Question

Suppose I want to write a generic function void f<T>(), which does one thing if T is a POD type and another thing if T is non-POD (or any other arbitrary predicate).

One way to achieve this would be to use a tag-dispatch pattern like the standard library does with iterator categories:

template <bool> struct podness {};
typedef podness<true> pod_tag;
typedef podness<false> non_pod_tag;

template <typename T> void f2(T, pod_tag) { /* POD */ }
template <typename T> void f2(T, non_pod_tag) { /* non-POD */ }

template <typename T>
void f(T x)
{
    // Dispatch to f2 based on tag.
    f2(x, podness<std::is_pod<T>::value>());
}

An alternative would be to use static member function of partially specialised types:

template <typename T, bool> struct f2;

template <typename T>
struct f2<T, true> { static void f(T) { /* POD */ } };

template <typename T>
struct f2<T, false> { static void f(T) { /* non-POD */ } };

template <typename T>
void f(T x)
{
    // Select the correct partially specialised type.
    f2<T, std::is_pod<T>::value>::f(x);
}

What are the pros and cons of using one method over the other? Which would you recommend?

Answer 1

I would like tag dispatch because:

• Easy to extend with new tags
• Easy to use inheritance (example)
• It is fairly common technique in generic programming

It seems tricky to me to add third variant in second example. When you’ll want to add, for example non-POD-of-PODs type you’ll have to replace bool in template <typename T, bool> struct f2; with something other (int if you like =) ) and replace all struct f2<T, bool-value> with struct f2<T, another-type-value>. So that for me the second variant looks hardly extensible. Please correct me if I wrong.