Link time error happened on C++ class codes [duplicate] - c++

Question

This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Undefined reference to WinMain (C++ MinGW)
(5 answers)
Closed last month.
In date.h, I wrote:
#ifndef DATE_H
#define DATE_H
#include <iostream>
#include <ctime>
class Date
{
int year,month,day;
public:
Date();
Date(int y, int m, int d)
: year(y), month(m), day(d) {}
~Date() = default;
int getYear() { return year; }
int getMonth() { return month; }
int getDay() { return day; }
bool isLeapyear() { return (year % 400 == 0) || (year % 4 == 0 & year % 100 != 0); }
void print() { std::cout << year << "." << month << "." << day << std::endl; }
};
#endif // DATE_H
and in date.cpp, I wrote:
#include "date.h"
Date::Date()
{
time_t ltime = time(NULL);
tm *today = localtime(&ltime);
year = today->tm_year + 1900;
month = today->tm_mon + 1;
day = today->tm_mday;
}
There are no compiling error with this code, but I got a link time error when I tried to run this code:
undefined reference to `WinMain'
collect2.exe: error: ld returned 1 exit status
What has caused the error and how can I fix it?

Answer 1

As other commented the problem is that you created a class which is not executable by itself. You created a library that you can use in other C++ programs, so it compiles fine but it cannot run by itself, that's why you get the error while trying to run the compiled binary. In C++ you have to execute the code of the classes from a main function.
The simplest way to fix it would be to add main.cpp:
#include "date.h"
int main(void){
Date d = Date(2023, 1, 23);
d.print();
}