ç$ P $＃如果pprocessor前pression问题 [英] C preprocessor #if expression question

问题描述

我对前pression我们可以在C语言与#IF preprocessor使用的类型有点糊涂。我尝试以下code和它不工作。请解释和可与preprocessor使用前pressions提供范例

 ＃包括LT＆;＆stdio.h中GT;
＃包括LT＆;＆CONIO.H GT;
＃包括LT＆;＆stdlib.h中GT;
INT C = 1;＃如果Ç== 1
＃定义检查（一）款（a == 1）：5
＃定义TABLE_SIZE 100
＃万一
诠释的main（）
{
诠释一个= 0，B;
的printf（a =％d个\\ N，一）;
B =检查（一）;
输出（A =％D \\ n，一，TABLE_SIZE）;
系统（暂停）;
返回0;
}
 

解决方案

在preprocessor不能使用变量来自前pressions C程序 - 它只能在preprocessor宏行事。所以，当你尝试在preprocessor使用 C 你没有得到你所期望的。

不过，你也别因为当preprocessor尝试计算未被定义为宏的标识符，它把标识符作为具有零价值得到一个错误。

所以，当你打这个片断：

 ＃如果Ç== 1
＃定义检查（一）款（a == 1）：5
＃定义TABLE_SIZE 100
＃万一
 

的 C 使用的preprocessor无关的变量 C 从C程序。在preprocessor看起来，看看是否有公司的C 为定义的宏。既然没有，则计算如下EX pression：

  0＃如果== 1
 

这是假的，当然。

既然你似乎没有使用变量 C 在程序中，你可以做以下获得一致的行为与你想什么：

 ＃定义的C 1＃如果Ç== 1
＃定义检查（一）款（a == 1）：5
＃定义TABLE_SIZE 100
＃万一
 

（请注意，我也做了与惯例宏名保持大写宏名。）

I am a bit confused on the type of expression we can use with #IF preprocessor in c language. I tried the following code and it isn't working. Please explain and provide examples for expressions that can be used with the preprocessor.

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
int c=1;

#if c==1
#define check(a) (a==1)?a:5
#define TABLE_SIZE 100
#endif
int main()
{
int a=0,b;
printf("a=%d\n",a);
b=check(a);
printf("a=%d %d\n",a,TABLE_SIZE);
system("PAUSE");
return 0;
}

解决方案

The preprocessor cannot use variables from the C program in expressions - it can only act on preprocessor macros. So when you try to use c in the preprocessor you don't get what you might expect.

However, you also don't get an error because when the preprocessor tries to evaluate an identifier that isn't defined as a macro, it treats the identifier as having a value of zero.

So when you hit this snippet:

#if c==1
#define check(a) (a==1)?a:5
#define TABLE_SIZE 100
#endif

The c used by the preprocessor has nothing to do with the variable c from the C program. The preprocessor looks to see if there's a macro defined for c. Since there isn't, it evaluates the following expression:

#if 0==1

which is false of course.

Since you don't appear to use the variable c in your program, you can do the following to get behavior in line with what you're trying:

#define C 1

#if C==1
#define check(a) (a==1)?a:5
#define TABLE_SIZE 100
#endif

(Note that I also made the macro name uppercase in keeping with convention for macro names.)

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