Question

这是我的代码，我不知道我错在哪里，什么是＆＃34;表达＆＃34;？

#define m(smth) (if(sizeof(smth) == sizeof(int)) {printf("%d", (int) smth);} else{puts((char*)smth);})

int main(void) {
    m("smth");
}

这里是输出：

/home/roroco/Dropbox/rbs/ro_sites/c/ex/ex2.c: In function ‘main’:
/home/roroco/Dropbox/rbs/ro_sites/c/ex/ex2.c:18:18: error: expected expression before ‘if’
 #define m(smth) (if(sizeof(smth) == sizeof(int)) {printf("%d", (int) smth);} else{puts((char*)smth);})
                  ^
/home/roroco/Dropbox/rbs/ro_sites/c/ex/ex2.c:21:5: note: in expansion of macro ‘m’
     m("smth");
     ^
make[3]: *** [ex/CMakeFiles/ex2.dir/ex2.c.o] Error 1
make[2]: *** [ex/CMakeFiles/ex2.dir/all] Error 2
make[1]: *** [ex/CMakeFiles/ex2.dir/rule] Error 2
make: *** [ex2] Error 2

Answer 1

(if(expression) {} else {})是无效的语法。

尝试

#define m(smth) if(sizeof(smth) == sizeof(int)) {printf("%d", (int) smth);} else{puts((char*)smth);}

Answer 2

你不能将C中的块括在大括号中（()） - 只需删除它们，你应该没问题。此外，使用大括号括起smth的呼叫可能不是一个坏主意：

#define m(smth) if(sizeof(smth) == sizeof(int)) {printf("%d", (int) (smth));} else{puts((char*)(smth));}

错误：'if'之前的预期表达式

2 个答案: