c - 为什么模数对 int 和 enum 的处理方式不同？

Question

如果运行以下代码，您会发现模数结果对于整数可能为负，但对于枚举始终为正。

这对我来说没问题(我用它来循环选项菜单列表)，但我想确保它是一个 ANSI 约束，而不是编译器实现。

#include <stdio.h>

enum targets {
    TARGET1,
    TARGET2,
    TARGET3,
    TARGET4,
    NUMTARGETS,
};

int main() {
    int i;
    enum targets selected_enum = TARGET3;
    int selected_int = TARGET3;

    for (i = 0; i < 10; i++) {
        selected_enum = (selected_enum - 1) % NUMTARGETS;
        printf("Selected_enum = %d\n", selected_enum); /* I only see positive values */
    }

    for (i = 0; i < 10; i++) {
        selected_int = (selected_int - 1) % NUMTARGETS;
        printf("Selected_int = %d\n", selected_int); /* I see negative values */
    }

    return 0;
}

Answer 1

首先你得写

selected_enum = ( enum targets )( ( selected_enum - 1 ) % NUMTARGETS );

代替

selected_enum = ( selected_enum - 1 ) % NUMTARGETS;

至于你得到的结果是实现定义的。

根据C标准(6.7.2.2枚举说明符)

4 Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined,128) but shall be capable of representing the values of all the members of the enumeration. The enumerated type is incomplete until immediately after the } that terminates the list of enumerator declarations, and complete thereafter.

And(6.3 转换，6.3.1.1 bool 值、字符和整数)

— The rank of any enumerated type shall equal the rank of the compatible integer type

例如 MS VC++ 2010 输出

Selected_enum = 1
Selected_enum = 0
Selected_enum = -1
Selected_enum = -2
Selected_enum = -3
Selected_enum = 0
Selected_enum = -1
Selected_enum = -2
Selected_enum = -3
Selected_enum = 0
Selected_int = 1
Selected_int = 0
Selected_int = -1
Selected_int = -2
Selected_int = -3
Selected_int = 0
Selected_int = -1
Selected_int = -2
Selected_int = -3
Selected_int = 0

看来您使用的编译器将枚举兼容类型定义为无符号类型。

尝试运行下面的代码

#include <stdio.h>

int main( void ) 
{
    enum targets { TARGET1, TARGET2, TARGET3, TARGET4, NUMTARGETS, };

    int i;
    enum targets selected_enum = TARGET3;
    int selected_int = TARGET3;

    for ( i = 0; i < 10; i++ ) 
    {
        if ( ( selected_enum = ( enum targets )( selected_enum - 1 ) ) >= 0 ) printf( "unsigned %u\n", +selected_enum );
        else  printf( "signed %d\n", selected_enum );
    }


    return 0;
}

在 www.ideone.com 上运行这个程序后，我得到了

unsigned 1
unsigned 0
unsigned 4294967295
unsigned 4294967294
unsigned 4294967293
unsigned 4294967292
unsigned 4294967291
unsigned 4294967290
unsigned 4294967289
unsigned 4294967288

而 MS VC++ 2010(和在线 MS VC++ 2014 编译器)输出

unsigned 1
unsigned 0
signed -1
signed -2
signed -3
signed -4
signed -5
signed -6
signed -7
signed -8