Why does const auto &p{nullptr} work while auto *p{nullptr} doesn't in C++17?

Question

This definition works:

const auto &b{nullptr};

while this fails:

auto *b{nullptr};

I have tried to compile this in Visual C++, GCC, and Clang. They all complain "cannot deduce type".

In the second case, shouldn't b be deduced to have some type like std::nullptr_t?

Answer 1

It's because you declare b to be a pointer, and initialize it to be a null pointer. But a null pointer to what type of data you don't say, so the compiler can't deduce the type.

If you want b to be a std::nullptr_t object, you should drop the asterisk:

auto b{nullptr};

Answer 2

decltype(nullptr) is std::nullptr_t.

so with

const auto &b{nullptr}; // auto is std::nullptr_t
// b is a reference to a temporary (with lifetime extension)

but nullptr is NOT a pointer (even if it is convertible to).

so auto *b{nullptr}; is invalid.

You might use instead

auto b{nullptr}; // auto is std::nullptr_t

Answer 3

nullptr is of type std::nullptr_t. As a nullptr does not point to anything, there is no corresponding pointee type for std::nullptr_t (you are not allowed to dereference a nullptr), hence

auto *b { nullptr};

requests a type that does not exist. If you want b to be of type nullptr_t simply write

auto b { nullptr};