c++ - 如何使用条件预处理器排除以下代码

Question

例如

typedef union
{
    struct
    { 
        unsigned fb_num                :  8 __attribute__ ((packed));
        unsigned stream_id             : 12 __attribute__ ((packed));
        unsigned lossless              :  1 __attribute__ ((packed));
        unsigned proto_idx             :  6 __attribute__ ((packed));
        unsigned da_logport            : 13 __attribute__ ((packed));
        unsigned sa_spare_dx           : 10 __attribute__ ((packed));
        unsigned da_spare_idx          : 10 __attribute__ ((packed));
        unsigned mcast_index           : 12 __attribute__ ((packed));/* ISH_B*/
        unsigned mesh_idx              :  8 __attribute__ ((packed));
        unsigned in_mirror             :  4 __attribute__ ((packed));
        unsigned vlan_idx              : 12 __attribute__ ((packed));
        unsigned dis_stp               :  1 __attribute__ ((packed));
        unsigned trkbal_idx            :  5 __attribute__ ((packed));
...

我想排除此代码部分：__attribute__ ((packed))，以便编译器将其读取为：

typedef union
{
    struct
    { 
        unsigned fb_num                :  8 ;
        unsigned stream_id             : 12 ;
        unsigned lossless              :  1 ;
        unsigned proto_idx             :  6 ;
        unsigned da_logport            : 13 ;
        unsigned sa_spare_dx           : 10 ;
        unsigned da_spare_idx          : 10 ;
        unsigned mcast_index           : 12 ;/* ISH_B*/
        unsigned mesh_idx              :  8 ;
        unsigned in_mirror             :  4 ;
        unsigned vlan_idx              : 12 ;
        unsigned dis_stp               :  1 ;
        unsigned trkbal_idx            :  5 ;
...

现在应该编译的宏是MCAST_SIMULATION：即如果MCAST_SIMULATION已定义，则排除提到的代码部分，否则不排除。我怎样才能做到这一点？

Answer 1

__attribute__ ((packed))用宏 替换文本（例如PACKED，并有条件地定义：

#ifndef MCAST_SIMULATION
#define PACKED __attribute__ ((packed))
#else
#define PACKED
#endif

Answer 2

你定义__attribute__()为无。

像这样：

#ifdef MCAST_SIMULATION
# define __attribute__(x)
#endif

Answer 3

您可以为此引入自己的宏。就像是：

#ifndef MCAST_SIMULATION
#define ATTRIB_PACKED __attribute__((packed))
#else
#define ATTRIB_PACKED
#endif

用法：

unsigned fb_num :  8 ATTRIB_PACKED ;