SFINAE AND enable_if<>
Before C++17, there were already ways to enable or disable templates as a whole – a compile-time if, partial specialization, SFINAE, and enable_if<>. Referring to our previous example, is_prime<>, where we determined the authenticity of a prime number, we can use partial specialization to choose at compile time between different type implementations. We can choose different implementations depending on the argument (we used a struct in the code because function templates do not support partial specialization):
template <std::size_t n, bool = IsPrime(n)>class hash_table;
template <std::size_t n>
class hash_table<n, true>
{
public:
    std::array<int, n> bucket;
};
template <std::size_t n>
class hash_table<n, false>
{
public:
    array<int, next_prime<n>::value> bucket;
// next_prime<size_t n> is a meta function which compute
// the next prime...
