基于char*设计一个字符串类MyString

myString.h
#ifndef __MYSTRING_H__
#define __MYSTRING_H__
#include <iostream>
using namespace std;
class MyString
{
    public:
        MyString();
        MyString(const char *_str);
        MyString(const MyString &_str);
        ~MyString();
    public:
        const MyString operator +(const MyString &_str);
        void operator =(const MyString &_str);  
    public:
        friend istream & operator >>(istream &_in, MyString &_str);
        friend ostream & operator <<(ostream &_out, MyString &_str);
    private:
        char *str;
        int length;
};
#endif

#include <iostream>
#include <string.h>
#include "myString.h"
using namespace std;
//constructor & destructor
MyString::MyString()
{
    str = new char;
    str[0] = '\0';
    length = 0;
}
MyString::MyString(const char *_str)
{
    length = strlen(_str);
    str = new char[length + 1]; 
    str[length] = '\0';
    strncpy(str, _str, length+1);
}
MyString::MyString(const MyString &_str)
{
    length = _str.length;
    str = new char[length + 1]; 
    str[length] = '\0';
    strncpy(str, _str.str, length+1);
    //cout << str << endl;
}
MyString::~MyString(){}
//operator
const MyString MyString::operator +(const MyString &_str)
{
    int newLength = length + _str.length;
    char * newStr = new char[newLength + 1]; 
    newStr[newLength] = '\0';
    strncpy(newStr, str, length);
    strncat(newStr, _str.str, _str.length);
    return MyString(newStr);
}
void MyString::operator =(const MyString &_str)
{
    str = _str.str;
    length = _str.length;
}
//friend
istream & operator >>(istream &_in, MyString &_str)
{
    _in >> _str.str;
    _str.length = strlen(_str.str);
    return _in;
}
ostream & operator <<(ostream &_out, MyString &_str)
{
    _out << _str.str << endl << "length: " << _str.length;
    return _out;
}

#include <iostream>
#include "myString.h"
using namespace std;
int main()
{
    MyString mStr;
    MyString mStr_0(mStr);
    MyString mStr_1("Hello");
    MyString mStr_2("World");
    cout << "mStr: " << mStr << endl;
    cout << "mStr_0: " << mStr_0 << endl;
    cout << "mStr_1: " << mStr_1 << endl;
    cout << "mStr_2: " << mStr_2 << endl;
    cout << "____________________" << endl;
    mStr_0 = mStr_1;
    mStr = mStr_1 + mStr_2;
    cout << mStr << endl;
    cout << mStr_0 << endl;
    return 0;
}

mStr: 
length: 0
mStr_0: 
length: 0
mStr_1: Hello
length: 5
mStr_2: World
length: 5
____________________
Hello
length: 5
HelloWorld
length: 10