C++ SFINAE (Substitution Failure Is Not An Error) Overload resolution with a large number of options
Example
If you need to select between several options,
enabling just one via enable_if<> can be quite cumbersome,
since several conditions needs to be negated too.
The ordering between overloads can instead be selected using inheritance, i.e. tag dispatch.
Instead of testing for the thing that needs to be well-formed, and also
testing the negation of all the other versions conditions, we instead test
just for what we need, preferably in a decltype in a trailing return.
This might leave several option well formed, we differentiate between those
using 'tags', similar to iterator-trait tags (random_access_tag et al).
This works because a direct match is better that a base class, which is better
that a base class of a base class, etc.
#include <algorithm>
#include <iterator>
namespace detail
{
// this gives us infinite types, that inherit from each other
template<std::size_t N>
struct pick : pick<N-1> {};
template<>
struct pick<0> {};
// the overload we want to be preferred have a higher N in pick<N>
// this is the first helper template function
template<typename T>
auto stable_sort(T& t, pick<2>)
-> decltype( t.stable_sort(), void() )
{
// if the container have a member stable_sort, use that
t.stable_sort();
}
// this helper will be second best match
template<typename T>
auto stable_sort(T& t, pick<1>)
-> decltype( t.sort(), void() )
{
// if the container have a member sort, but no member stable_sort
// it's customary that the sort member is stable
t.sort();
}
// this helper will be picked last
template<typename T>
auto stable_sort(T& t, pick<0>)
-> decltype( std::stable_sort(std::begin(t), std::end(t)), void() )
{
// the container have neither a member sort, nor member stable_sort
std::stable_sort(std::begin(t), std::end(t));
}
}
// this is the function the user calls. it will dispatch the call
// to the correct implementation with the help of 'tags'.
template<typename T>
void stable_sort(T& t)
{
// use an N that is higher that any used above.
// this will pick the highest overload that is well formed.
detail::stable_sort(t, detail::pick<10>{});
}
There are other methods commonly used to differentiate between overloads, such as exact match being better than conversion, being better than ellipsis.
However, tag-dispatch can extend to any number of choices, and is a bit more clear in intent.
