如何在嵌套类中重载 << 运算符

Question

我有三个类：ClientDatabase 和 Node 作为 Database 中的嵌套类。

Node 的数据是指向Client 对象的指针，Client 类有自己的重载<< 运算符。

我需要一个重载的<< 运算符来输出所有链表的数据。 我遇到的问题是无法使用<< 重载运算符遍历所有列表，我能做的最好的事情是使用getData 成员输出头节点的数据，出于某种原因Node::Print不会输出所有列表Client *data。

这是Database 类和提到的两个方法<< 和print()。

数据库.h

class Databse
{
    private:
        class Node //node as nested class
        {
            public:
                Node();
                void setNode(Client*&);
                Node* nextNode(Node*&);
                Client getData();
                void print (Node*);
            private:
                Client* data; //holds pointer to Client object
                Node* next; //holds pointer to next node in list
        };

        Node *head; //holds the head node
        int nClients;

    public:
        Databse();
        friend ostream& operator<<(ostream&, const Databse&);
        Node* getHead() const;
        ~Databsey();
};

Databse.cpp相关方法：

ostream& operator<<(ostream& out, const Databse& obj)
{   
    out << endl << "The databse holds" << obj.nClients << " clients:" << endl;
    out << obj.head->getData();        
    obj.head->print(obj.getHead());
    return out;
}

void Database::Node::print (Node* str)
{
    Node* current = str ;
    while (current->next)
    {
        cout << current->data;
        current = current->next;
    }
}

谢谢。

Answer 1

只需像添加任何其他类一样添加重载的operator<<，例如：

class Databse
{
    private:
        class Node //node as nested class
        {
            public:
                Node();
                void setNode(Client*&);
                Node* nextNode(Node*&);
                Client getData();
                friend ostream& operator<<( ostream&, const Node*& pNode );
            private:
                Client* data; //holds pointer to Client object
                Node* next; //holds pointer to next node in list
        };

        Node *head; //holds the head node
        int nClients;

    public:
        Databse();
        friend ostream& operator<<(ostream&, const Databse&);
        friend ostream& operator<<( ostream&, const Node*& pNode );
        Node* getHead() const;
        ~Databsey();
};

然后实现如下：

ostream& operator<<( ostream& out, const std::add_lvalue_reference<Databse::Node*>::type pNode )
{
    Databse::Node* pCurrNode = pNode;
    while( pCurrNode )
    {
        std::cout << pCurrNode->data;
        pCurrNode = pCurrNode->next;
    }

    return out;
}

Answer 2

修复接受 ostream 的打印

ostream& Database::Node::print (ostream& out, Node* str)
{
  Node* current = str ;
  while (current) //bugfix
  {
    out << current->data;
    current = current->next;
  }
  return out
}

然后你就可以打电话了

ostream& operator<<(ostream& out, const Databse& obj)
{   
  out << endl << "The databse holds" << obj.nClients << " clients:" << endl;
  out << obj.head->getData();        
  obj.head->print(out,obj.getHead());
  return out;
}

Answer 3

另一种方法。

1. 保持Database::Node:print() 非常简单——只打印对象上的数据。
2. 让ostream& operator<<(ostream& out, const Database& obj) 负责遍历列表中的节点并打印每个节点。

这是更新后的代码。

class Database
{
   private:
      class Node //node as nested class
      {
         public:
            Node();
            void setNode(Client*&);
            Node* nextNode(Node*&);

            // Need this to allow iteration of the list of nodes
            // of a Database object.
            Node* nextNode() const
            {
               return next;
            }
            Client getData();

            // Print the data associated with just one node.
            void print(ostream& out) const
            {
               cout << data;
            }

         private:
            Client* data; //holds pointer to Client object
            Node* next; //holds pointer to next node in list
      };

      Node *head; //holds the head node
      int nClients;

   public:
      Database();
      friend ostream& operator<<(ostream&, const Database&);
      Node* getHead() const;
      ~Database();
};

ostream& operator<<(ostream& out, const Database& obj)
{   
    out << endl << "The databse holds" << obj.nClients << " clients:" << endl;

    // Iterate over the nodes of the Database object
    // and print each one.
    Database::Node* current = obj.head;
    for ( ; current != NULL; current = current->nextNode() )
    {
       current->print(out);
       // Printing a whitespace between nodes makes the
       // output more readable.
       out << " ";
    }

    return out;
}