Predefined null pointer constant (since C23)

Syntax

nullptr (since C23)

Explanation

The keyword nullptr denotes a predefined null pointer constant. It is a non-lvalue of type nullptr_t. nullptr can be converted to a pointer types or bool, where the result is the null pointer value of that type or false respectively.

Keywords

nullptr

Example

Demonstrates that a copy of nullptr can also be used as a null pointer constant.

#include <stddef.h>
#include <stdio.h>
 
void g(int*)
{
    puts("Function g called");
}
 
#define DETECT_NULL_POINTER_CONSTANT(e) \
    _Generic(e,                         \
        void* : puts("void*"),          \
        nullptr_t : puts("nullptr_t"),  \
        default : puts("integer")       \
    )
 
int main()
{
    g(nullptr); // OK
    g(NULL); // OK
    g(0); // OK
 
    auto cloned_nullptr = nullptr;
    g(cloned_nullptr); // OK
 
    [[maybe_unused]] auto cloned_NULL = NULL;
//  g(cloned_NULL); // implementation-defined: maybe OK
 
    [[maybe_unused]] auto cloned_zero = 0;
//  g(cloned_zero); // Error
 
    DETECT_NULL_POINTER_CONSTANT(((void*)0));
    DETECT_NULL_POINTER_CONSTANT(0);
    DETECT_NULL_POINTER_CONSTANT(nullptr);
    DETECT_NULL_POINTER_CONSTANT(NULL); // implementation-defined
}

Possible output:

Function g called
Function g called
Function g called
Function g called
void*
integer
nullptr_t
void*

See also

implementation-defined null pointer constant
(macro constant)
(C23)
 the type of the predefined null pointer constant nullptr
(typedef)
C++ documentation for nullptr

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https://en.cppreference.com/w/c/language/nullptr