Как правильно заполнить итератор **random_access_iterator** для класса?

Помогите мне пожалуйста заполнить правильно итератор random_access_iterator для моего класса. p_str это char * p_str

/* Класс string только для строк  char или wchar_t */
template<class T = char>
class string 
{static_assert(std::is_same_v<T,char> || std::is_same_v<T,wchar_t>, " Err Type ");
 
/* Итератор  */
//------------------------------------------------------------------
private:
template<class T>
class string_iterator
{
public:
 
    using value_type        =   T;  
    using iterator_category =   std::random_access_iterator_tag;                    
    using difference_type   =   ptrdiff_t;
    using pointer           =   T*;
    using reference         =   T&;
    
                string_iterator()           noexcept = default;
    explicit    string_iterator(T *curr)    noexcept : current(curr){}   
    
    string_iterator& operator = ( const string_iterator &other)
    {
        if (this != &other)
        {
            current = other.current;
        }
        return *this;
    }
    string_iterator &operator ++()
    {
        ++current;
        return *this;
    }
    string_iterator operator ++(int i)
    {
        string_iterator tmp(current);
        ++current;
        return tmp;
    }
    string_iterator &operator --()
    {
        --current;
        return *this;
    }
    string_iterator operator --(int i)
    {
        string_iterator tmp(current);
        --current;
        return tmp;
    }
 
        // Операции, необходимые для RandomAccessIterator.
    reference operator[](difference_type n) const { auto tmp = *this; tmp += n; return *tmp; }
    string_iterator& operator+=(difference_type n) { /* что тут */return *this; }
    string_iterator& operator-=(difference_type n) { return *this += -n; }
 
        reference   operator *()    const   noexcept { return *current; }   
        pointer     operator ->()   const   noexcept { return current; }      
    
    bool operator == (const string_iterator &other )
    {
        return current == other.current;
    }
    bool operator != ( const string_iterator &other )
    {
        return !( *this == other );
    }
private:
    pointer current{0};
};
//------------------------------------------------------------------
 
 
public:
            
        using iterator          = string_iterator<T>;   
        using const_iterator    = string_iterator<const T>;
        using size_type         = size_t;
        using pointer           = T *;      
        using const_pointer     = const T *;
        using reference         = T &;
        using const_reference   = const T &;
        using value_type        = T; 
                        
 
        iterator begin()            noexcept { assert(p_str);   return iterator(p_str); }       
        iterator end()              noexcept { assert(p_str);   return iterator(p_str + leng); }           
        
        const_iterator cbegin()     const   noexcept { return const_iterator(p_str); }      
        const_iterator cend()       const   noexcept { return const_iterator(p_str + leng); }   
 
        iterator rbegin()           noexcept { assert(p_str);   return iterator(p_str + leng); }        
        iterator rend()             noexcept { assert(p_str);   return iterator(p_str); }          
        
        const_iterator crbegin()    const   noexcept {return const_iterator(p_str + leng);}     
        const_iterator crend()      const   noexcept {return const_iterator(p_str);}    
        
//------------------------------------------------------------------
private: