As far as I'm aware nullptr is a part of the core language.
Quoting C++11: (18.2/9)
nullptr_tis defined as follows:
namespace std { typedef decltype(nullptr) nullptr_t; }
and is defined in the header <cstddef>.
3 Answers
Because it can. A central aim in the C++ standardization process is to alter the core language as little as possible when adding to the language.
nullptr usurps the use of 0 to mean both a null pointer and, er, zero. Using 0 for both caused problems for obvious reasons, does f(0) call f(int) or f(int*)? So a brand new literal was added to the core language: nullptr. Its type is simply decltype(nullptr) so nullptr_t was added as a short cut:
namespace std {
using nullptr_t = decltype(nullptr);
}
answered Aug 23, 2019 at 14:48
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Thank you, but the nullptr itself can be a part of the STL... Is nullptr included in the core language to not require developers to include any header files?– embedcCommented Aug 23, 2019 at 14:54
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@embedc Any
stdtypedefs like e.g.std::stize_tare provided through headers, so what? Commented Aug 23, 2019 at 14:59 -
2@embedc
nullptris a literal like5,'a'or"hello". While I can't imagine what it is, I would bet there is a good technical reason why a null pointer literal is better than having anullptr-like object instd. Edit : For one thing, being a literal it's a prvalue so you can't take it's address. It would seem strange to me to be able to takenullptr's address. It would be like gettingtrue's address. Commented Aug 23, 2019 at 14:59 -
1No headers are required. It is a built-in keyword / pointer literal.– RonCommented Aug 23, 2019 at 15:03
The proposal that introduced nullptr, N2431, indicates in section 1.1 that it was desirable to not force users to include a header in order to use nullptr.
It also remarks, "We do not expect to see much direct use of nullptr_t in real programs". Thus, it was considered preferable to add nullptr_t to the library rather than create a new keyword only to be used for this obscure purpose. In addition, if you don't want to include the header, you can always just write decltype(nullptr) yourself.
answered Aug 23, 2019 at 15:52
From cppreference.com:
std::nullptr_t is the type of the null pointer literal, nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type.
If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.
You can then solve overloaded function call ambiguity with std::nullptr_t.
For example:
void Foo(int* ptr) {}
void Foo(double* ptr) {}
void Foo(std::nullptr_t ptr) {} // This overload is called if Foo(nullptr) is invoked
Read more about std::nullptr_t here: https://en.cppreference.com/w/cpp/types/nullptr_t
nullptr_tif you are willing to live withdecltype(nullptr).nullptr_tisn't a core concept, it's a helper.nullptr_tsometimes: "If two or more overloads accept different pointer types, an overload forstd::nullptr_tis necessary to accept a null pointer argument." (cppreference for std::nullptr_t).nullptr_tif you are willing to live withdecltype(nullptr). You can usedecltype(nullptr)instead ofnullptr_t.nullptr_tis just a convenient alias fordecltype(nullptr);. You should usenullptr_t, my comment is meant to indicate whynullptr_tmight not be part of the core language.nullptrpart of the language, it is much easier to ensure that it works as it should because you can easily have specific rules.