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

问题描述

当我尝试编译此代码时，此结构构造函数出现问题:

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;

当我来的时候出现这个错误:

when I come this error occurs:

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

最后一个问题是结构，但它在我的 main.cpp 中工作正常，这次它在头文件中并给了我这个问题.我正在使用 Code::Blocks 来编译这段代码

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

推荐答案

NULL 不是 C 或 C++ 语言中的内置常量.事实上，在 C++ 中它或多或少已经过时了，只需使用纯文字 0 代替，编译器会根据上下文做正确的事情.

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.

在较新的 C++(C++11 及更高版本)中，使用 nullptr(如评论中指出的，谢谢).

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

否则，添加

#include stddef.h>

获取NULL定义.

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