完整String.h:

#ifndef __MYSTRING__
#define __MYSTRING__

//1.
class String {//class with pointer members 必须有 copy Constructor【拷贝构造】 copy assignment operator【拷贝赋值】
private:
    char* m_data;

public:
    String( );
    String(const char* cstr);
    String(const String& str);//拷贝构造
    String& operator=(const String& str);//拷贝赋值
    ~String();
    char* get_c_str() const { return m_data; }//inline function
};

//2-1
inline
String::String( ) {
    //未定义初值
    m_data = new char[1];
    *m_data = '\0';
    
}

inline
String::String(const char* cstr) {
    if (cstr) {
        m_data = new char[strlen(cstr) + 1];
        strcpy(m_data, cstr);
    }
    else {//未定义初值
        m_data = new char[1];
        *m_data = '\0';
    }
}

inline 
String::~String() {
    delete[] m_data;
}

//2-2
inline
String::String(const String& str) {  //copy Constructor【拷贝构造】
    m_data = new char[strlen(str.m_data) + 1];
    strcpy(m_data, str.m_data);
}

//2-3
inline
String& String::operator=(const String& str) {  //copy assignment operator【拷贝赋值]
    if (this == &str) {//检测自我赋值【self assignment】
        return *this;
    }
    delete[] m_data;
    m_data = new char[ strlen(str.m_data) + 1];
    strcpy(m_data, str.m_data);
    return *this;
}

//2-4
#include <iostream>
ostream& operator << (ostream& os, const String& str) {
    os << str.get_c_str();
    return os;
}


#endif

测试代码main.cpp

#include <iostream>
#include <string.h>
#include <windows.h>

using namespace std;
#include "String.h"


int main() {
    String s1();
    String s2("hello");

    String s3(s2);
    String* p = new String("hello");

    std::cout << *p << std::endl;

    system("pause");
    return 0;
}