c++ - 当有多个构造函数时如何设置默认构造函数

Question

我正在学习 C++ 中的构造函数，我知道您可以声明多个构造函数。您可以在下面看到我下面有 2 个 Date 构造函数，但我想将 Date(long) 设置为默认构造函数。有人可以解释一下我会怎么做吗？

好吧，显然上面的问题很有道理。我正在解决书中的一个问题，这就是它所说的:

Modify Program 10.3 so that the only data member of the class is a long integer named yyyymmdd. Do this by substituting the declaration long yyyymmdd; for these existing declarations:

int month; int day; int year;

Using the same constructor prototypes currently declared in the class declaration section, rewrite them so that the Date(long) method becomes the default constructor, and the Date(int, int, int) method converts a month, day, and year into the correct form for the class data members.

程序 10.3:

#include <iostream>
#include <iomanip>
using namespace std;

class Date
{
    private:
        int month, day, year;
    public:
        Date(int=7, int=4, int=2012);
        Date(long);
        void showDate();
};

Date::Date(int mm, int dd, int yyyy)
{
   month = mm;
   day = dd;
   year = yyyy;
}

Date::Date(long yyyymmdd)
{
    year = int(yyyymmdd/10000);
    month = int( (yyyymmdd - year*10000)/100);
    day = int(yyyymmdd - year*10000 - month*100);
}

void Date::showDate()
{
    cout << "The date is "
         << setfill('0')
         << setw(2) << month << '/'
         << setw(2) << day << '/'
         << setw(2) << year % 100;
    cout << endl;
}

int main()
{
    Date a;
    Date b(4,1,1998);
    Date c = Date(20090515L);

    a.showDate();
    b.showDate();
    c.showDate();
    return 0;
}

Answer 1

您拥有的月/日/年构造函数的默认值使其成为默认构造函数(您可以不带参数调用它。您想要做的是取消这些默认值并将参数默认为 long 构造函数:

    Date(int, int, int);
    Date(long = 20120704);