:
  foo() { std::cout << "foo() " << this << std::endl; }
  foo(const foo&) { std::cout << "foo(const foo&) " << this << std::endl; }
  ~foo() { std::cout << "~foo() " << this << std::endl; }
};
/// @brief Mockup class with verbose construction and destruction.
class bar
{
public:
  bar() { std::cout << "bar() " << this << std::endl; }
  bar(const bar&) { std::cout << "bar(const bar&) " << this << std::endl; }
  ~bar() { std::cout << "~bar() " << this << std::endl; }
};
/// @brief Mockup factory function.
foo make_foo(bar& /* unused */)
{
  return foo();
}
BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  // Do not allow Foo to be explicitly created from its type.
  python::class_<foo>("Foo", python::no_init);
  python::class_<bar>("Bar", python::init<>());
  // Expose make_foo, that returns a foo object when provided a
  // bar object.  The bar object's lifetime will be extended to
  // be at least as long as that of the returned foo object.
  python::def("make_foo", &make_foo,
    python::with_custodian_and_ward_postcall<
      0, // custodian = returned Foo object
      1  // ward = provided Bar object
    >());
}

交互式用法：

>>> import example
>>> bar = example.Bar()
bar() 0x125ac30
>>> foo = example.make_foo(bar)
foo() 0x7fffa9b5efff
foo(const foo&) 0x7f1fcbe40090
~foo() 0x7fffa9b5efff
>>> bar = None
>>> foo = None
~foo() 0x7f1fcbe40090
~bar() 0x125ac30

请注意，尽管bar变量设置为 None ，但实际的守卫对象将保持活动状态，直到foo保管人被销毁。

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