没有默认构造函数？

Question

#include <iostream>
using namespace std;

//To know the car's make, model, color, license number, and the number of minutes the car has been parked
class ParkedCar
{
    string make, model, color, license_number;
    int minutes;
public:
    ParkedCar(string make = "...", string model = "...", string color = "...",
        string license_number = "...", int minutes = 0) : make{ make }, model{model},
        color{color}, license_number{license_number}, minutes{minutes} { this->make = make; }
};

//To know the number of minutes of parking time that has been purchased
class ParkingMeter
{
    int purchased_minutes;
public:
    ParkingMeter(int purchased_minutes) { this->purchased_minutes = purchased_minutes; }
};

//To report the make, model, color, and license number of the illegally parked car
//To report the amount of the fine, which is $25 for the first hour, or part of an
    //hour that the car is illegally parked, plus $10 for every additional hour or
    //part of an hour that the car is illegally parked
//To report the name and badge number of the police officer issuing the ticket
class ParkingTicket
{
    ParkedCar car;
    ParkingMeter meter;
    PoliceOfficer officer;
public:
    ParkingTicket(const ParkedCar &car, const ParkingMeter &meter, const PoliceOfficer &officer)
    {
        this->car = car;
        this->meter = meter;
        this->officer = officer;
    }
};

//To know the officer's name and badge number
//To examine a ParkedCar object and a ParkingMeter object, and determine whether the car's time has expired
//To issue a prking ticket (generate a ParkingTicket object) if the car's time has expired
class PoliceOfficer
{
    string name;
    int badge_number;
    ParkedCar car;
    ParkingMeter meter;
public:
    PoliceOfficer(const ParkedCar &car, const ParkingMeter &meter, string name = "...", int badge_number = 000000)
    {

        this->car = car;
        this->meter = meter;
    }

};

所以我正在为学校制作这个文件，一切都很顺利，直到我上 ParkingTicket 和 PoliceOfficer 课程。我正在尝试定义这些构造函数，它们都告诉我另一个类没有默认构造函数。我将如何解决这个问题？谢谢！ :)

Answer 1

因为你的构造函数

PoliceOfficer(const ParkedCar &car, const ParkingMeter &meter, string name = "...", int badge_number = 000000)
{

    this->car = car;
    this->meter = meter;
}

没有 car 或 meter 的成员初始化程序（主体中的赋值不算作初始化程序），就好像您编写了 : car(), meter() 初始化程序，它试图调用没有的默认构造函数不存在。您应该显式编写成员初始化器：

PoliceOfficer(const ParkedCar &car, const ParkingMeter &meter, string name = "...", int badge_number = 000000)
    : car(car), meter(meter)
{
}

（另外，你的对象模型有点搞笑：警察只有一个停车计时器？）

Answer 2

首先，ParkedCar（如您所见）确实有一个默认构造函数（所有参数都有默认值，因此可以不带参数调用它。

真正的解决方案是改变构造函数使用初始化列表：

   PoliceOfficer(const ParkedCar &car_, const ParkingMeter &meter_, string name = "...", int badge_number = 000000)
   : car(car_), meter(meter_)
   {
   }