Перегрузить класс String

#include <iostream>
#include <cstring>
 
using namespace std;
 
class String {
    int len;
    char *str;
 
public:
    String();
    String(const String&);
    String(char *str);
    ~String();
    String& operator=(String &);
    friend ostream& operator <<(ostream &c, String &x);
    friend istream& operator >>(istream &input, String &y);
    char operator[] (int index);
    int length(const String &s);
};
 
istream &operator>>(istream &input, String &y)
{
    char str[100001];
    input.getline(str, 100001);
    String tmp(str);
    y = tmp;
    return input;
}
 
ostream &operator<<(ostream &c, String &x)
{
    c << x.str;
    return c;
}
 
int String::length(const String &s){
    return s.len;
}
 
String::String(const String &x)
{
    this->len = x.len;
    this->str = new char[this->len + 1];
    strcpy(this->str, x.str);
    this->str[this->len] = '\0';
}
 
String &String::operator=(String &x)
{
    delete[] this->str;
    this->len = x.len;
    this->str = new char[this->len + 1];
    strcpy(this->str, x.str);
    this->str[this->len] = '\0';
    return (*this);
}
String::~String()
{
    delete[] str;
    this->len = 0;
}
 
String::String()
{
    this->str = new char[1];
    this->len = 0;
    str[0] = '\0';
}
 
String::String(char *str)
{
    this->len = strlen(str);
    this->str = new char[this->len + 1];
    strcpy(this->str, str);
    this->str[this->len] = '\0';
}
 
char String::operator[] (int index){
    return (char) this->str[index];
};
 
int main()
{
    String a;
    cin >> a;
    char buf = a[1];
    int len = a.length(a);
    a[0] = a[len - 1];
    a[len - 1] = buf;
    cout << a;
    return 0;
}

char& operator[] (int index);
//...
char& String::operator[] (int index){
    return this->str[index];
};

char buf = a[0];