c++ Derived class 改变一个基数 class 指针

Question

我有一个 user class，它有一个指向 data class 的成员指针。但我想实现一个扩展 user 的 derivedUser，但它还需要存储在 derivedData class 中的额外数据，它看起来像这样：

class Data {
    /*...the stored data...*/
}

class DerivedData : public Data {
    /*...the additional data...*/
}

class User {

    public:
        /*...some methods that use dp...*/

    protected:
         Data* dp;
}


class DerivedUser : public User {

    public:
        /*...even more methods that use ddp...*/

    protected:
         DerivedData* ddp;
}

但问题是：按照我设置的方式，DerivedUser class 会存储两个指向同一对象的不同类型的指针，这不是最优的。 DerivedUser 应该只存储一个指针，它应该知道它是 DerivedData 类型，如果给它错误的数据类型就会失败。问题是：我该如何实施？

我试过：

class DerivedUser : public User {

    public:
        /*...even more methods that use ddp...*/

    protected:
        DerivedData* ddp = dynamic_cast<DerivedData*>(dp);
}

Answer 1

在 DerivedUser class

中添加方法 DerivedData* GetData()

DerivedData* GetData() { return static_cast<DerivedData>(dp); }

如果 DerivedUser 的构造函数如下所示，您可以确保 dp 是 DerivedData*：

DerivedUser(DerivedData* d):User(d){}

完整代码如下：

class Data {
    /*...the stored data...*/
}

class DerivedData : public Data {
    /*...the additional data...*/
}

class User {

    public:
        User(Data* d):dp(d){}
        /*...some methods that use dp...*/

    protected:
        Data* dp;
}

class DerivedUser : public User {

    public:
        DerivedUser(DerivedData* d):User(d){}
        /*...even more methods that use ddp...*/

    protected:
        DerivedData* GetData(void)
        {
            return static_cast<DerivedData*>(dp);
        };
}

Answer 2

我看到您希望 DerivedUser 在其构造函数中包含 DerivedData。

由于多态性，父 class 可以引用它的子 class。所以这是合法的：

Data* dp = new DerivedData();

这是您正在寻找的解决方案：

class User {

public:
    /*...some methods that use dp...*/

    User(Data* dp){
        this->dp = dp;
    }

protected:
    Data* dp;
};

class DerivedUser : public User {

public:
    /*...even more methods that use ddp...*/

    DerivedUser(DerivedData *dp) : User(dp) {

    }
};

现在，DerivedUser 指向您的 DerivedData class

此处：

int main(){

    DerivedData* dp = new DerivedData();

    DerivedUser* user = new DerivedUser(dp);


    return 0;
}

c++ Derived class 改变一个基数 class 指针

c++ Derived class changing a base class pointer

c++

inheritance

pointers

dynamic-cast