c++派生类更改基类指针

Question

我有一个用户类，它有一个指向数据类的成员指针。但是我想实现一个扩展 user 的 derivedUser，但它还需要有存储在 derivedData 类 中的额外数据，它会看起来像这样:

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;
}

但这就是问题所在:按照我设置的方式，DerivedUser 类 将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，它就会失败。问题是:我该如何实现？

我试过:

class DerivedUser : public User {

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

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

Answer 1

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

由于多态性，父类可以引用它的子类。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 类

这里:

int main(){

    DerivedData* dp = new DerivedData();

    DerivedUser* user = new DerivedUser(dp);


    return 0;
}