使用单例模式实习string类

class  mystring
{
private:
    char *s;
    static mystring *self;
public:
    ~ mystring();
    const char *get_s() const;
    const void set_s(const char *s);
     static mystring *makestring(const char *s = NULL);
     static void deletestring();
protected:
     mystring();
    mystring(const mystring &it);  //构造函数
    mystring(const char *s);

};

mystring *mystring::self = NULL;

mystring::mystring():s(NULL)
{

}

mystring::mystring(const char *s)
{
    int len = strlen(s);
    this->s = new char[len +1];
    strcpy(this->s,s);
    this->s[len] = 0;
}


mystring::mystring(const mystring &it) //通过拷贝构造函数实现深拷贝
{
    int len = strlen(it.get_s());
    this->s = new char[len +1];
    strcpy(this->s, it.s);
    this->s[len] = 0;
}

mystring::~mystring()
{
    delete s;
    s = NULL;
}

const char *mystring::get_s() const
{
    return s;
}

const void mystring::set_s(const char *s)
{
    if(this->s == NULL)
    {
        int len = strlen(s);
        this->s = new char[len +1];
        strcpy(this->s, s);
        this->s[len] = 0;
    }
    else
    {
        int len1 = strlen(this->s);
        int len2 = strlen(s);
        if(len1 >= len2)
        {
            strcpy(this->s, s);
            this->s[len2] = 0;
        }
        else
        {
            delete []this->s;
            this->s = new char[len2+1];
            strcpy(this->s, s);
            this->s[len2] = 0;
        }
    }
}

mystring *mystring::makestring(const char *s)
{
    if(self == NULL)
    {
        if(s == NULL)
        {
            self = new mystring;
        }
        else
        {
            self = new mystring(s);
        }
    }
    return self;
}

void mystring::deletestring()
{
    if(self != NULL)
    {
        delete self;
        self = NULL;
    }
    
}