Understanding When and Why to Use iterator_traits

In generic programming, especially when writing STL-like containers and iterators, it’s essential to understand the role of iterator_traits and when you should or should not use it.

This guide explains:

• What iterator_traits is
• Why it is used in reverse_iterator
• Why it is not used in random_access_iterator
• The general rule for when to use iterator_traits
• Code examples and analogies

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📚 What Is iterator_traits?

iterator_traits is a traits class that extracts type information from an iterator type.
It gives you access to:

• value_type
• pointer
• reference
• difference_type
• iterator_category

Example:

template <typename Iterator>
void foo(Iterator it) {
    typedef typename ft::iterator_traits<Iterator>::value_type T;
    ...
}

This allows foo() to work with both:

• Class iterators (ft::random_access_iterator<T>)
• Raw pointers (T*)

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🧠 Why Use iterator_traits in reverse_iterator?

Because reverse_iterator is a generic adapter over any iterator type Iter:

template <typename Iter>
class reverse_iterator {
public:
    typedef typename ft::iterator_traits<Iter>::value_type        value_type;
    typedef typename ft::iterator_traits<Iter>::reference         reference;
    typedef typename ft::iterator_traits<Iter>::pointer           pointer;
    typedef typename ft::iterator_traits<Iter>::difference_type   difference_type;
    typedef typename ft::iterator_traits<Iter>::iterator_category iterator_category;
};

This works for:

• ft::random_access_iterator<T>
• const T*, T* (raw pointers)
• Any future custom iterator class

Why not Iter::value_type?

Because raw pointers like int* do not have member types like value_type or reference.

// Invalid for raw pointer
typename int*::value_type // ❌ compiler error

Traits fix that:

typename ft::iterator_traits<int*>::value_type // ✅ int

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🧱 Why Not Use iterator_traits in random_access_iterator?

Your ft::random_access_iterator<T> is the iterator being described.

template <typename T>
class random_access_iterator {
public:
    typedef T*       pointer;
    typedef T&       reference;
    typedef T        value_type;
    typedef std::ptrdiff_t difference_type;
    typedef std::random_access_iterator_tag iterator_category;
};

If you wrote this using iterator_traits:

typedef typename ft::iterator_traits<T>::value_type value_type; // ❌ invalid

It assumes T is already an iterator — but in your case, it's the value type, like int.

So this would be a circular, incorrect use of traits.

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🧠 Analogy: ID Cards

• random_access_iterator<T> is like creating your own ID card.
• iterator_traits<Iter> is like a scanner that reads ID cards.

So:

• random_access_iterator defines the traits (name, age, etc.).
• iterator_traits is used by others (like reverse_iterator) to extract those traits.

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✅ Summary: When to Use iterator_traits

Scenario	Use iterator_traits?	Why?
Inside random_access_iterator	❌ No	You're defining the iterator
Inside reverse_iterator	✅ Yes	You're adapting another iterator
Inside algorithms (advance)	✅ Yes	To support any iterator type (pointer, class)
With raw pointers (int*)	✅ Yes	Only traits can describe them

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🧪 Sanity Check

This should work:

ft::iterator_traits<ft::random_access_iterator<int> >::value_type // ✅ int
ft::iterator_traits<int*>::value_type                             // ✅ int

But this will fail:

typename int*::value_type // ❌ compiler error: no such member

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✅ Final Rule of Thumb

Use iterator_traits<T> when you don’t own or define the iterator T.

But when you're defining the iterator class itself — don't use iterator_traits inside it.

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Links:

https://www.youtube.com/watch?v=g9wTzeq0Y-g