Software Design with Traits and Tag Dispatching

Tag Dispatching enables it to choose a function based on the type characteristics. This decision takes place at compile time and is based on traits.


Tag dispatching is based on traits. Consequentially, I want to write a few words about traits.


Traits are class templates that provide characteristics of a generic type. They can extract one or more characteristics of a class template.

You may already assume it; the metafunctions from the type-traits library are typical examples of traits in C++. I have already written a few posts about them. Here are they:

  1. Type Checks
  2. Type Comparisons
  3. std::is_base_of
  4. Correctness
  5. Performance

 Before I directly jump in this post in tag dispatching, I want to introduce the iterator traits. The following code snippet shows their partial specialization for pointers:

struct iterator_traits<T*> { 
    using difference_type = std::ptrdiff_t; 
    using value_type = T; 
    using pointer = T*; 
    using reference = T&; 
    using iterator_category = std::random_access_iterator_tag; 


The iterator categories build the following hierarchy:


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    struct input_iterator_tag{}; 
    struct output_iterator_tag{}; 
    struct forward_iterator_tag: public input_iterator_tag{}; 
    struct bidirectional_iterator_tag: public forward_iterator_tag{}; 
    struct random_access_iterator_tag: public bidirectional_iterator_tag{}; 


     The various iterator categories correspond to the container of the Standard Template Library.


    The following relation holds for the iterator categories and their support operations. A random-access iterator is a bidirectional iterator, and a bidirectional iterator is a forward iterator. This means std::array, std::vector, and std::string support a random-access iterator but not std::list.

    Tag Dispatching

    Now, I can apply tag dispatching and implement a fine-tailored advance_ algorithm optimized for the used container. First of all, std::advance is already part of the standard template library:

    template< class InputIt, class Distance >
    void advance( InputIt& it, Distance n );            (until C++17)
    template< class InputIt, class Distance >
    constexpr void advance( InputIt& it, Distance n );  (since C++17)


    std::advance increments a given iterator it by n elements. If n is negative, the iterator is decremented. Consequentially, the container providing the iterator must be, in this case, bidirectional.

    Here is my implementation of advance_:


    // advance_.cpp
    #include <iterator>
    #include <forward_list>
    #include <list>
    #include <vector>
    #include <iostream>
    template <typename InputIterator, typename Distance>                    
    void advance_impl(InputIterator& i, Distance n, std::input_iterator_tag) {
        std::cout << "InputIterator used" << '\n'; 
        if (n >= 0) { while (n--) ++it; }
    template <typename BidirectionalIterator, typename Distance>              
    void advance_impl(BidirectionalIterator& i, Distance n, std::bidirectional_iterator_tag) {
        std::cout << "BidirectionalIterator used" << '\n';
        if (n >= 0) 
            while (n--) ++i;
            while (n++) --i;
    template <typename RandomAccessIterator, typename Distance>             
    void advance_impl(RandomAccessIterator& i, Distance n, std::random_access_iterator_tag) {
        std::cout << "RandomAccessIterator used" << '\n';
        i += n;                                                             // (5)
    template <typename InputIterator, typename Distance>                    // (4)
    void advance_(InputIterator& i, Distance n) {
        typename std::iterator_traits<InputIterator>::iterator_category category;    
        advance_impl(i, n, category);                                               
    int main(){
        std::cout << '\n';
        std::vector<int> myVec{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};               // (1)
        auto myVecIt = myVec.begin();                                               
        std::cout << "*myVecIt: " << *myVecIt << '\n';
        advance_(myVecIt, 5);
        std::cout << "*myVecIt: " << *myVecIt << '\n';
        std::cout << '\n';
        std::list<int> myList{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};                // (2)
        auto myListIt = myList.begin();                                             
        std::cout << "*myListIt: " << *myListIt << '\n';
        advance_(myListIt, 5);
        std::cout << "*myListIt: " << *myListIt << '\n';
        std::cout << '\n';
        std::forward_list<int> myForwardList{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; // (3)
        auto myForwardListIt = myForwardList.begin();                               
        std::cout << "*myForwardListIt: " << *myForwardListIt << '\n';
        advance_(myForwardListIt, 5);
        std::cout << "*myForwardListIt: " << *myForwardListIt << '\n';
        std::cout << '\n';


    I use in the example a std::vector (line 1), a std::list (line 2), and a std::forward_list (line 3). A std::vector supports a random-access iterator, a std::list bidirectional iterator, and a std::forward_list forward iterator. The call std::iterator_traits<InputIterator>::iterator_category category; in the function advance_  (line 4) determines the supported iterator category based on the given iterator. The final call advance_impl(i, n, category) finally dispatches to the most specialized overload of the implementation function advance_impl.

    To visualize the dispatch, I added a short message to implementation functions advance_impl.


    What are the advantages of such a fine-tuned advance implementation?

    1. Type safety: The compiler decides which version of advance_impl is used. Consequentially, you cannot invoke an implementation requiring a bidirectional iterator with a forward iterator. Backward iterating with a forward iterator is undefined behavior.
    2. Performance: Putting a forward or bidirectional iterator n position further requires n increment operation. Its complexity is, therefore, linear. This observation does not hold for a random access iterator: Pointer arithmetic such as i += n (line 5) is a constant operation.

    What’s next?

    In my next post, I bridge dynamic polymorphism (object orientation) with static polymorphism (templates) to introduce a sophisticated technique: type erasure.

    The Future of Modernes C++

    The type erasure post will be my last post about templates for now. To get the previous ones, use the TOC or the category Templates. Afterward, I will continue to write about C++20 and will peek into the C++23 future. If you have some exciting post ideas, please write me an e-mail: Rainer.Grimm@modernescpp.de.


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