为什么这个嵌套宏替换失败? [英] Why is this nested macro replacement failing?

问题描述

我正在尝试应用 X Macro 概念，以便可以将所有结构成员初始化为自定义默认(无效)值.我写了以下代码:

I am trying to apply the X Macro concept, in order to have the possibility to initialize all struct members to a custom default (invalid) value. I write the following code:

#define LIST_OF_STRUCT_MEMBERS_foo 
    X(a) 
    X(b) 
    X(c)

#define X(name) int name;
struct foo {
     LIST_OF_STRUCT_MEMBERS_foo
};
#undef X


#define X(name) -1,
static inline void foo_invalidate(struct foo* in) {
     *in = (struct foo){
     LIST_OF_STRUCT_MEMBERS_foo
     };
}
#undef X

#define X(name) -1,
#define foo_DEFAULT_VALUE  { LIST_OF_STRUCT_MEMBERS_foo }
#undef X

static struct foo test = foo_DEFAULT_VALUE;

但是，当我运行预处理器时，foo_DEFAULT_VALUE 的定义无法将 X(name) 调用替换为 -1,

However, when I run the preprocessor, the definition of foo_DEFAULT_VALUE fails to substitute the X(name) calls with -1,

预处理器输出:

struct foo {
     int a; int b; int c;
};

static inline void foo_invalidate(struct foo* in) {
     *in = (struct foo){
     -1, -1, -1, /*Here the substitution worked nicely*/
     };
}

static struct foo test = { X(a) X(b) X(c) }; /*Why this substitution failed?*/

我认为 C-macros 可以引用其他宏.你知道为什么替换失败吗?有什么解决办法吗?

I thought C-macros could refer to other macros. Do you know why that substitution fails? Is there any workaround?

我可以接受 foo_invalidate，但我不愿意放弃在初始化时直接使用值的一步.

I could live with foo_invalidate, but I am reluctant to give up one step from having a value to be used directly at initialization.

推荐答案

假设我们是预处理器，遇到了一行:

Let's pretend that we are the preprocessor and encountering the line:

static struct foo test = foo_DEFAULT_VALUE;

通过 1:

static struct foo test = { LIST_OF_STRUCT_MEMBERS_foo };

通过 2:

static struct foo test = { X(a) X(b) X(c) };

第 3 步:没有可扩展的内容，因为此行未定义 X.

Pass 3: Nothing to expand as X is undefined on this line.

一种解决方法是定义一个 const 变量(可能但不一定是 static)用作默认值:

One workaround could be defining a const variable (possibly but not necessarily static) to be used as default value:

#define X(name) -1,
static const struct foo foo_DEFAULT_VALUE = { LIST_OF_STRUCT_MEMBERS_foo };
#undef X

生成:

static const struct foo foo_DEFAULT_VALUE = { -1, -1, -1, };

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