c++ string class example

#include <iostream>
#include <cstring>

class String {
    char *p;
    unsigned int length;

public:
    // 1
    String()
        :p(new char[0]), length(0)
    {
        std::cout << "1, p="<< (void*)p << ", length=" << length << std::endl;
    }

    // 2
    String(char *p) :p(p), length(strlen(p)) { std::cout << "2" << std::endl; }

    // 3
    String(const String &s) {
        if(!s.p) throw "3 err";
        length = strlen(s.p);
        p = new char[length+1];
        strncpy(p, s.p, length + 1);
        std::cout << "3" << std::endl;
    }

    // 4
    ~String() {
        std::cout << "4" << std::endl;
    }

    // 5
    String operator+(String &s) {
        char *resp;
        if(!p || !s.p) {
            throw "5 err";
        }

        resp = new char[length + s.length + 1];
        strncpy(resp, p, length + 1);
        strncpy(resp + length, s.p, s.length + 1);
        std::cout << "5" << std::endl;
        return resp;
    }

    // 6
    char &operator[](int i) {
        std::cout << "6" << std::endl;
        return p[i];
    }

    // 7
    String &operator=(String &s) {
        if(&s == this) return *this;
        std::cout << "7" << std::endl;
        if(p) delete[] p;
        length = s.length;
        p = new char[length + 1];
        strncpy(p, s.p, length + 1);
        return *this;
    }

    String &operator=(const String &s) {
        if(&s == this) return *this;
        std::cout << "7rval" << std::endl;
        if(p) delete[] p;
        length = s.length;
        p = new char[length + 1];
        strncpy(p, s.p, length + 1);
        return *this;
    }
};

int main() {
    using namespace std;

    cout << "eleje" << endl;
    String s1("rejtvény");
    cout << "s1 létrehozás után" << endl;
    String s2;
    cout << "s2 után" << endl;
    String s3 = s2;
    cout << "s3 után" << endl;

    char c = s3[3];
    cout << "operator[] után" << endl;
    s2 = s3;
    cout << "operator= után" << endl;
    s2 = s3 + s2 + s1;
    cout << "operator+, operator= után" << endl;

    return 0;
}

namespace {
struct C {
    ~C() { std::cout << "main után" << std::endl; }
} _c;
}