如果没有给出宏的定义，则对宏有疑问 [英] Doubt on Macros , if definition of macro is not given

问题描述

此处"#define add"未给出定义.

当我对宏调用"add"进行评论时，它不会对o/p产生任何影响.

Here "#define add " no definition is given for it.

When I comment macro call "add", it make no impact on o/p.

//add
addP

结果
------
addadd1

Result
------
addadd1

add
//addP

结果
------
没有o/p

Result
------
no o/p

#include <stdio.h>

#define add
#define addP  \
 printf("add");   \
 printf("add1");



int main(void) {


add
addP

    return 0;
}

输出
======

addadd1

Output
======

addadd1

推荐答案

addP是一个宏，它会生成一些代码，如使用时所见. add是不执行任何操作但可以在预处理程序语句中进行测试的宏.尝试添加以下代码:

addP is a macro that generates some code, as you can see when you use it. add is a macro that does nothing but can be tested in preprocessor statements. Try adding the following code:

#ifdef add
    printf("add is defined");
#else
    printf("add is not defined");
#endif

现在编译并运行，首先使用#define add，然后不使用它.

now compile and run, first with #define add, and secondly without it.

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