切片问题

题目描述

如果一个基类引用实际上引用的是一个子类对象，对这个基类引用使用另一个子类对象进行赋值，则不会发生完整的子类到子类的赋值，而是只会发生基类间的赋值。由于发生了不完整的成员复制，仿佛是从完整的类中切下一片，因此被称为切片问题（Slicing problem）。

完成 Student 类的复制构造函数、复制赋值运算符重载和 Clone 成员函数，使程序获得符合预期的输出。当子类尝试 Clone 的对象并不是真正的子类对象时，将全局变量 error 置为 true。

关于输入

无

关于输出

见样例输出

参考答案

#include <iostream>

bool error = false;

class Person
{
    int age_;
public:
    Person(int age) : age_{ age } {}
    Person(const Person& another) : age_{ another.age_ } {}
    Person& operator=(const Person& another)
    {
        // 复制代价低，省略自赋值判断。
        age_ = another.age_;
        return *this;
    }
    virtual Person& Clone(const Person& another)
    {
        return operator=(another);
    }
    int GetAge() const { return age_; }
    virtual ~Person() = default;
};

class Student : public Person
{
    int studentID_;
public:
    Student(int age, int studentID) :
        Person{ age }, studentID_{ studentID }
    {
    }

    int GetID() const { return studentID_; }
    Student(const Student& another) : Person { another }, studentID_{ another.studentID_ } {}
    Student& operator=(const Student& another)
    {
        Person::operator=(another);
        studentID_ = another.studentID_;
        return *this;
    }
    virtual Person& Clone(const Person& another) override
    {
        if (dynamic_cast<const Student*>(&another) == nullptr)
        {
            error = true;
            return *this;
        }
        else {
            return operator=(dynamic_cast<const Student&>(another));
        }
    }
};

int main()
{
    const Person person{ 35 };
    Student student{ 15, 10086 };

    { // 测试Student之间的复制
        Student student2{ student };
        std::cout << student2.GetAge() << " " << student2.GetID() << "\n";
        Student student3{ 13, 10085 };
        student2 = student3;
        std::cout << student2.GetAge() << " " << student2.GetID() << "\n";
    }

    { // 测试Student向Person的复制和Clone
        Student student3{ 13, 10085 };

        Person& studentRef = student;
        studentRef = student3;

        std::cout << student.GetAge() << " " << student.GetID() << "\n";
        studentRef.Clone(student3);
        std::cout << student.GetAge() << " " << student.GetID() << "\n";
    }

    { // 测试错误复制
        std::cout << error << "\n";
        student.Clone(person);
        std::cout << student.GetAge() << " " << student.GetID() << "\n";
        std::cout << error << "\n";
    }

    return 0;
}