C++ Tag Dispatching


Example

A simple way of selecting between functions at compile time is to dispatch a function to an overloaded pair of functions that take a tag as one (usually the last) argument. For example, to implement std::advance(), we can dispatch on the iterator category:

namespace details {
    template <class RAIter, class Distance>
    void advance(RAIter& it, Distance n, std::random_access_iterator_tag) {
        it += n;
    }

    template <class BidirIter, class Distance>
    void advance(BidirIter& it, Distance n, std::bidirectional_iterator_tag) {
        if (n > 0) {
            while (n--) ++it;
        }
        else {
            while (n++) --it;
        }
    }

    template <class InputIter, class Distance>
    void advance(InputIter& it, Distance n, std::input_iterator_tag) {
        while (n--) {
            ++it;
        }
    }    
}

template <class Iter, class Distance>
void advance(Iter& it, Distance n) {
    details::advance(it, n, 
            typename std::iterator_traits<Iter>::iterator_category{} );
}

The std::XY_iterator_tag arguments of the overloaded details::advance functions are unused function parameters. The actual implementation does not matter (actually it is completely empty). Their only purpose is to allow the compiler to select an overload based on which tag class details::advance is called with.

In this example, advance uses the iterator_traits<T>::iterator_category metafunction which returns one of the iterator_tag classes, depending on the actual type of Iter. A default-constructed object of the iterator_category<Iter>::type then lets the compiler select one of the different overloads of details::advance. (This function parameter is likely to be completely optimized away, as it is a default-constructed object of an empty struct and never used.)

Tag dispatching can give you code that's much easier to read than the equivalents using SFINAE and enable_if.

Note: while C++17's if constexpr may simplify the implementation of advance in particular, it is not suitable for open implementations unlike tag dispatching.