錯誤：“if”之前的預期表達式 (error: expected expression before ‘if’)

問題描述

錯誤：“if”之前的預期表達式 (error: expected expression before ‘if’)

這是我的代碼，不知道哪裡錯了，什麼是“表達式”？

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

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

這裡是輸出：

/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

參考解法

方法 1:

(if(expression) {} else {}) is not valid syntax.

Try

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

方法 2:

You can't enclose a block in C in braces (()) ‑ just remove them, and you should be OK. Also, it probably won't be a bad idea to surround calls to smth with braces:

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

(by nwaicaethi、Almo、Mureinik)

參考文件

1. error: expected expression before ‘if’ (CC BY‑SA 2.5/3.0/4.0)