是否可以在函数内部使用#define? [英] Is it possible to use #define inside a function?

问题描述

例如，我看到了如下的源代码.我们可以在函数中使用 #define 吗?它是如何工作的?(更多信息:此代码是我从 openvswitch 源代码复制的代码):

For example, I saw source code like the following. Can we use #define in a function? How does it work? (more information: this code is what I copied from openvswitch source code):

void *
ofputil_put_action(enum ofputil_action_code code, struct ofpbuf *buf)
{
switch (code) {
case OFPUTIL_ACTION_INVALID:

#define OFPAT13_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME) case OFPUTIL_##ENUM:
#include "ofp-util.def"
    OVS_NOT_REACHED();

#define OFPAT10_ACTION(ENUM, STRUCT, NAME)                  \
case OFPUTIL_##ENUM: return ofputil_put_##ENUM(buf);
#define OFPAT11_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME)      \
case OFPUTIL_##ENUM: return ofputil_put_##ENUM(buf);
#define NXAST_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME)        \
case OFPUTIL_##ENUM: return ofputil_put_##ENUM(buf);
#include "ofp-util.def"
}
OVS_NOT_REACHED();
}

#define OFPAT10_ACTION(ENUM, STRUCT, NAME)                        \
void                                                        \
ofputil_init_##ENUM(struct STRUCT *s)                       \
{                                                           \
    memset(s, 0, sizeof *s);                                \
    s->type = htons(ENUM);                                  \
    s->len = htons(sizeof *s);                              \
}                                                           \
                                                            \
struct STRUCT *                                             \
ofputil_put_##ENUM(struct ofpbuf *buf)                      \
{                                                           \
    struct STRUCT *s = ofpbuf_put_uninit(buf, sizeof *s);   \
    ofputil_init_##ENUM(s);                                 \
    return s;                                               \
}
#define OFPAT11_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME) \
OFPAT10_ACTION(ENUM, STRUCT, NAME)
#define OFPAT13_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME) \
OFPAT10_ACTION(ENUM, STRUCT, NAME)
#define NXAST_ACTION(ENUM, STRUCT, EXTENSIBLE, NAME)            \
void                                                        \
ofputil_init_##ENUM(struct STRUCT *s)                       \
{                                                           \
    memset(s, 0, sizeof *s);                                \
    s->type = htons(OFPAT10_VENDOR);                        \
    s->len = htons(sizeof *s);                              \
    s->vendor = htonl(NX_VENDOR_ID);                        \
    s->subtype = htons(ENUM);                               \
}                                                           \
                                                            \
struct STRUCT *                                             \
ofputil_put_##ENUM(struct ofpbuf *buf)                      \
{                                                           \
    struct STRUCT *s = ofpbuf_put_uninit(buf, sizeof *s);   \
    ofputil_init_##ENUM(s);                                 \
    return s;                                               \
}
#include "ofp-util.def"

推荐答案

#define 是预处理程序指令:用于生成最终的C ++代码在将其处理给编译器之前，将生成一个可执行文件.因此，代码如下:

#define is a preprocessor directive: it is used to generate the eventual C++ code before it is handled to the compiler that will generate an executable. Therefore code like:

for(int i = 0; i < 54; i++) {
  #define BUFFER_SIZE 1024
}

不执行了54次(在预处理器级别):预处理器只是在 for 循环上运行(不知道 for 的作用是什么)循环是)，看到一个define语句，将 1024 与 BUFFER_SIZE 关联并继续.直到到达文件底部为止.

is not executed 54 times (at the preprocessor level): the preprocessor simply runs over the for loop (not knowing what a for loop is), sees a define statement, associates 1024 with BUFFER_SIZE and continues. Until it reaches the bottom of the file.

由于预处理器并不真正了解程序本身，因此您可以在任何地方编写 #define .

You can write #define everywhere since the preprocessor is not really aware of the program itself.

这篇关于是否可以在函数内部使用#define?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！