谁知道#ifndef 和#if !define 的区别？

[wangweicai]

谁知道#ifndef 和#if !define 的区别？
如果没有区别的话，应该怎么用？

[winco]

#ifdef identifier

#ifndef identifier

is equivalent to

#if defined identifier

#if !defined identifier

You can use the #ifdef and #ifndef directives anywhere #if can be used. The #ifdef identifier statement is equivalent to #if 1 when identifier has been defined, and it is equivalent to #if 0 when identifier has not been defined or has been undefined with the #undef directive. These directives check only for the presence or absence of identifiers defined with #define, not for identifiers declared in the C or C++ source code.

These directives are provided only for compatibility with previous versions of the language. The defined( identifier ) constant expression used with the #if directive is preferred.

The #ifndef directive checks for the opposite of the condition checked by #ifdef. If the identifier has not been defined (or its definition has been removed with #undef), the condition is true (nonzero). Otherwise, the condition is false (0).

从上面MSDN上的话可以看出，他们没有区别：）
用法：
用来定义一个变量，或者头文件，
防止重复定义和重复引用。

[qing_li73]

The #ifdef and #ifndef Directives
The #ifdef and #ifndef directives perform the same task as the #if directive when it is used with defined( identifier ).

Syntax

#ifdef identifier

#ifndef identifier

is equivalent to

#if defined identifier

#if !defined identifier

You can use the #ifdef and #ifndef directives anywhere #if can be used. The #ifdef identifier statement is equivalent to #if 1 when identifier has been defined, and it is equivalent to #if 0 when identifier has not been defined or has been undefined with the #undef directive. These directives check only for the presence or absence of identifiers defined with #define, not for identifiers declared in the C or C++ source code.

These directives are provided only for compatibility with previous versions of the language. The defined( identifier ) constant expression used with the #if directive is preferred.

The #ifndef directive checks for the opposite of the condition checked by #ifdef. If the identifier has not been defined (or its definition has been removed with #undef), the condition is true (nonzero). Otherwise, the condition is false (0).

Microsoft Specific

The identifier can be passed from the command line using the /D option. Up to 30 macros can be specified with /D.

This is useful for checking whether a definition exists, because a definition can be passed from the command line. For example:

// PROG.CPP
#ifndef test // These three statements go in your source code.
#define final
#endif

CL /Dtest prog.cpp // This is the command for compilation.

END Microsoft Specific

[wangweicai]

用汉语讲一下好吗？
看得我头都晕了，都没看明白。

[riverboat]

基本上一样，不过后者的应用范围更大，可以支持多个预编译变量的检查：
#if (!defined(_DEBUG) && defined(USE_MYLIB))
..........
#endif

这种情况用前一种方式就只能写一个嵌套的条件判断：
#ifndef _DEBUG
#ifdef USE_MYLIB
................
#endif
#endif

[riverboat]

BTW，你的defined写掉了一个d
：）

[wangweicai]

riverboat(诸葛不亮) ：
对不起，
谢了。

[hudgan]

study

[zhaohangcom]

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