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NULL

零

Defined in header ​​
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。

DRApplied toBehavior as publishedCorrect behavior
CWG 903C++11constant expressions with zero value such as 1-1 were allowedonly the literal zero is allowed

另见

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

C文档为NULL

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