Real-world use of X-Macros

Question

I just learned of X-Macros. What real-world uses of X-Macros have you seen? When are they the right tool for the job?

Answer 1

Chromium has an interesting variation of a X-macro at dom_code_data.inc. Except it's not just a macro, but an entirely separate file. This file is intended for keyboard input mapping between different platforms' scancodes, USB HID codes, and string-like names.

The file contains code like:

DOM_CODE_DECLARATION {

  //            USB     evdev    XKB     Win     Mac   Code
  DOM_CODE(0x000000, 0x0000, 0x0000, 0x0000, 0xffff, NULL, NONE), // Invalid
...
};

Each macro invocation actually passes in 7 arguments, and the macro can choose which arguments to use and which to ignore. One usage is to map between OS keycodes and platform-independent scancodes and DOM strings. Different macros are used on different OSes to pick the keycodes appropriate for that OS.

// Table of USB codes (equivalent to DomCode values), native scan codes,
// and DOM Level 3 |code| strings.
#if defined(OS_WIN)
#define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
  { usb, win, code }
#elif defined(OS_LINUX)
#define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
  { usb, xkb, code }
#elif defined(OS_MACOSX)
#define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
  { usb, mac, code }
#elif defined(OS_ANDROID)
#define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
  { usb, evdev, code }
#else
#define DOM_CODE(usb, evdev, xkb, win, mac, code, id) \
  { usb, 0, code }
#endif
#define DOM_CODE_DECLARATION const KeycodeMapEntry usb_keycode_map[] =
#include "ui/events/keycodes/dom/dom_code_data.inc"
#undef DOM_CODE
#undef DOM_CODE_DECLARATION