今天，練習程式設計時，照著書本敲程式碼，居然出現了問題，太詭異了。而且出現的是：error LNK2019: 無法解析的外部符號

這樣詭異的問題，著實困擾了我10分鐘。然後我找到了解決辦法——補充預設不帶引數的建構函式的函式體。

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方法如下：

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1.出問題的程式碼如下

#include "stdafx.h"
#include <iostream>


using namespace std;

class DayofYear
{
public:
	DayofYear(int the_month, int the_day);

	DayofYear();

	void input();

	void output();

	int get_month();

	int get_day();

private:
	void check_date();
	int month;
	int day;
};

bool equal(DayofYear date1, DayofYear date2);

int _tmain(int argc, _TCHAR* argv[])
{
	DayofYear today, bach_birthday(3, 21);
	cout << "Enter today's date: \n";
	today.input();
	cout << "Today's date is ";
	today.output();
	cout << "J. S. Bach's birthday is ";
	bach_birthday.output();
	if ( equal(today, bach_birthday))
	{
		cout << "Happy Birthday Johann Sebastian!\n";
	}
	else
	{
		cout << "Happy Unbirthday Johann Sebastian!\n";

	}

	system("pause");
	return 0;
}

bool equal(DayofYear date1, DayofYear date2)
{
	return ( date1.get_month() == date2.get_month() &&
		date1.get_day() == date2.get_day() );
}

DayofYear::DayofYear(int the_month, int the_day)
	: month(the_month), day(the_day)
{
	check_date();

}

//DayofYear::DayofYear()
//{
//
//}

int DayofYear::get_month()
{
	return month;
}

int DayofYear::get_day()
{
	return day;
}

void DayofYear::input()
{
	cout << "Enter the month as a number: ";
	cin >> month;
	cout << "Enter the day of the month: ";
	cin >> day;
}

void DayofYear::output()
{
	cout << "month = " << month
		<< ", day = " << day << endl;
}


void DayofYear::check_date()
{
	
}
 

2.解決問題的程式碼如下，只改了一個地方，就是去掉上面程式碼的註釋

// ConsoleApplication1.cpp : 定義控制檯應用程式的入口點。
//

#include "stdafx.h"
#include <iostream>


using namespace std;

class DayofYear
{
public:
	DayofYear(int the_month, int the_day);

	DayofYear();

	void input();

	void output();

	int get_month();

	int get_day();

private:
	void check_date();
	int month;
	int day;
};

bool equal(DayofYear date1, DayofYear date2);

int _tmain(int argc, _TCHAR* argv[])
{
	DayofYear today, bach_birthday(3, 21);
	cout << "Enter today's date: \n";
	today.input();
	cout << "Today's date is ";
	today.output();
	cout << "J. S. Bach's birthday is ";
	bach_birthday.output();
	if ( equal(today, bach_birthday))
	{
		cout << "Happy Birthday Johann Sebastian!\n";
	}
	else
	{
		cout << "Happy Unbirthday Johann Sebastian!\n";

	}

	system("pause");
	return 0;
}

bool equal(DayofYear date1, DayofYear date2)
{
	return ( date1.get_month() == date2.get_month() &&
		date1.get_day() == date2.get_day() );
}

DayofYear::DayofYear(int the_month, int the_day)
	: month(the_month), day(the_day)
{
	check_date();

}

DayofYear::DayofYear()//是的，沒錯，就是這裡！！！！！！！！！！！
{

}

int DayofYear::get_month()
{
	return month;
}

int DayofYear::get_day()
{
	return day;
}

void DayofYear::input()
{
	cout << "Enter the month as a number: ";
	cin >> month;
	cout << "Enter the day of the month: ";
	cin >> day;
}

void DayofYear::output()
{
	cout << "month = " << month
		<< ", day = " << day << endl;
}


void DayofYear::check_date()
{
	
}
 

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