NULL

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

Type support

Basic types
Fixed width integer types(C++11)
Fixed width floating-point types(C++23)
ptrdiff_t size_t max_align_t (C++11)     byte (C++17)	nullptr_t (C++11) offsetof NULL
Numeric limits
numeric_limits
C numeric limits interface
Runtime type information
type_info type_index (C++11)	bad_typeid bad_cast

 

Defined in header <clocale>
Defined in header <cstddef>
Defined in header <cstdio>
Defined in header <cstdlib>
Defined in header <cstring>
Defined in header <ctime>
Defined in header <cwchar>
#define NULL /* implementation-defined */

The macro NULL is an implementation-defined null pointer constant.

Possible implementation

#define NULL 0// since C++11#define NULL nullptr

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.

Example

Run this code

#include <cstddef>#include <iostream>#include <type_traits>#include <typeinfo>   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';   ifconstexpr(std::is_same_v<decltype(NULL), std::nullptr_t>)std::cout<<"NULL implemented with type std::nullptr_t\n";elsestd::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

See also

nullptr(C++11)	the pointer literal which specifies a null pointer value
nullptr_t (C++11)	the type of the null pointer literal nullptr (typedef)
C documentation for NULL