NULL
Defined in header <cstddef> | | |
|---|---|---|
Defined in header <cstring> | | |
Defined in header <cwchar> | | |
Defined in header <ctime> | | |
Defined in header <cstdio> | | |
Defined in header <clocale> | | |
Defined in header <cstdlib> | | |
#define NULL /*implementation-defined*/ | | |
宏NULL是实现定义的空指针常量，它可能是。
an integral constant expression rvalue of integer type that evaluates to zero. | (until C++11) |
|---|---|
an integer literal with value zero, or a prvalue of type std::nullptr_t. | (since C++11) |
空指针常量可能是隐式转换指向任何指针类型；这种转换将导致该类型的空指针值。如果空指针常量具有整数类型，则可以将其转换为类型的prvalue。std::nullptr_t...
可能的实施
#定义空0//自C++11#定义空nullptr
*。
注记
在C中，宏NULL可能有这种类型void*，但这在C++中是不允许的。
例
二次
#include <cstddef>
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;
}二次
缺陷报告
以下行为更改缺陷报告追溯应用于先前发布的C++标准。
DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
CWG 903 | C++11 | constant expressions with zero value such as 1-1 were allowed | only the literal zero is allowed |
另见
nullptr | the pointer literal which specifies a null pointer value (C++11) |
|---|---|
nullptr_t (C++11) | the type of the null pointer literal nullptr (typedef) |
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