为什么复制构造函数被调用两次？

Question

为什么我Queue<listEntry<int> > queue2(intList);被两次调用复制构造函数？

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

template <class dataType>
class listEntry
{

      private:
              dataType data;
      public:
              listEntry *next;
              listEntry *prev;

              dataType getData() { return this->data; }

              listEntry();
              listEntry(dataType data) { this->data = data; }
              ~listEntry() {};    

};

template <class dataType>
class List
{

      private:
              dataType *head;
              dataType *tail;
              int count;
      public:
             dataType *getHead() { return this->head; }
             dataType *getTail() { return this->tail; }

             void addToTail(dataType *newEntry);
             void addToHead(dataType *newEntry);

             int getCount() { return count; }

             void printListForward();

             List();
             List(const List<dataType> &li);
             ~List();  

};

template <class dataType>
class Queue: public List<dataType>
{

      public:
             void enQueue(dataType *newEntry);

             Queue():List<dataType>() { return; }
             Queue(List<dataType> li):List<dataType>(li) { return; }

};


void addIntegersToList(Queue<listEntry<int> > *queue);

int main()
{
    cout << "Queue of Integers: " << endl;
    cout << "Default Queue Constructor: " << endl;
    //Create an intance of a queue using the default queue constructor
    Queue<listEntry<int> > queue;       
    addIntegersToList(&queue);          
    queue.printListForward();
    cout << endl; 

    cout << "List parameter Queue Constructor: " << endl;
    //Create a new list that will be used to initiate an instance of a queue
    List<listEntry<int> > intList;
    //Add some data to that list
    intList.addToTail(new listEntry<int>(11));
    //Now create a new instance of a queue with a list as its creation parameter
    Queue<listEntry<int> > queue2(intList);   
    //addIntegersToList(&queue2);               
    //queue2.printListForward();                

    while (true);   

}

template <class dataType>
List<dataType>::List()
{

    this->head = NULL;
    this->tail = NULL;
    this->count = 0;                      

}

template <class dataType>
void List<dataType>::addToTail(dataType *newEntry)
{

     newEntry->next = NULL;
     newEntry->prev = NULL;

     if (this->head == NULL)
     {

         this->head = newEntry;
         this->tail = this->head;
         this->count = this->count + 1;               

     }
     else
     {

         this->tail->next = newEntry;
         newEntry->prev = this->tail;
         this->tail = newEntry;
         this->count = this->count + 1;    

     }

     dataType *temp = this->tail;

}

template <class dataType>
void List<dataType>::addToHead(dataType *newEntry)
{

     newEntry->next = NULL;
     newEntry->prev = NULL;

     if (this->head == NULL)
     {

         this->head = newEntry;
         this->tail = this->head;              

     }
     else
     {

         this->head->prev = newEntry;
         newEntry->next = this->head;
         this->head = newEntry;

     } 

     this->count = this->count + 1;    

}

template <class dataType>
void List<dataType>::printListForward()
{

    cout << "Head: ";
    dataType *temp = this->head;

    while (temp)
    {

          cout << temp->getData() << " -> ";
          temp = temp->next;      

    }

    cout << "      :Tail" << endl;

}

void addIntegersToList(Queue<listEntry<int> > *queue)
{

     int i = 0;

     for (i = 1; i <= 10; i++)    
         queue->enQueue(new listEntry<int>(i));    

}

template <class dataType>
void Queue<dataType>::enQueue(dataType *newEntry)
{

     this->addToTail(newEntry);     

}

template <class dataType>
List<dataType>::~List()
{

    while (this->head)
    {

          dataType *next = this->head->next;
          delete this->head;
          this->count = this->count - 1;
          this->head = next;      

    }  

}

template <class dataType>
List<dataType>::List(const List<dataType> &li)
{

    tail = new dataType;
    *tail = *li.tail;
    head = new dataType;
    *head = *li.head;

    cout << "head: " << head << endl;
    cout << "tail: " << tail << endl;  

    count = li.count;          
    cout << "count: " << count << endl;

}

template <class dataType>
listEntry<dataType>::listEntry()
{

    this->next = NULL;
    this->prev = NULL;
    this->data = 0;                      

}

Answer 1

因为您正在按值将List传递给Queue构造函数。试试这个

Queue(const List<dataType>& li):List<dataType>(li) { return; }