請(qǐng)問：#if *與 if(*)有什么區(qū)別呢？

77h2_eleven

RT，是不是一樣？

yecheng_110

不一樣
一個(gè)是預(yù)處理機(jī)制里用的
一個(gè)C語言用的

ychang0918

剛剛看GNU Coding Standards
Richard Stallman,
last updated June 10, 2008

3.5 Conditional Compilation
When supporting configuration options already known when building your program we
prefer using if (... ) over conditional compilation, as in the former case the compiler is
able to perform more extensive checking of all possible code paths.
For example, please write
if (HAS_FOO)
...
else
...
instead of:
#ifdef HAS_FOO
...
#else
...
#endif
A modern compiler such as GCC will generate exactly the same code in both cases, and
we have been using similar techniques with good success in several projects. Of course, the
former method assumes that HAS_FOO is defined as either 0 or 1.
While this is not a silver bullet solving all portability problems, and is not always appropriate,
following this policy would have saved GCC developers many hours, or even days,
per year.
In the case of function-like macros like REVERSIBLE_CC_MODE in GCC which cannot be
simply used in if( ...) statements, there is an easy workaround. Simply introduce another
macro HAS_REVERSIBLE_CC_MODE as in the following example:
#ifdef REVERSIBLE_CC_MODE
#define HAS_REVERSIBLE_CC_MODE 1
#else
#define HAS_REVERSIBLE_CC_MODE 0
#endif

[ 本帖最后由 ychang0918 于 2008-7-25 21:57 編輯 ]

醉臥水云間

不一樣

xi2008wang

#if是條件編譯，在預(yù)處理階段處理，預(yù)處理完成后根本就沒有這樣這樣的代碼了
下一階段才是編譯，處理if分支語句

xi2008wang

原帖由 ychang0918 于 2008-7-25 21:56 發(fā)表
剛剛看GNU Coding Standards
Richard Stallman,
last updated June 10, 2008

3.5 Conditional Compilation
When supporting configuration options already known when building your program we
prefer ...

找到個(gè)翻譯
如果已經(jīng)知道配置選項(xiàng),那么在編譯你的程序時(shí),應(yīng)該使用if(...)而不是條件編譯,因?yàn)樵谇耙环N情況下,編譯器能夠?qū)λ�有可能的路徑進(jìn)行廣泛的檢查.
比如: 要寫
if(HAS_FOO)
...
else
...
而不是:
#ifdef HAS_FOO
...
#else
...
#endif

對(duì)于這兩種情況,現(xiàn)代的編譯器比如GCC將會(huì)生成完全相同的代碼,并且我們已經(jīng)在幾個(gè)項(xiàng)目中成功地使用了類似的技術(shù).當(dāng)然,前一種方法假定HAS_FOO被定義為0或1.
盡管它并不象銀彈(silver bullet)那樣解決了所有的可移植性問題,并且并不是總是很恰當(dāng),但是遵照這種策裸可以每年節(jié)省GCC開發(fā)者許多小時(shí),甚至許多天.
在處理GCC中的類似函數(shù)的宏比如REVERSIBLE_CC_MODE時(shí),不能簡(jiǎn)單地使用if(>...)語句.這里有一種簡(jiǎn)單的方法. 再引入另外一個(gè)宏HAS_REVERSIBLE_CC_MODE宏即可. 例如:
#ifdef REVERSIBLE_CC_MODE
#define HAS_REVERSIBLE_CC_MODE 1
#else
#define HAS_REVERSIBLE_CC_MODE 0
#endif

77h2_eleven

原帖由 xi2008wang 于 2008-7-25 22:10 發(fā)表

找到個(gè)翻譯
如果已經(jīng)知道配置選項(xiàng),那么在編譯你的程序時(shí),應(yīng)該使用if(...)而不是條件編譯,因?yàn)樵谇耙环N情況下,編譯器能夠?qū)λ�有可能的路徑進(jìn)行廣泛的檢查.
比如: 要寫
if(HAS_FOO)
...
els ...

多謝，知道了~~~

langue

簡(jiǎn)單說來，你做主的時(shí)候，用 #if；否則，用 if。

langue

原帖由 yecheng_110 于 2008-7-25 13:51 發(fā)表
不一樣
一個(gè)是預(yù)處理機(jī)制里用的
一個(gè)C語言用的

預(yù)處理器也是C語言編譯系統(tǒng)的一部分。

慮而后能得

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學(xué)習(xí)