c++ - What's the point of unnamed non-type template parameters?

Question

According to the reference, the name of a non-type template parameter is optional, even when assigning a default value (see (1) and (2)). Therefore these template structs are valid:

template <int> struct Foo {};
template <unsigned long = 42> struct Bar {};

I haven't seen a possibility of accessing the values of the non-type parameters. My question is: What's the point of unnamed/anonymous non-type template parameters? Why are the names optional?

Answer 1

First, we can split declaration from definition. So name in declaration is not really helpful. and name might be used in definition

template <int> struct Foo;
template <unsigned long = 42> struct Bar;

template <int N> struct Foo {/*..*/};
template <unsigned long N> struct Bar {/*..*/};

Specialization is a special case of definition.

Then name can be unused, so we might omit it:

template <std::size_t, typename T>
using always_t = T;

template <std::size_t ... Is, typename T>
struct MyArray<std::index_sequence<Is...>, T>
{
    MyArray(always_t<Is, const T&>... v) : /*..*/
};

or used for SFINAE

template <typename T, std::size_t = T::size()>
struct some_sized_type;