关于NULL的问题 [英] A question about NULL

问题描述

我是C ++的新手（在今年夏天开始学习），我

有以下问题（对不起，如果它听起来愚蠢）：

在许多代码示例和源文件中，我看到正在使用NULL表达式（对于

示例，int * someInt = NULL;）。我自己使用了类似的初始化，即使我没有定义NULL，它也可以正常工作。但是什么是NULL究竟？它是由编译器定义的

常量吗？以下

两种初始化指针的方法有什么区别吗？

//示例1

int * myInt = NULL;


//示例2

int * myInt = 0;


如果没有差异，我为什么要使用NULL而不是0？

提前致谢。


Dmitry

PS这是我在新闻组中的第一篇文章:)

Hi,
I''m new to C++ (started learning in the beginning of this summer), and I
have the following question (sorry if it sounds stupid):
In many code samples and source files, I see NULL expression being used (for
example, int* someInt=NULL; ). I used similar initialization myself, and it
works fine even if I don''t define NULL. But what is "NULL" exactly? Is it a
constant defined by compiler? Is there any difference between the following
two ways to initialize a pointer?
// example 1
int * myInt = NULL;

// example 2
int * myInt = 0;

If there is no difference, why would I use NULL instead of 0?
Thanks in advance.

Dmitry
P.S. This is my first post in the newsgroup :)

推荐答案

" Dmitry D" < WO **** @ telus.net>在消息中写道

news：xv ********************* @ news0.telusplanet.net ...

"Dmitry D" <wo****@telus.net> wrote in message
news:xv*********************@news0.telusplanet.net ...

我是C ++的新手（在今年夏天开始学习），我有以下问题（对不起，如果听起来很愚蠢）：
很多代码示例和源文件，我看到使用
的NULL表达式（例如，int * someInt = NULL;）。我自己使用了类似的初始化，并且
它工作正常，即使我没有定义NULL。但是什么是NULL究竟？它是
编译器定义的常量吗？两种初始化指针的方法之间的
有什么区别吗？
//示例1
int * myInt = NULL;

//示例2
int * myInt = 0;

Hi,
I''m new to C++ (started learning in the beginning of this summer), and I
have the following question (sorry if it sounds stupid):
In many code samples and source files, I see NULL expression being used (for example, int* someInt=NULL; ). I used similar initialization myself, and it works fine even if I don''t define NULL. But what is "NULL" exactly? Is it a constant defined by compiler? Is there any difference between the following two ways to initialize a pointer?
// example 1
int * myInt = NULL;

// example 2
int * myInt = 0;

在C中，NULL的定义如下：


＃ define NULL（void *）0

在C ++中，NULL定义如下：


#define NULL 0

如果你尝试初始化一个指向void的指针类型的指针，你会用C ++编译器得到错误：


int * p =（无效*）0; //确定''C *，C ++中的错误

int * p = 0; //在C ++中运行，'C'中的错误'

C ++专门禁止NULL的''C''样式定义。为什么？也许

为''C''样式分配一个''类型'指针'NULL可以被认为是

类似于指定派生类型的''指针''到'''指向基地的指针

类型''，这是错误的。然后，再次指定一个''指向一个类型的指针''

到零值是更正确的;除此之外''void''不是所有类型的基础

类型？你可以说它有点混乱（我这样做）。然后再次获得
，C ++并不是真正的OO语言;这是'C'的OO扩展的包袱。

本质上只是便携式汇编程序。为了正确地完成工作，你需要一种语言，它具有适用于所有类型的通用基本类型，并且每种派生类型都需要Null

实例。

In ''C'', NULL is defined like this:

#define NULL (void*)0
In C++, NULL is defined like this:

#define NULL 0
If you try and initialise a pointer to a type with a pointer to void, you''ll
get errors with a C++ compiler:

int* p = (void*)0; // OK in ''C*, error in C++
int* p = 0; // OK in C++, error in ''C''
C++ specifically forbids the ''C'' style definition of NULL. Why? Perhaps
assigning a ''pointer to a type'' to the ''C'' style NULL could be considered
similar to assigning a ''pointer to a derived type'' to a ''pointer to base
type'', which would be wrong. Then again, is assigning a ''pointer to a type''
to the value zero is any more correct; besides which ''void'' is not the base
type of all types? You could say that it''s a bit of shambles (I do). Then
again, C++ isn''t a real OO language; it''s a baggage of OO extensions to ''C''
which is, essentially, just portable assembler. To do the job properly, you
would need a language that had a common base type for all types and Null
instances of every derived type.

Dmitry D写道：

Dmitry D wrote:

我是C ++的新手（在今年夏天开始学习），我是
有以下问题（对不起，如果它听起来很愚蠢）：
在许多代码示例和源文件中，我看到正在使用NULL表达式（例如，对于
示例，int * someInt = NULL;）。我自己使用了类似的初始化，即使我没有定义NULL，它也能正常工作。但是什么是NULL究竟？它是由编译器定义的常量吗？以下两种初始化指针的方法有什么区别吗？
//示例1
int * myInt = NULL;

//示例2
int * myInt = 0;

如果没有区别，为什么我会使用NULL而不是0？
在此先感谢。

Dmitry

Hi,
I''m new to C++ (started learning in the beginning of this summer), and I
have the following question (sorry if it sounds stupid):
In many code samples and source files, I see NULL expression being used (for
example, int* someInt=NULL; ). I used similar initialization myself, and it
works fine even if I don''t define NULL. But what is "NULL" exactly? Is it a
constant defined by compiler? Is there any difference between the following
two ways to initialize a pointer?
// example 1
int * myInt = NULL;

// example 2
int * myInt = 0;

If there is no difference, why would I use NULL instead of 0?
Thanks in advance.

Dmitry
P.S. This is my first post in the newsgroup :)

NULL通常是一个宏


#define NULL 0

有时（但仅限于C作为C ++


#define NULL（（void *）0）

'' 0''是一个特殊情况，意思是


int * ptr = 0; //开心的编译器


完全合法但是


int * ptr = 1; //非法 - 胡思乱想的编译器


错误：从`int''无效转换为`int *''

为什么是NULL？有些人认为可读性。有人会认为NULL是

而不是'0'，例如我*可以*（但我永远不会定义

NULL和（（void *）1）。


NULL通常在其中一个标准头文件中定义。

NULL is usually a macro

#define NULL 0

and sometimes (but only for C as C++

#define NULL ((void *) 0)
''0'' is a special case in the sense that

int * ptr = 0; // happy compiler

Is perfectly legal ... but

int * ptr = 1; // Illegal - cranky compiler

error: invalid conversion from `int'' to `int*''
Why NULL? Some would argue readability. Some would argue that NULL is
not neccassarily ''0'', for example I *could* (but I would never) define
NULL and ((void *) 1).

NULL is usually defined in one of the standard header files.

Dmitry D在新闻中写道：xv ********************* @ news0.telusplanet.net:

Dmitry D wrote in news:xv*********************@news0.telusplanet.net :

我是C ++的新手（开始学习ning在今年夏天开始），并且我有以下问题（对不起，如果它听起来很愚蠢）：
在许多代码示例和源文件中，我看到使用了NULL表达式（例如，int * someInt = NULL; ）。我自己使用了类似的初始化，即使我没有定义NULL也能正常工作。但是什么是
NULL究竟？它是由编译器定义的常量吗？以下两种初始化指针的方法之间是否有任何区别？
//示例1
int * myInt = NULL;

//示例2
int * myInt = 0;

如果没有区别，为什么我会使用NULL而不是0？
提前致谢。

Hi,
I''m new to C++ (started learning in the beginning of this summer), and
I have the following question (sorry if it sounds stupid):
In many code samples and source files, I see NULL expression being
used (for example, int* someInt=NULL; ). I used similar initialization
myself, and it works fine even if I don''t define NULL. But what is
"NULL" exactly? Is it a constant defined by compiler? Is there any
difference between the following two ways to initialize a pointer?
// example 1
int * myInt = NULL;

// example 2
int * myInt = 0;

If there is no difference, why would I use NULL instead of 0?
Thanks in advance.

NULL是#define'的宏使用它你必须包含

中的一个定义它的系统头，但不要使用它。 br />

在C ++中，NULL通常定义为0或0L，因此在您给出的2个示例中没有

的差异。

在C中，NULL通常被定义为（（void *）0），这是更多

typesafe"在C.


因此，如果你有一些C编译器和C ++编译器可以看到的代码，那么宏NULL很有用，否则使用0表示C ++。


Rob。

-
http://www.victim-prime.dsl.pipex.com/

NULL is a #define''s macro to use it you have to include one of
the system header''s that defines it, but don''t use it.

In C++ NULL is usually defined to be 0 or 0L, so there is no
difference in the 2 examples you gave.

In C NULL is usually defined to be ((void *)0), this is more
"typesafe" in C.

So the macro NULL is useful if you have some code that is going to
bee seen by a C compiler and a C++ compiler, otherwise use 0 for C++.

Rob.
--
http://www.victim-prime.dsl.pipex.com/

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