简而言之，创建一个守卫类型，它将在其构造函数和析构函数中初始化和关闭遗留库，然后通过每个公开对象中的智能指针管理守卫。
有一些微妙的细节可能会使销毁过程的正确性变得困难：
- 对象和模块中的对象的破坏顺序
Py_Finalize()是随机的。
- 没有一个模块的最终确定。特别是，动态加载的扩展模块不会被卸载。
- 遗留 API 只有在使用它的所有对象都被销毁后才应该关闭。然而，对象本身可能不知道彼此。
为此，Boost.Python 对象需要协调何时初始化和关闭旧版 API。这些对象还需要对使用遗留 API 的遗留对象拥有所有权。使用单一职责原则，可以将职责划分为几个类。
可以使用资源获取初始化(RAII) 习语来初始化和关闭传统 AP。例如，使用以下legacy_api_guard，当legacy_api_guard构造对象时，它将初始化旧版 API。当legacy_api_guard对象被破坏时，它将关闭旧版 API。
/// @brief Guard that will initialize or shutdown the legacy API.
struct legacy_api_guard
{
legacy_api_guard() { legacy::initialize(); }
~legacy_api_guard() { legacy::shutdown(); }
};
由于多个对象需要共同管理何时初始化和关闭遗留 API，因此可以使用智能指针（例如std::shared_ptr）来负责管理守卫。以下示例延迟初始化和关闭旧版 API：
/// @brief Global shared guard for the legacy API.
std::weak_ptr<legacy_api_guard> legacy_api_guard_;
/// @brief Get (or create) guard for legacy API.
std::shared_ptr<legacy_api_guard> get_api_guard()
{
auto shared = legacy_api_guard_.lock();
if (!shared)
{
shared = std::make_shared<legacy_api_guard>();
legacy_api_guard_ = shared;
}
return shared;
}
最后，将嵌入到 Boost.Python 对象中的实际类型需要在创建遗留对象的实例之前获取遗留 API 保护的句柄。此外，在销毁时，应在销毁遗留对象后释放遗留 API 保护。实现此目的的一种非侵入式方法是在将遗留类型暴露给 Boost.Python 时使用提供自定义HeldType 。公开类型时，需要抑制默认的 Boost.Python 生成的初始化程序，因为将使用自定义工厂函数来提供对对象创建的控制：
/// @brief legacy_object_holder is a smart pointer that will hold
/// legacy types and help guarantee the legacy API is initialized
/// while these objects are alive. This smart pointer will remain
/// transparent to the legacy library and the user-facing Python.
template <typename T>
class legacy_object_holder
{
public:
typedef T element_type;
template <typename... Args>
legacy_object_holder(Args&&... args)
: legacy_guard_(::get_api_guard()),
ptr_(std::make_shared<T>(std::forward<Args>(args)...))
{}
legacy_object_holder(legacy_object_holder& rhs) = default;
element_type* get() const { return ptr_.get(); }
private:
// Order of declaration is critical here. The guard should be
// allocated first, then the element. This allows for the
// element to be destroyed first, followed by the guard.
std::shared_ptr<legacy_api_guard> legacy_guard_;
std::shared_ptr<element_type> ptr_;
};
/// @brief Helper function used to extract the pointed to object from
/// an object_holder. Boost.Python will use this through ADL.
template <typename T>
T* get_pointer(const legacy_object_holder<T>& holder)
{
return holder.get();
}
/// Auxiliary function to make exposing legacy objects easier.
template <typename T, typename ...Args>
legacy_object_holder<T>* make_legacy_object(Args&&... args)
{
return new legacy_object_holder<T>(std::forward<Args>(args)...);
}
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
python::class_<
legacy::Test, legacy_object_holder<legacy::Test>,
boost::noncopyable>("Test", python::no_init)
.def("__init__", python::make_constructor(
&make_legacy_object<legacy::Test>))
;
}
下面是一个完整的示例，演示了使用自定义 HeldType 以非侵入式延迟保护具有共享管理的资源：
#include <iostream> // std::cout, std::endl
#include <memory> // std::shared_ptr, std::weak_ptr
#include <boost/python.hpp>
/// @brief legacy namespace that cannot be changed.
namespace legacy {
void initialize() { std::cout << "legacy::initialize()" << std::endl; }
void shutdown() { std::cout << "legacy::shutdown()" << std::endl; }
class Test
{
public:
Test() { std::cout << "legacy::Test::Test()" << std::endl; }
virtual ~Test() { std::cout << "legacy::Test::~Test()" << std::endl; }
};
void use_test(Test&) {}
} // namespace legacy
namespace {
/// @brief Guard that will initialize or shutdown the legacy API.
struct legacy_api_guard
{
legacy_api_guard() { legacy::initialize(); }
~legacy_api_guard() { legacy::shutdown(); }
};
/// @brief Global shared guard for the legacy API.
std::weak_ptr<legacy_api_guard> legacy_api_guard_;
/// @brief Get (or create) guard for legacy API.
std::shared_ptr<legacy_api_guard> get_api_guard()
{
auto shared = legacy_api_guard_.lock();
if (!shared)
{
shared = std::make_shared<legacy_api_guard>();
legacy_api_guard_ = shared;
}
return shared;
}
} // namespace
/// @brief legacy_object_holder is a smart pointer that will hold
/// legacy types and help guarantee the legacy API is initialized
/// while these objects are alive. This smart pointer will remain
/// transparent to the legacy library and the user-facing Python.
template <typename T>
class legacy_object_holder
{
public:
typedef T element_type;
template <typename... Args>
legacy_object_holder(Args&&... args)
: legacy_guard_(::get_api_guard()),
ptr_(std::make_shared<T>(std::forward<Args>(args)...))
{}
legacy_object_holder(legacy_object_holder& rhs) = default;
element_type* get() const { return ptr_.get(); }
private:
// Order of declaration is critical here. The guard should be
// allocated first, then the element. This allows for the
// element to be destroyed first, followed by the guard.
std::shared_ptr<legacy_api_guard> legacy_guard_;
std::shared_ptr<element_type> ptr_;
};
/// @brief Helper function used to extract the pointed to object from
/// an object_holder. Boost.Python will use this through ADL.
template <typename T>
T* get_pointer(const legacy_object_holder<T>& holder)
{
return holder.get();
}
/// Auxiliary function to make exposing legacy objects easier.
template <typename T, typename ...Args>
legacy_object_holder<T>* make_legacy_object(Args&&... args)
{
return new legacy_object_holder<T>(std::forward<Args>(args)...);
}
// Wrap the legacy::use_test function, passing the managed object.
void legacy_use_test_wrap(legacy_object_holder<legacy::Test>& holder)
{
return legacy::use_test(*holder.get());
}
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
python::class_<
legacy::Test, legacy_object_holder<legacy::Test>,
boost::noncopyable>("Test", python::no_init)
.def("__init__", python::make_constructor(
&make_legacy_object<legacy::Test>))
;
python::def("use_test", &legacy_use_test_wrap);
}
交互使用：
>>> import example
>>> test1 = example.Test()
legacy::initialize()
legacy::Test::Test()
>>> test2 = example.Test()
legacy::Test::Test()
>>> test1 = None
legacy::Test::~Test()
>>> example.use_test(test2)
>>> exit()
legacy::Test::~Test()
legacy::shutdown()
请注意，基本的整体方法也适用于非惰性解决方案，其中遗留 API 在导入模块时被初始化。需要使用 ashared_ptr而不是 a weak_ptr，并使用以下方法注册清理功能atexit.register()：
/// @brief Global shared guard for the legacy API.
std::shared_ptr<legacy_api_guard> legacy_api_guard_;
/// @brief Get (or create) guard for legacy API.
std::shared_ptr<legacy_api_guard> get_api_guard()
{
if (!legacy_api_guard_)
{
legacy_api_guard_ = std::make_shared<legacy_api_guard>();
}
return legacy_api_guard_;
}
void release_guard()
{
legacy_api_guard_.reset();
}
...
BOOST_PYTHON_MODULE(example)
{
// Boost.Python may throw an exception, so try/catch around
// it to initialize and shutdown legacy API on failure.
namespace python = boost::python;
try
{
::get_api_guard(); // Initialize.
...
// Register a cleanup function to run at exit.
python::import("atexit").attr("register")(
python::make_function(&::release_guard)
);
}
// If an exception is thrown, perform cleanup and re-throw.
catch (const python::error_already_set&)
{
::release_guard();
throw;
}
}
请参阅此处进行演示。