宏扩展错误 [英] Error in macro expansion

问题描述

我一直试图理解宏扩展，发现第二个printf给出了一个错误.我期望第二个print语句生成与第一个相同的输出.我知道有一些函数可以进行字符串连接.我发现很难理解为什么第一个打印语句有效而第二个打印语句无效.

I have been trying to understand macro expansion and found out that the second printf gives out an error. I am expecting the second print statement to generate the same output as the first one. I know there are functions to do string concatenation. I am finding it difficult to understand why first print statement works and the second doesn't.

#define CAT(str1, str2) str1 str2

void main()
{
    char *string_1 = "s1", *string_2 = "s2";
    printf(CAT("s1", "s2"));
    printf(CAT(string_1, string_2));
}

推荐答案

尝试手动"进行预处理:

Try to do the preprocessing "by hand":

CAT 应该带有2个输入变量，并一个接一个地打印它们，并在它们之间留一个空格.所以...如果我们对您的代码进行预处理，它将变成:

CAT is supposed to take 2 input variables, and print them one after the other, with a space between. So... if we preprocess your code, it becomes:

void main()
{
    char *string_1 = "s1", *string_2 = "s2";
    printf("s1" "s2");
    printf(string_1 string_2);
}

编译器将"s1""s2" 自动链接为"s1s2" ， string_1 string_2 是无效的语法.

While "s1" "s2" is automatically concatenated to "s1s2" by the compiler, string_1 string_2 is invalid syntax.

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