如何将manager的引用传递给handler对象？

Question

我想要做的是编写一个小型的Manager/Handler类.经理分发和管理Handles.这样的句柄可以是例如简单的文件句柄.

如果消费者想要获得已经存在的句柄,那么管理者只需返回一个shared_ptr.如果句柄不存在,管理器将创建一个新句柄,然后返回shared_ptr.

在Manager内部,那些shared_ptr存储在一个简单的STL-Map中.如果分配的最后一个shared_ptr被删除,我希望我的经理删除相关的map-element,以便处理程序对象自动被破坏.

这听起来有点像垃圾收集(例如工作线程,它检查指针的使用次数),但我相信它可以更优雅地完成.

如何将管理器实例的引用传递给处理程序对象？(例如,像将unique_ptr(this)传递给新处理程序的构造函数)

#include <memory>
#include <iostream>
#include <map>

using namespace std;

/*
 * Simple handler class, that actually does nothing.
 * This could be e.g. a Filehandler class or sth. like that
 */
class Handler {
private:
    int i;
public:
    Handler(int i) :i(i) {}
    ~Handler() {}
    // Say who you are.
    void print(void) { cout << "I am handler # " << i << endl; }
};

/*
 * This is the "manager" class, that manages all handles. A handle is identified
 * by an integer value. If a handle already exists, the Manager returns a shared_ptr,
 * if it does not exist, the manager creates a new handle.
 */
class Manager {
private:
    map<int, shared_ptr<Handler> > handles;
public:
    Manager() {}
    ~Manager() {}

    shared_ptr<Handler> get_handler(int identifier) {
        shared_ptr<Handler> retval;
        auto it = handles.find(identifier);

        if(it != handles.end() ) {
            retval = it->second;
        } else {
            retval = shared_ptr<Handler>(new Handler(identifier));
            handles.insert( pair<int, shared_ptr<Handler>>(identifier, retval) );
        }

        return retval;
    }
};

int main(int argc, char** argv) {
    Manager m;

    // Handler 13 doesn't exist, so it gets allocated
    auto h = m.get_handler(13);
    // Manager knows about handler 13, so it returns the already existing shared_ptr
    auto i = m.get_handler(13);

    h.reset(); // Well... Let's assume we don't need h any more...
    // do some stuff...
    i->print();
    // ...
    i.reset(); // We also loose i. This is exactly the point where i want the manager to forget about the handle 13

    return 0;
}

Answer 1

您可能希望在管理器中保留非拥有指针以跟踪现有句柄,并shared_ptr通过自定义删除器提供拥有权.自定义删除器将确保在对象最终被销毁时删除管理器中相应的观察指针.

我称这种模式为跟踪工厂,这就是它的工作原理.给定一个object类(Handler在您的情况下):

class object
{

public:

    size_t get_id() const
    {
        return _id;
    }

private:

    friend class tracking_factory;

    object(size_t id) : _id(id) { }

    size_t _id = static_cast<size_t>(-1);

};

我定义了一个类,它创建实例object并将非拥有引用存储weak_ptr到它们中.这个类是唯一object可以通过其创建实例的类- 这就是构造函数object是私有的,并且tracking_factory被声明为friend能够访问它:

class tracking_factory
{

public:

    std::shared_ptr<object> get_object(size_t id, 
                                       bool createIfNotFound = true)
    {
        auto i = std::find_if(
            begin(_objects),
            end(_objects),
            [id] (std::pair<size_t const, std::weak_ptr<object>> const& p) 
            -> bool
        {
            return (p.first == id);
        });

        if (i != end(_objects))
        {
            return i->second.lock();
        }
        else if (createIfNotFound)
        {
            return make_object(id);
        }
        else
        {
            return std::shared_ptr<object>();
        }
    }

    size_t count_instances() const
    {
        return _objects.size();
    }

private:

    std::shared_ptr<object> make_object(size_t id)
    {
        std::shared_ptr<object> sp(
            new object(id),
            [this, id] (object* p)
        {
            _objects.erase(id);
            delete p;
        });

        _objects[id] = sp;

        return sp;
    }

    std::map<size_t, std::weak_ptr<object>> _objects;

};

然后,程序的其余部分将通过以下内容获取shared_ptrs :如果已经创建了具有所需特征的对象(此处为成员),则将返回一个没有实例化新对象的对象.以下是一些测试功能的代码:objectobject_factoryidshared_ptr

#include <iostream>

int main()
{
    tracking_factory f;

    auto print_object_count = [&f] ()
    {
        std::cout << "Number of objects: " << f.count_instances() << std::endl;
    };

    print_object_count();

    auto p1 = f.get_object(42);

    print_object_count();

    {
        auto p2 = f.get_object(42);

        print_object_count();

        p1 = f.get_object(0);

        print_object_count();
    }

    print_object_count();

    p1.reset();

    print_object_count();
}

最后,这是一个实例.