Why is the nested macro not unfolded?-CodePudding

I am using nested macro under gcc-9.3.1.

Here is the code.

#define A(_name) _name, _name, _name

#define P(_1, _2, _name, ...) _name

#define B(...) P(__VA_ARGS__, A(something))(__VA_ARGS__)

B(k)

I expected the B(k) converted to P(k, something, something, something)(k) at first and then converted into something(k).

However, compilier told me that two arguments were too few for P, which meant that A(something) was not unfolded.

Why is that? And how can I make it unfolded?

CodePudding user response：

B(k) expands to P(k, A(something))(k) which then undergoes recursive expansion. The first thing it finds there is P, which doesn't have enough arguments.

If you want this to work the way I think you do, you need to arrange for A to expand before P. You can do that by adding explicit indirect EXPAND macros:

#define EXPAND(...)   __VA_ARGS__

#define B(...) EXPAND(P EXPAND((__VA_ARGS__, A(something))))(__VA_ARGS__)

This way, P won't be recognized as a macro (no following () until after the inner EXPAND is expanded (which will also expand A). You need the outer EXPAND to explicitly expand P afterwards.