C++ 重载有关问题

//in file sequence.h-----------------------------
#include<iostream>
typedef int ItemType;
using namespace std;

class Sequence{
friend ostream &operator<<( ostream & out, const Sequence &aSeq);
public:
  /*Default constructor:   constructs an empty        
    Sequence object where:
     capacityS == maxSize (default value 1024); 
itemPtr points to a dynamic array with 
capacityS as its capacity and 
     sizeS is set to 0. ------------*/
   Sequence(int maxSize = 1024);
  /*Destructor:  the memory dynamically allocated     
   by the constructor for the array pointed to by 
   itemPtr has been returned to the heap.-----*/
   ~Sequence();
  /* Copy constructor: constructs a copy of                
     origSeq.-----------------------*/
   Sequence(const Sequence & origSeq);
  
   /* Assignment operator: assigns a copy of the 
rightSeq to the current object. A const 
reference to this Sequence is returned.----*/
   const Sequence & operator=(const Sequence& rightSeq);
     
  /* Returns true if the sequence is empty, false   
if not.----------------*/
   bool empty() const;
  /* The value item is inserted into the sequence at the position 
determined by pos, provided there is room (i.e. sizeS < capacityS) and 
pos is a legal position (i.e. 0 <= pos <= sizeS).159.234-2012 Semester 2
2
   When pos is illegal a message is display and the sequence is left 
unchanged.When there is no room for a new element the program 
terminates.---------*/
   void insert(ItemType item, int pos);
  
   /* The value at the position determined by pos is removed, provided 
pos is a legal position.  Otherwise a message is display and the 
sequence is left unchanged.--------------------------- */
   void erase(int pos);
/* The sequence represented by this Sequence object 
has been inserted 
into out.----------- */ 
   void display(ostream & out) const;
private:
   int sizeS;          // current size of Sequence
   int capacityS;      // current capacity of dynamic array
   ItemType * itemPtr; // pointer to dynamic array
};
//info about the authors of sequence.cpp code
void info();

//Secquence main function
#include <iostream>
#include <cstdlib>
#include "sequence.h"

using namespace std;
//Sequence seq;

Sequence::Sequence(int maxSize){
    capacityS = maxSize;
    sizeS=0;
//    seq.itemPtr=(ItemType*) malloc(itemPtr, maxSize * sizeof(int));    
}

Sequence::~Sequence(){
}

Sequence::Sequence(const Sequence & origSeq){    
}

const Sequence & operator=(const Sequence& rightSeq){
    
}

void Sequence::insert(ItemType item, int pos){
    if(item>capacityS){
        cout<<"Illegal location to insert -- "<<item<<".  Sequence unchanged. ***"<<endl;
    }
    else{
       sizeS++;
       itemPtr[sizeS - 1] = item;
    }
}

bool Sequence::empty() const{
    if(itemPtr==NULL){
        return true;
    }
    else {return false;}
}

//delete the element in the current position
void Sequence::erase (int pos){
    if(pos>capacityS){
        cout<<"Illegal location. Sequence unchanged."<<endl;
    }
    else{
       for(int i=pos; i<sizeS-1; i++){
            itemPtr[i]=itemPtr[i+1];
           sizeS--;
        }
   }
}