什么是`std :: nullptr_t`模板参数的正确用例？

Question

Today I came to know that C++ allows non-type template parameters of type std::nullptr_t :

template<std::nullptr_t N> struct A { };

template<std::nullptr_t N> void f() { } 

For the life of me, I cannot come up with any sensible use-case for these. Can anyone please come up with a rationale for this?

Answer 1

It seems this is allowed to avoid the need to special case template using a pointer type and a value for std::nullptr_t . That, the use case would look look something like this:

template <typename T, T Ptr>
struct pointer_object {
    static T get_pointer() { return Ptr; }
};

int int_ptr(0);

typedef pointer_object<int*, &int_ptr> int_ptr_t;
typedef pointer_object<std::nullptr_t, nullptr> null_ptr_t;

That is, pointer values can be template arguments and, thus, nullptr should be, too.

Answer 2

I guess it's most useful in a setting like this:

template <typename T, T Value> struct Foo;

Foo<int, 10> x;
Foo<std::nullptr_t, nullptr> y;

No harm in that.

(Maybe std::integral_constant is an example of this.)