使用成员布尔函数比较对象数组数据的问题排查

需要用Person类的operator==成员函数比较对象数组中人员的出生日期，同时利用operator<成员函数找出出生日期最早的人员，但程序仅输出数组内容后便退出，无法实现预期功能，尝试编写operator<逻辑未成功，附上代码：

#include <iostream>
#include <string>

using namespace std;

class Birth_date
{
private:
    unsigned int day{};
    unsigned int month{};
    unsigned int year{};

public:
    Birth_date() {}
    
    Birth_date(int day, int month, int year)
    {
        this->day = day;
        this->month = month;
        this->year = year;
    }
    
    Birth_date(const Birth_date& birth) 
    {
        day = birth.day;
        month = birth.month;
        year = birth.year;
    }

    void set_day(int day)
    {
        this->day = day;
    }
    void set_month(int month)
    {
        this->month = month;
    }
    void set_year(int year)
    {
        this->year = year;
    }

    int get_day()
    {
        return day;
    }
    int get_month()
    {
        return month;
    }
    int get_year()
    {
        return year;
    }

    friend bool operator == (const Birth_date& a, const Birth_date& b)
    {
        return (a == b);
    }
    bool operator < (const Birth_date& bd)
    {
        return day < bd.day && month < bd.month && year < bd.year;
    }
    bool operator > (const Birth_date& bd)
    {
        return day > bd.day && month > bd.month && year > bd.year;
    }
    
    ostream& Output(ostream& out)
    {
        out << "Date of birth: " << day << "/" << month << "/" << year << endl;
        return out;
    }
};

class Person
{
private:
    string name;
    Birth_date dob;

public:
    Person() {}

    Person(string name, Birth_date dob)
    {
        this->name = name;
        this->dob = dob;
    }

    Person(const Person& p)
    {
        name = p.name;
        dob = p.dob;
    }

    string get_name()
    {
        return name;
    }
    Birth_date get_dob()
    {
        return dob;
    }

    bool operator < (const Person& p)const
    {
        Birth_date bd1 = p.dob;
        Birth_date bd2 = p.dob;

        return (bd1 < bd2);
    }
    bool operator == (const Person& p)const
    {
        return (dob == p.dob);
    }

    const ostream& output(ostream& out)
    {
        out << "Name: " << name << "\t" ;
        dob.Output(out);
        return out;
    }

};

int main()
{
    Person students[5] = { Person("Ivan Petkov",Birth_date(10,6,1999)),
                           Person("Gabe Trent",Birth_date(20,12,1996)),
                           Person("Maggy Sommer",Birth_date(5,2,2000)),
                           Person("Cameron Dallas",Birth_date(1,4,2001)),
                           Person("Catherine Crumb",Birth_date(28,8,2000)) };

    for (int i = 0; i < 5; i++)
    {
        students[i].output(cout);
    }

    cout << "\n";


    for (int i = 0; i < 5; i++)
    {
        for (int x = i + 1; x < 5; x++)
        {
            if (students[i].operator==(students[x]))
            {
                cout << "Students " << students[i].get_name()
                    << "and " << students[x].get_name()
                    << "have the same date of birth." << endl;
            }
            else
                cout << "Nobody has the same date of birth." << endl;
        }
    }

    return 0;
}

1. Birth_date类operator==递归死循环 #

原代码中operator==直接返回a == b，会无限递归调用自身，导致程序崩溃。正确实现应该逐一比较年、月、日：

friend bool operator == (const Birth_date& a, const Birth_date& b)
{
    return (a.year == b.year && a.month == b.month && a.day == b.day);
}

2. Birth_date类operator<逻辑错误 #

原代码用&&连接年月日的比较，只有当前对象的年、月、日全部小于另一个对象时才返回true，这不符合日期比较逻辑。正确逻辑是先比年份，年份小则日期更早；年份相同比月份，月份小则更早；月份相同比日期：

bool operator < (const Birth_date& bd) const // 加上const保证常量对象可调用
{
    if (year != bd.year)
        return year < bd.year;
    if (month != bd.month)
        return month < bd.month;
    return day < bd.day;
}

同时给函数加上const修饰，确保常量对象能调用该运算符。

3. Person类operator<逻辑错误 #

原代码中bd1和bd2都是p.dob，比较同一个对象永远返回false，正确应该比较当前对象的dob和参数p的dob：

bool operator < (const Person& p)const
{
    return this->dob < p.dob;
}

4. 主函数重复输出问题 #

原双重循环中，每对不相等的学生都会输出“Nobody has the same date of birth”，导致大量冗余输出。应该先标记是否找到相同出生日期的学生，循环结束后统一提示：

bool hasSameDob = false;
for (int i = 0; i < 5; i++)
{
    for (int x = i + 1; x < 5; x++)
    {
        if (students[i] == students[x]) // 直接用==运算符，无需显式调用operator==
        {
            cout << "Students " << students[i].get_name()
                 << " and " << students[x].get_name()
                 << " have the same date of birth." << endl;
            hasSameDob = true;
        }
    }
}
if (!hasSameDob)
    cout << "Nobody has the same date of birth." << endl;

5. 添加寻找最早出生日期的逻辑 #

遍历数组，用operator<找到最早的学生：

Person earliestStudent = students[0];
for (int i = 1; i < 5; i++)
{
    if (students[i] < earliestStudent)
    {
        earliestStudent = students[i];
    }
}
cout << "\nStudent with earliest date of birth:" << endl;
earliestStudent.output(cout);

#include <iostream>
#include <string>

using namespace std;

class Birth_date
{
private:
    unsigned int day{};
    unsigned int month{};
    unsigned int year{};

public:
    Birth_date() {}
    
    Birth_date(int day, int month, int year)
        : day(static_cast<unsigned int>(day)), 
          month(static_cast<unsigned int>(month)), 
          year(static_cast<unsigned int>(year)) {}
    
    Birth_date(const Birth_date& birth) 
        : day(birth.day), month(birth.month), year(birth.year) {}

    void set_day(int day) { this->day = static_cast<unsigned int>(day); }
    void set_month(int month) { this->month = static_cast<unsigned int>(month); }
    void set_year(int year) { this->year = static_cast<unsigned int>(year); }

    unsigned int get_day() const { return day; }
    unsigned int get_month() const { return month; }
    unsigned int get_year() const { return year; }

    friend bool operator == (const Birth_date& a, const Birth_date& b)
    {
        return (a.year == b.year && a.month == b.month && a.day == b.day);
    }
    
    bool operator < (const Birth_date& bd) const
    {
        if (year != bd.year)
            return year < bd.year;
        if (month != bd.month)
            return month < bd.month;
        return day < bd.day;
    }
    
    bool operator > (const Birth_date& bd) const
    {
        return bd < *this;
    }
    
    ostream& Output(ostream& out) const
    {
        out << "Date of birth: " << day << "/" << month << "/" << year;
        return out;
    }
};

class Person
{
private:
    string name;
    Birth_date dob;

public:
    Person() {}

    Person(string name, Birth_date dob)
        : name(move(name)), dob(dob) {}

    Person(const Person& p)
        : name(p.name), dob(p.dob) {}

    string get_name() const { return name; }
    Birth_date get_dob() const { return dob; }

    bool operator < (const Person& p) const
    {
        return dob < p.dob;
    }
    
    bool operator == (const Person& p) const
    {
        return dob == p.dob;
    }

    ostream& output(ostream& out) const
    {
        out << "Name: " << name << "\t";
        dob.Output(out);
        out << endl;
        return out;
    }
};

int main()
{
    Person students[5] = { 
        Person("Ivan Petkov", Birth_date(10,6,1999)),
        Person("Gabe Trent", Birth_date(20,12,1996)),
        Person("Maggy Sommer", Birth_date(5,2,2000)),
        Person("Cameron Dallas", Birth_date(1,4,2001)),
        Person("Catherine Crumb", Birth_date(28,8,2000)) 
    };

    cout << "All students:" << endl;
    for (const auto& student : students)
    {
        student.output(cout);
    }

    cout << "\nChecking for same date of birth:" << endl;
    bool hasSameDob = false;
    for (int i = 0; i < 5; i++)
    {
        for (int x = i + 1; x < 5; x++)
        {
            if (students[i] == students[x])
            {
                cout << "Students " << students[i].get_name()
                     << " and " << students[x].get_name()
                     << " have the same date of birth." << endl;
                hasSameDob = true;
            }
        }
    }
    if (!hasSameDob)
        cout << "Nobody has the same date of birth." << endl;

    cout << "\nStudent with earliest date of birth:" << endl;
    Person earliestStudent = students[0];
    for (int i = 1; i < 5; i++)
    {
        if (students[i] < earliestStudent)
        {
            earliestStudent = students[i];
        }
    }
    earliestStudent.output(cout);

    return 0;
}

内容的提问来源于stack exchange，提问作者Heiru