关于抽象类new的问题

[qq_41780568]

//=====================
//idate.h
//=====================
#ifndef IDATE_H_INCLUDED
#define IDATE_H_INCLUDED
#include <iostream>
using namespace std;
class IDate
{
protected:
          virtual int ymd2i() = 0 ;
public:
          virtual ~IDate(){}
          virtual IDate& operator+(int n) = 0 ;
          int operator-( IDate& d)
          {
               return ymd2i()-d.ymd2i();
          }
          virtual IDate& operator+=(int n) = 0;
          virtual IDate& operator++() = 0;
          virtual void print(ostream& o)const = 0 ;
};
IDate& createDate(int y,int m,int d);
IDate& createDate(int n);
IDate& createDate(const string s);
inline ostream& operator<<(ostream& o,const IDate& d)
{
          d.print(o);
          return o;
}
#endif // IDATE_H_INCLUDED

//====================
//date.h
//====================
#ifndef DATE_H_INCLUDED
#define DATE_H_INCLUDED
#include "idate.h"
#include <iostream>
#include <iomanip>
using namespace std;
class Date : public IDate
{
     int year;
     int month;
     int day;
protected:
     int ymd2i()const ;
     void i2ymd(int n);
     static const int tians[];
     bool isLeapyear()const
     {
          return !(year%4) && (year%100) || !(year%400);
     }
public:
     Date(const string s);
     Date(int n)
     {
          i2ymd(n);
     }
     Date(int y,int m,int d):year(y),month(m),day(d){}
     ~Date(){}
     void print(ostream& o)const;
     Date& operator + (int n)
     {
          return *new Date(ymd2i()+n);
     }
     Date& operator+=(int n)
     {
          i2ymd(ymd2i()+n);
          return *this;
     }
     Date& operator++()
     {
          return *this += 1;
     }
};


#endif // DATE_H_INCLUDED

//===============
//date.cpp
//===============
#include "date.h"
#include <stdlib.h>
#include <iostream>
#include <iomanip>
using namespace std;
const int Date::tians[]= {0,31,59,120,150,181,212,242,273,303,334};
Date::Date(const string s)
{
    year = atoi(s.substr(0,4).c_str());
    month = atoi(s.substr(5,2).c_str());
    day = atoi(s.substr(8,2).c_str());
}
void Date ::i2ymd(int absDay)
{
     absDay = absDay > 0 && absDay < 3650000 ? absDay : 1;
     int n = absDay;
     for(year = 1;n>isLeapyear()+365;n-=isLeapyear()+365,year++);
     for(month = 1;(month < 13 && n>(isLeapyear()&&month > 2))+tians[month];month++)
          day = n - (isLeapyear() && month > 2) - tians[month - 1];
}
int Date::ymd2i()const
{
     int absDay = (year-1)*365 + (year - 1)/4 - (year-1)/100 + (year-1)/400;
     return absDay += tians[month - 1] + (isLeapyear() && month > 2) + day;
}
void Date::print(ostream& o)const
{
     o << setfill('0') << setw(4) << year << "-"
      << setw(2) << month << '-' << setw(2) << day << "\n" << setfill(' ');
}
IDate& createDate(int y,int m,int d)
{
     return  *new Date(y,m,d);
}
IDate& createDate(int n)
{
     return  *new Date(n);
}
IDate& createDate(const string s)
{
     return *new Date(s);
}

//======================
//main.cpp
//======================
#include "date.h"
#include <iostream>
void fn(IDate& d1,IDate& d2)
{
    std::cout << d2 - d1 << endl;
    std::cout << ++d;
}
int main()
{
    IDate& rd1 = createDate(2005,1,6);
    IDate& rd2 = createDate(2005,2,3);
    fn(rd1,rd2);
    delete &rd1;
    delete &rd2;
}

源代码如上，编译时四个出现new的地方都报错了:error: invalid new-expression of abstract class type ‘Date’
就是说抽象类不允许new表达式，网上说我有纯虚函数没有实现，我不太明白，idate.h里面五个纯虚函数都实现了啊，请问是哪里的问题。|

[paschen]

抽象类（即带有纯虚函数的类）不能创建类对象，纯虚函数比如你上面的： virtual IDate& operator+(int n) = 0 ;

[mstlq]

楼主编码器太古老？

Date& operator + (int n)
     {
          return *new Date(ymd2i()+n);
     }
     Date& operator+=(int n)
     {
          i2ymd(ymd2i()+n);
          return *this;
     }
     Date& operator++()
     {
          return *this += 1;
     }

改成

IDate& operator + (int n)
     {
          return *new Date(ymd2i()+n);
     }
     IDate& operator+=(int n)
     {
          i2ymd(ymd2i()+n);
          return *this;
     }
     IDate& operator++()
     {
          return *this += 1;
     }

试试？

[wallesyoyo]

你的IDate里面的纯虚函数ymd2i，是非const类型的，而你Date里面的实现版本是const类型的。所以你没有实现IDate里面给出的非const版本的纯虚函数ymd2i，你的Date还是被认为是抽象类。 把你IDate里面的ymd2i加上const。

[mstlq]

看漏cv限定符号，3楼说的对