The key to implementing a custom iterator in C is to follow the structure and specifications of the standard library iterator. 1. Understand the basic requirements of iterators: it needs to support *it, it->, it, it and ==, != operations, and provide five type aliases to be compatible with STL; 2. Define container and iterator classes: such as the MyArray container and its internal iterator class in the example, implement constructors, dereferences, member access, prefix/suffixes and comparison operators; 3. Notes: Ensure that all necessary type aliases are defined, avoid dangling pointers, distinguish pre-suffixes, and maintain begin/end consistency. If you need random access, you need to implement operators, operators-, etc. and set corresponding iterator_category; 4. Usage scenarios and extensions: suitable for non-standard container traversal, resource secure access, virtual collection generation, etc., and can support const_iterator and reverse iteration in combination with template programming. As long as you clarify the required functions and implement them in accordance with the specifications, you can build an efficient and reliable custom iterator.

The key to implementing a custom iterator in C is to understand how iterators in the standard library work and write them according to certain structures and specifications. Although it seems a bit complicated, it is not difficult to master the basic structure and necessary interfaces.

1. Understand the basic requirements of iterators

The iterator of C is essentially an object that supports some basic operations, such as:

• *it : access the current element
• it-> : access the member of the current element (if the element is an object)
• it or it : move to the next element
• == and != : determine whether the two iterators are equal or inequality

For STL compatibility, your custom iterator is best inherited from std::iterator (recommended before C 17), or manually provide five type aliases: value_type , difference_type , pointer , reference , iterator_category .

For example, if you are writing a forward read-only iterator, you can set iterator_category to std::forward_iterator_tag ; if it is a read-write bidirectional iterator, you can set it to std::bidirectional_iterator_tag .

2. Define a simple container and iterator class

Let's start with a simple example: a fixed-size array container and its iterator.

 template <typename T>
class MyArray {
    T data[10];
public:
    // Iterator class defines class iterator {
        T* ptr;
    public:
        using value_type = T;
        using difference_type = std::ptrdiff_t;
        using pointer = T*;
        using reference = T&;
        using iterator_category = std::random_access_iterator_tag;

        iterator(T* p) : ptr(p) {}
        reference operator*() const { return *ptr; }
        pointer operator->() const { return ptr; }
        iterator& operator () { ptr; return *this; } // prefix iterator operator int) { iterator tmp(*this); ptr; return tmp; } // suffix bool operator==(const iterator& other) const { return ptr == other.ptr; }
        bool operator!=(const iterator& other) const { return ptr != other.ptr; }
    };

    iterator begin() { return iterator(&data[0]); }
    iterator end() { return iterator(&data[10]); }
};

This way you can use this container like you would with vector:

 MyArray<int> arr;
for (auto it = arr.begin(); it != arr.end(); it) {
    *it = 42; // Modify each element}

3. Precautions and common mistakes

• Don't forget to define all necessary type aliases : otherwise, some STL algorithms may not be checked.
• Avoid dangling pointers or references : If your container's internal data changes dynamically, make sure the data pointed to by the iterator is still valid.
• Pay attention to the difference between pre and postfixes when overloading operators : the suffix version needs to return the old value, so it is usually a little worse than the prefix in efficiency.
• Try to maintain consistency : for example, begin() returns the position of the first element, end() returns the position after the last element.

If you intend to support random access (such as it n or it[n] ), you also need to implement operator , operator- , operator[] , and correctly set iterator_category to std::random_access_iterator_tag .

4. Usage scenarios and extension suggestions

The most common uses of custom iterators include:

• Traversing non-standard containers (such as trees, graphs, linked lists, etc.)
• Provides secure access to resources (such as file flows, network data flows)
• Implement lazy evaluation or "virtual" collections of data generated on demand

If you want to make iterators more general, you can consider combining template programming to support const versions ( const_iterator ), or even support reverse iteration ( rbegin() / rend() ).

Basically that's it. It's a bit cumbersome to write, but it's clear in logic and clear in structure. As long as you figure out which operations you want to support and what categories you inherit, the rest is to implement operator overload step by step.

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