我可以附加到预处理器宏吗? [英] Can I append to a preprocessor macro?

问题描述

在标准 C 或 GNU 扩展中是否有任何方法可以将内容附加到宏定义中?例如，给定一个定义为
的宏#define List foo bar
我可以附加 bas 以便它 List 像我定义它一样扩展
#define List foo bar bas?

Is there any way in standard C—or with GNU extensions—to append stuff to a macro definition? E.g., given a macro defined as
#define List foo bar
can I append bas so that it List expands as if I’d defined it
#define List foo bar bas?

我希望我能做这样的事情:

I was hoping I could do something like this:

#define List    foo bar bas

#define List_   Expand(List)
#undef List
#define List    Expand(List_) quux

但我不知道如何定义 Expand() 宏，所以它会做我想做的事.

but I can’t figure out how to define the Expand() macro so it’ll do what I want.

动机:我正在按照这些思路与受歧视/标记的工会一起玩:

Motivation: I’m playing with discriminated/tagged unions along these lines:

struct quux_foo { int x; };
struct quux_bar { char *s; };
struct quux_bas { void *p; };

enum quux_type {quux_foo, quux_bar, quux_bas};

struct quux {
    enum quux_type type;
    union {
        struct quux_foo foo;
        struct quux_bar bar;
        struct quux_bas bas;
    } t;
};

我认为这是 X-macro 的好地方.如果我定义一个宏
#define quux_table X(foo) X(bar) X(bas)
枚举和结构可以这样定义，并且永远不会不同步:

I figure this is a good place for the X-macro. If I define a macro
#define quux_table X(foo) X(bar) X(bas)
the enumeration & structure can be defined thus, and never get out of sync:

#define X(t) quux_ ## t,
enum quux_type {quux_table};
#undef X

#define X(t) struct quux_ ## t t;
struct quux {
    enum quux_type type;
    union {quux_table} t;
};
#undef X

当然，quux_* 结构可能会不同步，所以我想做这样的事情，只是合法的:

Of course, the quux_* structures can get out of sync, so I’d like to do something like this, only legally:

struct quux_foo { int x; };
#define quux_table quux_table X(foo)

struct quux_bar { char *s; };
#define quux_table quux_table X(bar)

struct quux_bas { void *p; };
#define quux_table quux_table X(bas)

(嗯，我真正想要做的事情是
member_struct(quux, foo) { int x;};
但我很清楚不能从宏中(重新)定义宏.)

(Well, what I really want to be able to do is something like
member_struct(quux, foo) { int x; };
but I’m well aware that macros cannot be (re)defined from within macros.)

无论如何，这是我的激励例子.有没有办法做到这一点?

Anyhow, that’s my motivating example. Is there a way to accomplish this?

Boost.Preprocessor 示例很好，如果您能告诉我如何使 X-macro 技术与该库一起工作.

Boost.Preprocessor examples are fine, if you can show me how to make the X-macro technique work with that library.

推荐答案

实际上，没有.

宏是惰性求值的.当你#define List_Expand(List)时，它的替换列表是Expand、(、List 和 <代码>).没有任何方法可以将宏扩展为替换列表.

Macros are lazily evaluated. When you #define List_ Expand(List), its replacement list is the sequence of four tokens Expand, (, List, and ). There isn't any way to expand a macro into a replacement list.

所有宏替换都发生在调用宏时.

All macro replacement takes place when a macro is invoked.

我建议使用 Boost.Preprocessor 库进行自动代码生成.这有点工作，但你可以完成一些 相当令人印象深刻的东西使用它.它应该与 C 完全兼容.

I'd recommend looking at using the Boost.Preprocessor library for automatic code generation. It's a bit of work, but you can accomplish some fairly impressive things using it. It should be fully compatible with C.

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