String& operator+=(const String& s) {
    for (size_t i = 0; i < s.sz; ++i) {
        push_back(s[i]);
    }
    return *this;
}

String& operator+=(const char c) {
    push_back(c);
    return *this;
}

String& operator+=(const char& add_chr) {
    push_back(add_chr);
    return *this;
}

String substr(size_t start, size_t count) const {
    String res(count, '\0');
    for (size_t i = start; i < start + count; ++i) {
        res[i - start] = str[i];
    }
    return res;
}

//это вне функции
String operator+(const char lstr, const String& rstr) {
    String copy(1, lstr);
    copy += rstr;
    return copy;
}

String operator+(const String& lstr, const char rstr) {
    String copy = lstr;
    copy.push_back(rstr);
    return copy;
}

String operator+(const String& lstr, const String& rstr) {
    String copy = lstr;
    copy += rstr;
    return copy;
}

bool operator==(const String& lhs, const String& rhs) {
    if (lhs.length() != rhs.length()) {
        return false;
    }
    for (size_t i = 0; i < lhs.length(); ++i) {
        if (lhs[i] != rhs[i]){
            return false;
        }
    }
    return true;
}