为什么NULL未声明？ [英] Why is NULL undeclared?

问题描述

我在尝试编译此代码时遇到此问题：

  typedef struct Node 
 {
 Node（int data）// 
 {
 this-> data = data; 
 previous = NULL; //编译器在这里指示
 next = NULL; 
} 
 
 int data; 
 Node * previous; 
 Node * next; 
} NODE; 
 

当我遇到此错误时：

  \linkedlist\linkedlist.h ||在构造函数`Node :: Node（int）'： 
 \linkedlist\linkedlist.h | 9 |错误：`NULL'未在此范围中声明| 
 || ===构建完成：1错误，0警告=== | 
  

最后一个问题是struct，但是它在我的main.cpp中工作得很好，它在一个头文件，并给我这个问题。我使用Code :: Blocks编译此代码

解决方案

NULL 不是C或C ++语言中的内置常量。事实上，在C ++中它或多或少过时，只是使用一个简单的文本 0 ，而不是根据上下文编译器会做正确的事情。

在新的C ++（C ++ 11及更高版本）中，使用 nullptr （如注释中指出， >

否则，添加

#include< stddef.h >

以获取 NULL 定义。

I have a problem with this struct contructor when I try to compile this code:

typedef struct Node
{
    Node( int data ) //
    {
        this->data = data;
        previous = NULL; // Compiler indicates here
        next = NULL;
    }

    int data;
    Node* previous;
    Node* next;
} NODE;

when I come this error occurs:

\linkedlist\linkedlist.h||In constructor `Node::Node(int)':|
\linkedlist\linkedlist.h|9|error: `NULL' was not declared in this scope|
    ||=== Build finished: 1 errors, 0 warnings ===|

Last problem was the struct, but it worked fine when it was in my main.cpp, this time it's in a header file and is giving me this problem. I am using Code::Blocks to compile this code

解决方案

NULL is not a built-in constant in the C or C++ languages. In fact, in C++ it's more or less obsolete, just use a plain literal 0 instead, the compiler will do the right thing depending on the context.

In newer C++ (C++11 and higher), use nullptr (as pointed out in a comment, thanks).

Otherwise, add

#include <stddef.h>

to get the NULL definition.

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