将派生类作为参数传递给方法，它是具有智能指针的基类

Question

我最近阅读了Head First设计模式。本书显示了Java相关代码。但是，我尝试将Java代码转换为C ++。在《观察者模式》一章中，我在转换某处时陷入困境。相关代码如下。

class Subject{ //Publisher
public:
    virtual void registerObserver(ObserverPtr o) = 0;
    virtual void removeObserver(ObserverPtr o) = 0;
    virtual void notifyObservers() = 0;
};
using SubjectPtr = std::shared_ptr<Subject>;

订户类接口：

class Observer{
public:
    virtual void update(double temp, double humidity, double pressure) = 0;
};

using ObserverPtr = std::shared_ptr<Observer>;

显示元素界面：

class DisplayElement{
public:
    virtual void display() = 0;
};

using DisplayElementPtr = std::shared_ptr<DisplayElement>;

和CurrentConditionsDisplay

class CurrentConditionsDisplay : public Observer, public DisplayElement{
    SubjectPtr m_weatherData;
    double temperature;
    double humidity;
public:
    CurrentConditionsDisplay(SubjectPtr weatherData);
    void update(double temperature, double humidity, double pressure);
    void display();

};

using CurrentConditionsDisplayPtr = std::shared_ptr<CurrentConditionsDisplay>;

我的问题：

在CurrentCurrentConditionsDisplay类的构造函数中，我希望Publisher（Subject）注册CurrentCurrentConditionsDisplay。

//constructor
CurrentConditionsDisplay::CurrentConditionsDisplay(SubjectPtr weatherData) :
    temperature(0),
    humidity(0)
{
    m_weatherData = weatherData;

    /* How should I pass 'this' ? */
    m_weatherData->registerObserver(???????); //parameter type is ObserverPtr.

}

由于参数类型为ObserverPtr，应该如何传递“指针”？

Answer 1

我建议使用工厂方法，例如：

std::shared_ptr<CurrentConditionsDisplay>
MakeCurrentConditionsDisplay(SubjectPtr weatherData)
{
    auto res = std::make_shared<CurrentConditionsDisplay>(weatherData);

    weatherData->registerObserver(res);
    return res;
}

如果您坚持要在构造函数中执行此操作，则可以使用std::enable_shared_from_this：

std::enable_shared_from_this

class CurrentConditionsDisplay :
    public std::enable_shared_from_this<CurrentConditionsDisplay>,
    public Observer,
    public DisplayElement
{
    SubjectPtr m_weatherData;
    double temperature = 0;
    double humidity = 0;
public:
    explicit CurrentConditionsDisplay(SubjectPtr weatherData) :
        m_weatherData(weatherData)
    {
        m_weatherData->registerObserver(shared_from_this());
    }

    void update(double temperature, double humidity, double pressure) override;
    void display() override;
};

不能从构造函数中调用