gcc 预处理器在 #if

Question

我遇到了奇怪的问题，因为 gcc 预处理器在 #if 指令上失败。

示例代码：

#include "stdio.h"

enum BoardTypes{
    Board_IO_WithAdc,
    Board_IO_WithEnc,
    Board_PakelcIn,
    Board_PakelcOutP,
    Board_PakelcOutV,
};

#define BoardAdr Board_PakelcIn

int main(void){

    #if ((BoardAdr==Board_PakelcOutV) || (BoardAdr==Board_PakelcOutP))
        printf("Board_PakelcOutV or Board_PakelcOutP\n");
    #endif

    #if (BoardAdr==Board_PakelcIn)
        printf("Board_PakelcIn\n");
    #endif
}

预期结果是Board_PakelcIn，但得到了Board_PakelcOutV或Board_PakelcOutP，Board_PakelcIn。

显然这里出了问题。

Answer 1

所有宏都会扩展为未定义的内容，被视为 0。

预处理器没有你的

enum

的概念，所以你的代码本质上就是：

#include "stdio.h"

#define BoardAdr Board_PakelcIn

int main(void){

    #if ((BoardAdr==Board_PakelcOutV) || (BoardAdr==Board_PakelcOutP))
        printf("Board_PakelcOutV or Board_PakelcOutP\n");
    #endif

    #if (BoardAdr==Board_PakelcIn)
        printf("Board_PakelcIn\n");
    #endif
}

将

BoardAdr

替换为

Board_PakelcIn

后，我们得到：

#include "stdio.h"

int main(void){

    #if ((Board_PakelcIn==Board_PakelcOutV) || (Board_PakelcIn==Board_PakelcOutP))
        printf("Board_PakelcOutV or Board_PakelcOutP\n");
    #endif

    #if (Board_PakelcIn==Board_PakelcIn)
        printf("Board_PakelcIn\n");
    #endif
}

现在，所有

Board_PakelcIn

、

Board_PakelcOutV

和

Board_PakelcOutP

都未定义，因此

0

，所以我们剩下：

#include "stdio.h"

int main(void){

    #if ((0==0) || (0==0))
        printf("Board_PakelcOutV or Board_PakelcOutP\n");
    #endif

    #if (0==0)
        printf("Board_PakelcIn\n");
    #endif
}