Question

I am a beginner with OOP->I have a class Date with 3 private variable members and a should print the date in 2 ways:

1. 12/25/2010
2. December 25 2010

The following code it gives the error:

date.obj : error LNK2019: unresolved external symbol “public: __thiscall Date::Date(void)” (??0Date@@QAE@XZ) referenced in function “public: void __thiscall Date::printDate(void)” (?printDate@Date@@QAEXXZ)
What i am doing wrong?
date.h

#include<iostream>
#include<string>
#ifndef DATE_H
#define DATE_H
class Date
{
private:
    int day;
    int month;
    int year;
public:
    Date();
    Date(int d, int m, int y)
    {
        day=d;
        month=m;
        year=y;
    }
    int getDay() const {return day;}
    int getMonth() const {return month;}
    int getYear() const {return year;}
    void printDate(void);
};
#endif

date.cpp

#include"date.h"
#include<iostream>
#include<string>
const int NR=12;
void Date::printDate()
{
    Date newDate;
    std::string Months[]={"January","February", "March" , "April", "May", "June", "July", "August", "September", "Octomber", "November", "December"};
    int position;
    std::string month;
    position=newDate.getMonth();
    for(int i=0;i<NR;i++)
    {
        if(i==position)
        {
            month=Months[i];
        }
    }
    std::cout<<month<<" "<<newDate.getDay()<<" "<<newDate.getYear()<<std::endl;
}

main.cpp

#include "date.h"
#include <iostream>
int main()
{
    int d;
    int m;
    int y;
    std::cout<<"Enter day: ";
    std::cin>>d;
    std::cout<<"Enter month: ";
    std::cin>>m;
    std::cout<<"Enter years: ";
    std::cin>>y;
    Date newDate(d,m,y);
    std::cout<<newDate.getMonth()<<"/"<<newDate.getDay()<<"/"<<newDate.getYear()<<std::endl;
    newDate.printDate();
}

Answer 1

The error is quite clear: you declared constructors for your Date class but didn’t define them inside the cpp file.

You should add definitions for these constructors. They could just look like this:

Date::Date() {}

or maybe

Date::Date() {
  d = 1;
  m = 1;
  y = 1970;
}

which at least wouldn’t print nonsense if you call

Date myDate;
myDate.printDate();

EDIT:

As suggested by Mat, you should use constructor initializer lists when you can. Your other constructor which uses parameters would look like this with an initialization list:

Date(int d, int m, int y) : 
   day(d), month(m), year(y) {}

In your case, your constructor calls the empty constructor on day, month and year and then assigns values to them, whereas when using initialization lists, the Date constructor calls constructors with parameters for day, month and year.