Code for a basic random access iterator based on pointers?

Question

I've never implemented STL-like iterators and I try to understand how to implement a very basic thing based on pointers. Once I will have this class I will be able to modify it to do more complicated things. Consequently, this is a first step, and I need it to be rock solid to understand how to write my own iterators (without boost).

I have written the following code and I know that there are errors in it. Can you help me to design correctly a Random Access Iterator class inspired from that :

template<Type> class Container<Type>::Iterator : public std::iterator<random_access_iterator_tag, Type>
{
    // Lifecycle:
    public:
        Iterator() : _ptr(nullptr) {;}
        Iterator(Type* rhs) : _ptr(rhs) {;}
        Iterator(const Iterator &rhs) : _ptr(rhs._ptr) {;}

    // Operators : misc
    public:
        inline Iterator& operator=(Type* rhs) {_ptr = rhs; return *this;}
        inline Iterator& operator=(const Iterator &rhs) {_ptr = rhs._ptr; return *this;}
        inline Iterator& operator+=(const int& rhs) {_ptr += rhs; return *this;}
        inline Iterator& operator-=(const int& rhs) {_ptr -= rhs; return *this;}
        inline Type& operator*() {return *_ptr;}
        inline Type* operator->() {return _ptr;}
        inline Type& operator[](const int& rhs) {return _ptr[rhs];}

    // Operators : arithmetic
    public:
        inline Iterator& operator++() {++_ptr; return *this;}
        inline Iterator& operator--() {--_ptr; return *this;}
        inline Iterator& operator++(int) {Iterator tmp(*this); ++_ptr; return tmp;}
        inline Iterator& operator--(int) {Iterator tmp(*this); --_ptr; return tmp;}
        inline Iterator operator+(const Iterator& rhs) {return Iterator(_ptr+rhs.ptr);}
        inline Iterator operator-(const Iterator& rhs) {return Iterator(_ptr-rhs.ptr);}
        inline Iterator operator+(const int& rhs) {return Iterator(_ptr+rhs);}
        inline Iterator operator-(const int& rhs) {return Iterator(_ptr-rhs);}
        friend inline Iterator operator+(const int& lhs, const Iterator& rhs) {return Iterator(lhs+_ptr);}
        friend inline Iterator operator-(const int& lhs, const Iterator& rhs) {return Iterator(lhs-_ptr);}

    // Operators : comparison
    public:
        inline bool operator==(const Iterator& rhs) {return _ptr == rhs._ptr;}
        inline bool operator!=(const Iterator& rhs) {return _ptr != rhs._ptr;}
        inline bool operator>(const Iterator& rhs) {return _ptr > rhs._ptr;}
        inline bool operator<(const Iterator& rhs) {return _ptr < rhs._ptr;}
        inline bool operator>=(const Iterator& rhs) {return _ptr >= rhs._ptr;}
        inline bool operator<=(const Iterator& rhs) {return _ptr <= rhs._ptr;}

    // Data members
    protected:
        Type* _ptr;
};

Thank you very much.

Answer 1

Your code has the following issues:

• You do not follow the Rule of Three/Five. The best option in your situation is not to declare any custom destructors, copy/move constructors or copy/move assignment operators. Let's follow so called Rule of Zero.
• Iterator(Type* rhs) could be private and the Container could be marked as Iterator's friend, but that's not strictly necessary.
• operator=(Type* rhs) is a bad idea. That's not what type safety is about.
• Post-in(de)crementation should return Iterator, not Iterator &.
• Adding two iterators has no meaning.
• Subtracting two iterators should return a difference, not a new iterator.
• You should use std::iterator<std::random_access_iterator_tag, Type>::difference_type instead of const int &.
• If a method does not modify an object, it should be marked const.

Useful resource: RandomAccessIterator @ cppreference.com

Here is a fixed version of your code:

template<typename Type>
class Container<Type>::Iterator : public std::iterator<std::random_access_iterator_tag, Type>
{
public:
    using difference_type = typename std::iterator<std::random_access_iterator_tag, Type>::difference_type;
    
    Iterator() : _ptr(nullptr) {}
    Iterator(Type* rhs) : _ptr(rhs) {}
    Iterator(const Iterator &rhs) : _ptr(rhs._ptr) {}
    /* inline Iterator& operator=(Type* rhs) {_ptr = rhs; return *this;} */
    /* inline Iterator& operator=(const Iterator &rhs) {_ptr = rhs._ptr; return *this;} */
    inline Iterator& operator+=(difference_type rhs) {_ptr += rhs; return *this;}
    inline Iterator& operator-=(difference_type rhs) {_ptr -= rhs; return *this;}
    inline Type& operator*() const {return *_ptr;}
    inline Type* operator->() const {return _ptr;}
    inline Type& operator[](difference_type rhs) const {return _ptr[rhs];}
    
    inline Iterator& operator++() {++_ptr; return *this;}
    inline Iterator& operator--() {--_ptr; return *this;}
    inline Iterator operator++(int) const {Iterator tmp(*this); ++_ptr; return tmp;}
    inline Iterator operator--(int) const {Iterator tmp(*this); --_ptr; return tmp;}
    /* inline Iterator operator+(const Iterator& rhs) {return Iterator(_ptr+rhs.ptr);} */
    inline difference_type operator-(const Iterator& rhs) const {return _ptr-rhs.ptr;}
    inline Iterator operator+(difference_type rhs) const {return Iterator(_ptr+rhs);}
    inline Iterator operator-(difference_type rhs) const {return Iterator(_ptr-rhs);}
    friend inline Iterator operator+(difference_type lhs, const Iterator& rhs) {return Iterator(lhs+rhs._ptr);}
    friend inline Iterator operator-(difference_type lhs, const Iterator& rhs) {return Iterator(lhs-rhs._ptr);}
    
    inline bool operator==(const Iterator& rhs) const {return _ptr == rhs._ptr;}
    inline bool operator!=(const Iterator& rhs) const {return _ptr != rhs._ptr;}
    inline bool operator>(const Iterator& rhs) const {return _ptr > rhs._ptr;}
    inline bool operator<(const Iterator& rhs) const {return _ptr < rhs._ptr;}
    inline bool operator>=(const Iterator& rhs) const {return _ptr >= rhs._ptr;}
    inline bool operator<=(const Iterator& rhs) const {return _ptr <= rhs._ptr;}
private:
    Type* _ptr;
};