运算符重载中类定义的对象前置的++可以输出

运算符重载中 类定义的对象前置的++可以输出，后置++输出不了，请问是为什么啊？

c++

#include <iostream>
#include <string>
using namespace std;

class my_integer
{
    friend ostream& operator<< (ostream& cout, my_integer& i);
public:
    my_integer()
    {
        m_num = 0;
    }
    my_integer& operator++ ()
    {
        this->m_num++;
        return *this;
    }
    my_integer operator++ (int)
    {
        my_integer temp = *this;
        m_num++;
        return temp;
    }
private:
    int m_num;
};
 
ostream& operator<< (ostream& cout, my_integer& i)
{
    cout << "m_num:  " << i.m_num;
    return cout;
}

void test01()
{
    my_integer i;
    cout << ++i<< endl;
    cout << i << endl;
}
void test02()
{
    my_integer j;
    
    cout << j++ << endl;                        // 为什么输出不了j++啊？？？
    cout << j << endl;
}
int main()
{
    test01();
    test02();
    system("pause");
    return 0;
}

```****

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