tag dispatching based on type traits

tags.cc

#include <iostream>

struct fast_speed_tag {};
struct slow_speed_tag {};

template <typename T>
struct traits { // default
    typedef slow_speed_tag speed;
};

template <>
struct traits<int> { // we would do the same for long etc
    typedef fast_speed_tag speed;
};

template <typename T>
void work_dispatch (const T &val, const slow_speed_tag&) {
    std::cout << "slow" << std::endl;
}

template <typename T>
void work_dispatch (const T &val, const fast_speed_tag&) {
    std::cout << "fast" << std::endl;
}

template <typename T>
void work (const T &val) {
    work_dispatch(val, typename traits<T>::speed());
}

int main () {
    int x;
    float y;

    work(x); // fast
    work(y); // slow

    return 0;
}