STL string input / output overload 2

1、 Custom header file （Mystring.h）

//#pragma once
#ifndef _MYSTRING
#define _MYSTRING

#include<iostream>
using namespace std;
class stu
{
private:
    // String first address
    char* m_str;
    // The number of strings
    size_t m_l;
public:
    stu();// No arguments structure .
    stu(size_t length ,char ch);// take length Characters ch, Assign into object . 
    stu(const char* str);// Initialize objects with strings
    stu(const char* str,size_t length);// Use one character before length Assign a length
    stu(stu& str,size_t index,size_t length);// Copy the middle string of another object into the current object
    stu(const stu& str);// Copy structure
public:
    // Get the first address of the string .
    const char* c_str() const;
    // Get the number of characters of the string .
    const size_t size() const;
    // Set the number of characters .
    void s_l(size_t length);
    // Set the string content .
    char* &GetMstr();
public:
    // Destructor
    ~stu();
public:
    // Output overload
    friend ostream& operator <<(ostream& os, stu& str);
    // Input overload
    friend istream& operator >>(istream& is, stu& str);
};
// Output overload
ostream& operator <<(ostream& os, stu& str);
// Input overload
istream& operator >>(istream& is, stu& str);

#endif