referrence:
http://gcc.gnu.org/onlinedocs/cpp/Argument-Prescan.html#Argument-Prescan
Procedure
macro_expand (MACRO(ARGV)) /* 帶參宏擴展過程,宏爲MACRO, 參數爲ARGV */ { /* 如果宏本身的定義爲含有#, ##的宏,那麼直接把參數ARGV當成字符串,代入宏,得到結果,退出 */ if ((MACRO contain "#") || (MACRO contain "##")) { stringified ARGV || ARGV pasted with other tokens } /* 如果宏不含#,##, 並且宏的參數ARGV本身也是個宏,那麼先將宏參數擴展,宏本身不做任何改變,參數先不要代入 */ else if ( ARGV is still an macro, which ARGV == NEW_MACRO (NEW_ARGV)) { /* that is to say macro_expand (ARGV) ARGV == NEW_MACRO (NEW_ARGV) 遞歸這個過程 */ MACRO ( macro_expand (NEW_MACRO(NEW_ARGV)) ) } /* 宏的參數爲普通參數,不是一個宏,代入之,得到結果,從內往外代入,逐層得到結果。*/ else /* ARGV is a plain argument */ { expand the MACRO with argument ARGV } }
three examples
First example:
#define AFTERX(x) X_ ## x
#define XAFTERX(x) AFTERX(x)
#define TABLESIZE 1024
#define BUFSIZE TABLESIZE
then AFTERX(BUFSIZE) expands to X_BUFSIZE,
and XAFTERX(BUFSIZE) expands to X_1024.
(Not to X_TABLESIZE. Prescan always does a complete expansion.)
Second example:
If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros.
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)
==> "foo"
xstr (foo)
==> xstr (4)
==> str (4)
==> "4"
s is stringified when it is used in str, so it is not macro-expanded first.
But s is an ordinary argument to xstr, so it is completely macro-expanded before xstr itself is expanded (see Argument Prescan).
Therefore, by the time str gets to its argument, it has already been macro-expanded.
Third example:
#define a(x) b(x) + 1
#define b(x) c(x)
#define c(x) #x
#define TABLE SIZE + 2
#define SIZE 5
/* right */
a(TABLE)
==> a(SIZE + 2)
==> a(5 + 2)
==> b(5 + 2) + 1
==> c(5 + 2) + 1
==> "5 + 2" + 1
/* wrong */
a(TABLE)
==> b(TABLE) + 1
==> c(TABLE) + 1
==> "TABLE" + 1
本例爲本人原創,已經通過代碼測試。
主要表明,是先進行參數擴展,然後才進行參數替換,而不是先宏擴展,再參數替換。
測試代碼參見我空間的代碼。