C/C++ Macro expansion : from two to four parameters
I'm trying to convert a two parameter macro into a four parameter one. From
the definition of LIST
which I can't change, I'd like to call ALL
,
providing the two extra parameters.
#define LIST(x) \
x(p1, p2) \
x(p3, p4) \
x(...
#define ALL(A, B, C, D) .../... do stuff with A, B, C and D
Here's what I tried so far:
#define RIGHT(C, D) C, D)
#define LEFT(A, B) (A, B, RIGHT
But when I'm testing it (gcc -E text.cpp
) with LIST( ALL LEFT (q1, q2) )
I
get the following error:
error: unterminated argument list invoking macro "ALL"
Taking a closer look at it, this is why:
LIST
is substituted which give for the first lineALL LEFT (q1, q2)(p1, p2)
LEFT
is substituted toALL (q1, q2, RIGHT(p1, p2)
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- The preprocessor tries to expand
ALL
, but doesn't find the closing parenthesis, there's the error...
How can I do otherwise?
Thanks.
EDIT:
To be more precise, with the given input:
#define LIST(x) \
x("p1", "p2") \
x("p3", "p4") \
x("p5", "p6")
#define ALL(A, B, C, D) {A, B, C, D}
I want to find a way to get, as a result:
{"q1", "q2", "p1", "p2"}
{"q1", "q2", "p3", "p4"}
{"q1", "q2", "p5", "p6"}
I see two anwers to this:
1) Pass in the name of a helper macro when using LIST:
#define to_all(c, d) ALL("q1", "q2", c, d)
LIST(to_all)
2) Let LIST expand to helper macros, which is expected to exist when used (this is a variant of something named x-macros). This, however, would require you to redefine list:
// Somewhere "central"
#define LIST() \
LIST_HELPER("p1", "p2") \
LIST_HELPER("p3", "p4") \
LIST_HELPER("p5", "p6")
#define ALL(A, B, C, D) {A, B, C, D}
// When used:
#define LIST_HELPER(c, d) ALL("q1", "q2", c, d)
LIST()
#undef LIST_HELPER
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