派生类 C++ 中的向上转换变量

Question

如何更改派生类中继承变量的类型？我有以下课程：

class Position;
class StonePosition;
class Position {
public:
    Position() {}
};
class StonePosition : public Position {
    int count;
public:
    StonePosition(const int count) { this->count = count; }
    int getCount() { return this->count; }
    void setCount(int count) { this->count = count; }
    friend ostream& operator<<(ostream&, StonePosition);
};
class Board {
protected:
    Position* crrPos;
public:
    Board() { }
    Position* getCrrPos() { return crrPos; }
    void setCrrPos(Position* pos) { crrPos=pos; }
};
class StoneBoard : public Board {
public:
    StoneBoard(const int &count) { this->crrPos=new StonePosition(count); } //<----------------
    StonePosition* getCrrPos() { return (StonePosition*)crrPos; }
    void setCrrPos(StonePosition* pos) { crrPos=pos; }
};

用箭头标记问题的位置。我需要在 StoneBoard 类中将变量的类型从 Position 更改为 StonePosition。 我找到了一个可以使用向上转换的选项，但它仅适用于单个方法，我需要更改整个类的变量。

问题解决了，看看我的回答。

Answer 1

变量"crrPos"不是Position类型，而是指向Position的指针类型，这很重要，因为指向Position的指针可以指向Position或从Position派生的类而不会丢失任何内容。

如果你设计好你的类，并利用虚函数，你通常可以完全避免上投的需要。

#include <iostream>
class Base {
public:
    virtual void foo() { std::cout << "Base::foo()n"; }
    virtual bool isDerived() const { return false; }
};
class Derived : public Base {
public:
    void foo() override { std::cout << "Derived::foo()n"; }
    bool isDerived() const { return true; }
};
int main() {
    Base* crrPos = new Derived;
    crrPos->foo();
    bool isDerived = crrPos->isDerived();
    std::cout << isDerived << 'n';
    delete crrPos;
}

现场演示：http://ideone.com/UKcBaA

Answer 2

问题已经解决，我只使用投影（（StonePosition*）Position*）：

#include <iostream>
using namespace std;
class Position;
class StonePosition;
class Position {
public:
    Position() {}
};
class StonePosition : public Position {
    int count;
public:
    StonePosition(const int count) { this->count = count; }
    int getCount() { return this->count; }
    void setCount(int count) { this->count = count; }
    friend ostream& operator<<(ostream&, StonePosition);
};
template <typename TPos> class TBoard { 
protected:
    TPos* crrPos;
public:
    TBoard() { }
    TPos* getCrrPos() { return crrPos; }
    void setCrrPos(TPos* pos) { crrPos=pos; }
};
class Board {
protected:
    Position* crrPos;
public:
    Board() { }
    Position* getCrrPos() { return crrPos; }
    void setCrrPos(Position* pos) { crrPos=pos; }
};
class StoneBoard : public Board {
public:
    StoneBoard(const int &count) { this->crrPos=new StonePosition(count); } 
    Position* getCrrPos() { return crrPos; }
    void setCrrPos(Position* pos) { crrPos=pos; }
};
int main(){
    StoneBoard s(7);
    cout<<((StonePosition*)s.getCrrPos())->getCount();//<----right here
    system("pause");
    return 0;
}

而且它的工作很好:)