C++类组合 - 何时执行构造函数和析构函数

请帮忙！我知道这个问题很长，但我真的没有人可以帮助我理解这个概念。

我有一本书中的代码，但我真的根本无法理解输出。该代码是关于两个类组合的示例，以及构造函数和析构函数的执行顺序。

很抱歉冗长的例子：

// Fig. 10.8: Date.h
// Date class definition; Member functions defined in Date.cpp
#ifndef DATE_H
#define DATE_H
class Date
{
    public:
    static const unsigned int monthsPerYear = 12; // number of months in a year
    explicit Date( int = 1, int = 1, int = 1900 ); // default constructor
    void print() const; // print date in month/day/year format
    ~Date(); // provided to confirm destruction order
private:
    unsigned int month; // 1-12 (January-December)
    unsigned int day; // 1-31 based on month
    unsigned int year; // any year
    // utility function to check if day is proper for month and year
    unsigned int checkDay( int ) const;
}; // end class Date
#endif
// Fig. 10.9: Date.cpp
// Date class member-function definitions.
#include <array>
#include <iostream>
#include <stdexcept>
#include "Date.h" // include Date class definition
using namespace std;
// constructor confirms proper value for month; calls
// utility function checkDay to confirm proper value for day
Date::Date( int mn, int dy, int yr )
{
    if ( mn > 0 && mn <= monthsPerYear ) // validate the month
        month = mn;
    else
        throw invalid_argument( "month must be 1-12" );
    year = yr; // could validate yr
    day = checkDay( dy ); // validate the day
    // output Date object to show when its constructor is called
    cout << "Date object constructor for date ";
    print();
    cout << endl;
} // end Date constructor
    // print Date object in form month/day/year
void Date::print() const
{
    cout << month << '/' << day << '/' << year;
} // end function print
// output Date object to show when its destructor is called
Date::~Date()
{
    cout << "Date object destructor for date ";
    print();
    cout << endl;
} // end ~Date destructor
// utility function to confirm proper day value based on // month and     year; handles leap years, too
unsigned int Date::checkDay( int testDay ) const
{
    static const array < int, monthsPerYear + 1 > daysPerMonth =
        { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
    // determine whether testDay is valid for specified month
    if ( testDay > 0 && testDay <= daysPerMonth[ month ] )
        return testDay;
    // February 29 check for leap year
    if ( month == 2 && testDay == 29 && ( year % 400 == 0 || ( year % 4 == 0 && year % 100 != 0 ) ) )
        return testDay;
    throw invalid_argument( "Invalid day for current month and year" );
} // end function checkDay
// Fig. 10.10: Employee.h
// Employee class definition showing composition. // Member functions     defined in Employee.cpp.
#ifndef EMPLOYEE_H
#define EMPLOYEE_H
#include <string>
//#include "Date.h" // include Date class definition using namespace std;
class Employee
{
public:
    Employee( const string &, const string &, const Date &, const Date & );
    void print() const;
    ~Employee(); // provided to confirm destruction order
private:
    string firstName; // composition: member object
    string lastName; // composition: member object
    const Date birthDate; // composition: member object
    const Date hireDate; // composition: member object 23
}; // end class Employee
#endif
// Fig. 10.11: Employee.cpp
// Employee class member-function definitions.
#include <iostream>
#include "Employee.h" // Employee class definition
#include "Date.h" // Date class definition
using namespace std;
// constructor uses member initializer list to pass initializer
// values to constructors of member objects
Employee::Employee( const string &first, const string &last, const Date &dateOfBirth, const Date &dateOfHire )
: firstName( first ), lastName( last ), birthDate( dateOfBirth ), hireDate( dateOfHire )
{
    // output Employee object to show when constructor is called
    cout << "Employee object constructor: " << firstName << ' ' << lastName << endl;
} // end Employee constructor
// print Employee object
void Employee::print() const
{
    cout << lastName << ", " << firstName << " Hired: ";
    hireDate.print();
    cout << " Birthday: ";
    birthDate.print();
    cout << endl;
} // end function print
// output Employee object to show when its destructor is called
Employee::~Employee()
{
    cout << "Employee object destructor: " << lastName << ", " << firstName << endl;
} // end ~Employee destructor
// Fig. 10.12: fig10_12.cpp
// Demonstrating composition--an object with member objects.
#include <iostream>
//#include "Employee.h" // Employee class definition
using namespace std;
int main()
{
    Date birth( 7, 24, 1949 );
    Date hire( 3, 12, 1988 );
    Employee manager( "Bob", "Blue", birth, hire );
    cout << endl;
    manager.print();
}//endmain

我知道代码太长了，但真的想不出更容易做到这一点的方法。输出如下。

Date object constructor for date 7/24/1949
Date object constructor for date 3/12/1988
Employee object constructor: Bob Blue
Blue, Bob  Hired: 3/12/1988  Birthday: 7/24/1949
Employee object destructor: Blue, Bob
Date object destructor for date 3/12/1988
Date object destructor for date 7/24/1949
Date object destructor for date 3/12/1988
Date object destructor for date 7/24/1949

现在，我的问题是...为什么日期对象的构造函数只执行两次，而析构函数执行四次？据我了解，Date 对象的构造函数也应该执行四次： 1. 在"出生日期（7， 24， 1949）"上 2. 在"日期雇用（3， 12， 1988）"在主要 3. 在员工中的"连续出生日期"上。 4. 在员工.h 中的"持续日期雇用日期"

我的 #3 和 #4 的原因是我无法想象 Employee 对象构造函数在没有相同类型的空变量（或在这种情况下为 Date 对象）的情况下复制"出生"和"雇用"。因此，我的理解是，在 Employee 标头中，在创建私有成员 Date 对象时，应为每个对象执行一次显式默认 Date 构造函数（将 1/1/1900 作为成员变量 mn、dy、yr）。