Redefinition of class constructor error - c++

Question

I am trying to compile my code and I keep getting the error:
redefinition of 'SimpleDate::SimpleDate(int, int, int)
I think the error is usually because of not adding #ifndef, #define, and #endif but I did add those.
simple_date.h:
#ifndef SIMPLE_DATE_H
#define SIMPLE_DATE_H
#include <string>
class SimpleDate
{
int _year, _month, _day;
public:
// create a new 'SimpleDate' object with the given year, month, and day
// throws an invalid_argument error if the date is invalid
SimpleDate(int year, int month, int day) {};
// returns the current year
int year() const { return 0; };
// returns the current month
int month() const { return 0; };
// returns the current day of the month
int day() const { return 0; };
// return string formatted as year-month-day with day an month 0 prefixed
// i.e. 2000-01-01 is Jan 01, 2000
std::string to_string() const { return ""; };
// comparison operators
bool operator==(const SimpleDate& lhs) const { return true; };
bool operator!=(const SimpleDate& lhs) const { return true; };
bool operator> (const SimpleDate& lhs) const { return true; };
bool operator< (const SimpleDate& lhs) const { return true; };
bool operator>=(const SimpleDate& lhs) const { return true; };
bool operator<=(const SimpleDate& lhs) const { return true; };
// returns 'true' if current year is a leap year
bool is_leap() const { return true; };
// returns the day of the week; 0 = Sunday
// HINT: To calculate the day of the week, you need to have a known day. Use
// 1970-01-01 which was a Thursday. Make sure to reference where you found
// the algorithm or how you came up with it.
int wday() const { return 0; };
// returns the day of the year; Jan 1st = 1
int yday() const { return 0; };
// add one day to current date
SimpleDate& incr_day() { SimpleDate dd {2016, 1, 1}; };
SimpleDate& operator++() { SimpleDate dd {2016, 1, 1}; };
// add n day(s) to current date
SimpleDate& operator+=(int n) { SimpleDate dd {2016, 1, 1}; };
// subtract one day from current date
SimpleDate& decr_day() { SimpleDate dd {2016, 1, 1}; };
SimpleDate& operator--() { SimpleDate dd {2016, 1, 1}; };
// subtract n day(s) from current date
SimpleDate& operator-=(int n) { SimpleDate dd {2016, 1, 1}; };
// add one month to current date
SimpleDate& incr_month() { SimpleDate dd {2016, 1, 1}; };
// subtract one month from current date
SimpleDate& decr_month() { SimpleDate dd {2016, 1, 1}; };
// add one year to current date
SimpleDate& incr_year() { SimpleDate dd {2016, 1, 1}; };
// subtract one year from current date
SimpleDate& decr_year() { SimpleDate dd {2016, 1, 1}; };
// returns 'true' if current date is a Monday
bool is_monday() const { return true; };
// returns 'true' if current date is a Tuesday
bool is_tuesday() const { return true; };
// returns 'true' if current date is a Wednesday
bool is_wednesday() const { return true; };
// returns 'true' if current date is a Thursday
bool is_thursday() const { return true; };
// returns 'true' if current date is a Friday
bool is_friday() const { return true; };
// returns 'true' if current date is a Saturday
bool is_saturday() const { return true; };
// returns 'true' if current date is a Sunday
bool is_sunday() const { return true; };
};
#endif
simple_date.cpp:
#include "simple_date.h"
SimpleDate::SimpleDate(int year, int month, int day)
{
std::cout >> year;
if (year < 1970 || year > 2020)
{
throw invalid_argument("Don't care about years less than 1970 or greater than 2020");
}
else
{
_year = year;
}
if (month < 1 || month > 12)
{
throw invalid_argument("Not a real month");
}
else
{
_month = month;
}
if (day < 1 || day>31)
{
throw invalid_argument("Not a real day");
}
else if ((_month == 2 || _month == 4 || _month == 6 || _month == 9 || _month == 11) && day > 30)
{
throw invalid_argument("Not a real day for this month");
}
else if (_month == 2 && day == 29)
{
if (this.is_leap())
_day = day;
else
hrow invalid_argument("Not a real day for this month and year");
}
else
_day = day;
}

Answer 1

You defined the constructor in the header file, here:
SimpleDate(int year, int month, int day) {};
Those innocent-looking braces turned this declaration into a definition. And then compiler got very upset when it later encountered the real definition of this constructor. Just remove them:
SimpleDate(int year, int month, int day);