a label can only be part of statement and a declaratioin is not a statement

參考資料：

　　https://stackoverflow.com/questions/18496282/why-do-i-get-a-label-can-only-be-part-of-a-statement-and-a-declaration-is-not-a

問題背景：

　　寫了一段code，如下：

#include<stdio.h>

int main()
{
    int a;switch (a) {
        case 0:
            break;
        case 1:
            int aa;
            break;
        case 2:
            break;
        default:
            break;
    }

    return 0;
}

　　如上所示code編譯時候會報錯，錯誤提示“a label can only be part of statement and a declaratioin is not a statement”。

問題原因：

　　引用一段話解釋為“Prior to C99, all declarations had to precede all statements within a block, so it wouldn't have made sense to have a label on a declaration. C99 relaxed that restriction, permitting declarations and statement to be mixed within a block, but the syntax of a labeled-statement was not changed. – Keith Thompson”。

　　翻譯過來就是，C99之前，在一代碼塊中所有的定義都必須在聲明之前，所以，一個標簽在定義之前是沒有意義的。C99放寬了這個限制，允許代碼塊中混合定義、聲明，但這這個標簽不能在定義之前的限制沒有改變。

解決方法：

#include<stdio.h>

int main()
{
    int a;
    switch (a) {
        case 0:
            break;
        case 1:; //加一個空的‘;’，標示空聲明
            int aa;
            break;
        case 2:
            break;
        default:
            break;
    }

    return 0;
}

 

#include<stdio.h>

int main()
{
    int a;
    int aa;    //定義放到 switch 之前
    switch (a) {
        case 0:
            break;
        case 1:
            break;
        case 2:
            break;
        default:
            break;
    }

    return 0;
}