c++ string을 나만의 것으로 구현

#include<iostream>
#include<string>
#include<cstring>
#pragma warning ( disable : 4996 )
using namespace std;
 
class Mystring {
 
    char *a; // 문자열을 만들기 위해서 포인터를 사용한다.
    int len; // 문자열의 길이를 잡기 위해서 사용한다.
 
public :
 
    Mystring() {
 
        this ->a = new char[50];
        len = 0;
 
    }
    Mystring(const char *ch) { // 
        
 
        len = strlen(ch);
        a = new char[len+1];
        strcpy_s(a,len+1,ch);
 
    }
 
    Mystring(int num, const char ch) { // 
 
        len = num;
        a = new char[len + 1];
 
        for (int i = 0; i < len; i++) {
 
            a[i] = ch;
        }
        a[len] = NULL;
 
    }
 
 
    Mystring(char c[], int num) {
 
        len = num;
        a = new char[len+1];
        char x[20];
 
        for (int i = 0; i < len; i++) {
            x[i] = c[i];
            
        }
        x[len - 1] = NULL;
        strcpy(a,x);
    
    }
    
    Mystring(char c[], char s[]) {
 
        len = 5;
        a = new char[len + 1];
        char x[20];
        for (int i = 0; i < len; i++) {
            x[i] = c[i];
 
        }
        x[len - 1] = NULL;
        strcpy(this->a, x);
        
    }
 
    Mystring(Mystring &my) {
 
        len = my.len;
        this->a = new char[len + 1];
 
        strcpy_s(this->a, len + 1, my.a);
 
    }
    ~Mystring(){
    
        delete []a;
    
    }
 
    Mystring & operator = (const Mystring &aa) {
 
        len = strlen(aa.a);
        delete []this->a; // 기존에 있던 two가 가리키는 값을 지워 주려고 delete를 사용. 
        this->a = new char[len + 1];
        strcpy_s(this->a, len + 1, aa.a);
        return *this;
    }
 
 
    /*
    Mystring & operator += (const Mystring &my) {
 
        
        char *tmp;
        int len1 = strlen(my.a);
        tmp = new char[len1+1];
        strcpy(tmp, my.a);
    
        len = strlen(this->a)+ len1;
        this->a = new char[len + 1];
 
        strcpy(this->a, strcat(this->a, my.a));
 
        a[len - 1] = NULL;
        return *this;
    }
    */
    Mystring & operator+=(const Mystring &aa) {
 
        char *cpy;
        cpy = new char[strlen(this->a) + 1];
        strcpy(cpy, this->a);
 
        len = strlen(this->a) + strlen(aa.a) + 1;
        this->a = new char[len];
 
        strcpy(this->a, strcat(cpy, aa.a));
        a[len - 1] = NULL;
 
        return *this;
    }
    
    char & operator [] (const int num) {
 
        return this->a[num];
    }
 
    Mystring & operator +(const Mystring &aa) {
 
        char *tmp;
        len = strlen(this->a) + strlen(aa.a) + 1; //30
        tmp = new char[len+1];
        strcpy(tmp,this->a);
        delete []this->a;
        this->a = new char[len+1];
        strcpy(this->a, tmp);
        strcat(this->a, strcpy(tmp, aa.a));
 
        return *this;
 
    }
    friend ostream & operator <<(ostream & out, Mystring & a);
    friend istream & operator >> (istream& in, Mystring &aa);
    
};
ostream & operator <<(ostream & out, Mystring & aa) {
 
    out << aa.a;
    return out;
}
 
istream & operator >> (istream& in, Mystring &aa) {
 
    in >> aa.a;
    return in;
}
 
 
 
int main()
{
    
    Mystring one("lottery winner!");//생성자함수호출
    cout << one << endl;  //출력연산자함수호출
 
    Mystring two(20, '$');//생성자함수호출
    cout << two << endl;//출력연산자함수호출
 
    Mystring three(one); // 복사생성자호출
    cout << three << endl; //출력연산자함수호출
 
 
    one += "oops"; // +=연산자함수호출 ( strcat함수 )
    cout << one << endl;//문자열결합 
 
 
    two = "sorry!that was";//대입연산자함수 호출 
    cout << two << endl;
 
    three[0] = 'p';//[]첨자연산자함수 호출 
    cout << three << endl;
    
 
    Mystring four;
    four = two + three;
    cout << four << endl;
 
 
    char alls[] = "all's well that ends well";
    Mystring five(alls, 20); //생성자호출
    cout << five << "!\n";
 
    
    Mystring six(alls + 6, alls + 10);  //생성자
    cout << six << ",";
 
 
    Mystring seven(&five[6], &five[10]);  //생성자
    cout << seven << "...\n";
 
 
    Mystring eight;
    cout << "문자열 입력하세요 :";
    cin >> eight;  // >>연산자호출
    cout << " 입력한 문자열은 \"" << eight << "\" 입니다 " << endl;
 
}