NULL - cppreference.com (original) (raw)
| Defined in header | | | | -------------------------------------------------------------------------- | | | | Defined in header | | | | Defined in header | | | | Defined in header | | | | Defined in header | | | | Defined in header | | | | Defined in header | | | | #define NULL /* implementation-defined */ | | |
The macro NULL is an implementation-defined null pointer constant.
[edit] Possible implementation
#define NULL 0 // since C++11 #define NULL nullptr
[edit] Notes
In C, the macro NULL may have the type void*, but that is not allowed in C++ because null pointer constants cannot have that type.
[edit] Example
#include #include #include #include class S; int main() { int* p = NULL; int* p2 = static_cast<std::nullptr_t>(NULL); void(*f)(int) = NULL; int S::*mp = NULL; void(S::*mfp)(int) = NULL; auto nullvar = NULL; // may trigger a warning when compiling with gcc/clang std::cout << "The type of nullvar is " << typeid(nullvar).name() << '\n'; if constexpr(std::is_same_v<decltype(NULL), std::nullptr_t>) std::cout << "NULL implemented with type std::nullptr_t\n"; else std::cout << "NULL implemented using an integral type\n"; {}(p, p2, f, mp, mfp); // < suppresses "unused variable" warnings }
Possible output:
The type of nullvar is long NULL implemented using an integral type