c++ - 从对象外部操作链表

Question

我想从一个对象外部操作一个链表，但它并没有像我想象的那样工作。

情况是这样的:我有一个对象，它具有指向第一个条目的基类指针，以标记链表的开头。

class theObject {
public:
    theObject() : mFirstEntry(0), mLastEntry(0) {}
    ~theObject() {}

    template<class T>
    void addEntry(const std::string &blah, const std::string &blub, const T &hui)
    {
        child<T> *newEntry = new child<T>(blah, blub, hui);

        if (mFirstEntry) {
            mLastEntry->setNext(newEntry);
            mLastEntry = newEntry;
        }
        else {
            mFirstEntry = newEntry;
            mLastEntry = newEntry;
        }
    }

    base * getFirstEntry() const
    {
        return mFirstEntry;
    }

    void printEntrys() const
    {
        base *data = mFirstEntry;
        while(data) {
            std::cout << data->getBlah() << data->getBlub() << std::endl;
            data = data->getNext();
        }
    }

private:
    base *mFirstEntry;
    base *mLastEntry;
};

class base {
public:
    base() : mBlah(""), mBlub(""), mNext(0) {}

    base(const std::string &blah, const std::string &blub) : mBlah(blah), mBlub(blub), mNext(0) {}

    virtual ~base()
    {
        if (mNext) {
            delete mNext;
        }
    }

    void setNext(base *next)
    {
        mNext = next;
    }

    base * getNext() const
    {
        return mNext;
    }

    std::string getBlah() const
    {
        return mBlah;
    }

    std::string getBlub() const
    {
        return mBlub;
    }

protected:
    std::string mBlah;
    std::string mBlub;
    base *mNext;
};

链表的每个条目都是子类型，它是一个模板并继承基类。

template<class T>
class child : public base {
public:
    inline child(const std::string &blah, const std::string &blub, const T &hui, base *next = 0) : mHui(hui), base(blah, blub)
    {
        if(next) {
            mNext = next;
        }
    }

    inline child(const child &r)
    {
        *this = r;
    }

    inline const child & operator = (const child &r)
    {
        if (this == &r) return *this;

        mBlah = r.mBlah;
        mBlub = r.mBlub;
        mNext = r.mNext;
        mHui = r.mHui;

        return *this;
    }

    inline const T getData() const
    {
        return mHui;
    }

protected:
    T mHui;
};

现在我用几个条目填充对象

int main(int argc, char* argv[])
{
    theObject data;
    int a(0), b(1), c(2), d(3);
    const std::string blah("blah"), blub("blub");
    data.addEntry(blah, blub, a);
    data.addEntry(blah, blub, b);
    data.addEntry(blah, blub, c);
    data.addEntry(blah, blub, d);

    std::cout << "Original entries" << std::endl;
    data.printEntrys();

然后我想修改链表

    base *stuff = data.getFirstEntry();
    std::cout << "Changed in stuff list" << std::endl;
    while(stuff) {
        stuff = new child<double>("noBlah", "noBlub", 3.14, stuff->getNext());
        std::cout << stuff->getBlah() << stuff->getBlub() << std::endl;
        stuff = stuff->getNext();
    }

并希望操纵原件......但它接缝我只操纵了一个拷贝

    std::cout << "linked list in data object should now be stuff list" << std::endl;
    data.printEntrys();

    return 0;
}

有人知道为什么它不起作用吗？ getFirstEntry() 返回一个指针，所以我想我会操纵它指向的对象。

最好的问候，本

Answer 1

你通过 base *stuff = data.getFirstEntry(); 得到一个指向第一个元素的指针，然后使用 stuff = new child<double>("noBlah", "noBlub", 3.14, stuff->getNext()); 将它指向一个新对象.换句话说，您更改的是指针，而不是指向的值。

我建议您查看有关指针的教程。曾经有人推荐过这个视频:Binky Pointer Fun