赋值运算符重载，内存重复释放

问题遇到的现象和发生背景

在练习赋值运算符重载，重复释放问题

问题相关代码，请勿粘贴截图

#include <iostream>
using namespace std;
class Person
{
    friend ostream &operator<<(ostream &cout, Person p);

public:
    Person(int age, double height);
    ~Person();
    Person &operator=(Person &p);

private:
    int m_age;
    double m_height;
    int *m_grade;
};
Person::Person(int age, double height)
{
    m_age = age;
    m_height = height;
    m_grade = new int(100);
}
Person::~Person()
{
    if (m_grade != NULL)
    {
        delete m_grade;
        m_grade = NULL;
    }
}
ostream &operator<<(ostream &cout, Person p)
{
    cout << "age=" << p.m_age << "\t"
         << "height=" << p.m_height << "\tgrade=" << *p.m_grade;

    return cout;
}
Person &Person::operator=(Person &p)
{
    this->m_age = p.m_age;
    this->m_height = p.m_height;
    if (this->m_grade != NULL)
    {
        delete this->m_grade;
        this->m_grade = NULL;
    }
    this->m_grade = new int(*p.m_grade);
    return *this;
}
int main()
{
    Person p1(19, 179.8);
    Person p2(15, 170);
    p2 = p1;
    cout << p2 << endl;

    return 0;
}

运行结果及报错内容

重复释放内存

我的解答思路和尝试过的方法

运算符重载，重新开辟内存

我想要达到的结果

正常运行

一、你没有提供拷贝构造函数
二、你的operator<< 参数应该是const 引用而不是值

可以查看手册：c++-运算符重载 中的内容