C++ Programming – Why Address of nullptr Cannot Be Taken

c++c++11nullptrstd

In the C++11 standard, I don't understand the reason why taking the address of nullptr is disallowed whereas one is allowed to take the address of their own std::nullptr_t instances. Aside from the fact that nullptr is a reserved keyword, is there any designated reasoning for this decision?

Simply because it amuses me, I attempted to circumnavigate this restriction with the following function:

decltype(nullptr)* func(const decltype(nullptr) &nref) noexcept
{
    return const_cast<decltype(nullptr)*>(reinterpret_cast<const decltype(nullptr)*>(&nref));
}

I had to use reinterpret_cast on the parameter because without it I was getting the hysterical error:

error: invalid conversion from 'std::nullptr_t*' to 'std::nullptr_t*' [-fpermissive]

When I call this function by passing nullptr directly I get a different address each time. Is nullptr dynamically assigned an address just-in-time for comparisons and such? Or (probably more likely) perhaps is the compiler forcing a temporary copy of the underlying object?

Of course none of this is vital information, I just find it interesting why this particular restriction was implemented (and subsequently why I am seeing the behavior I am).

Best Answer

It's the same as not being able to take the address of 5 even though you can take the address of an int after giving it the value 5. It doesn't matter that there's no alternative value for a nullptr_t to have.

Values don't have addresses; objects do.

A temporary object is generated when you pass such a value to a const & parameter, or otherwise bind a value to a const reference, such as by static_cast< T const & >( … ) or declaring a named reference T const & foo = …;. The address you're seeing is that of the temporary.

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