Extracting first parameter of __VA_ARGS__ - c++

Question

Suppose that I have a macro:
#define FOO(a, ...) if (a) foo(a, ## __VA_ARGS__)
This works well:
FOO(a) will be transformed to if (a) foo(a)
FOO(a, <some_parameters>) will be transformed to if (a) foo(a, <some_parameters>)
Is it possible to modify this macro, so only the first parameter of __VA_ARGS__ (if exists) passed to foo? So, I need:
FOO(a) to be transformed to if (a) foo(a)
FOO(a, b, <some_parameters>) to be transformed to if (a) foo(a, b)
I've tried to solve this with the same idea as BOOST_PP_VARIADIC_SIZE has, but it turned out this macro returns 1 for BOOST_PP_VARIADIC_SIZE() (empty arguments), which is not expected (I expected 0).
Note, that I need a solution, where b and <some_parameters> are evaluated only when bool(a) is true.

Answer 1

I propose a variadic macro with a generic lambda as a solution.
The Important points are as follows:
It is difficult to pass both a and __VA_ARGS__ to a lambda as passed arguments in macro because when __VA_ARGS__ is empty
[](){...}(a, __VA_ARGS__)
becomes
[](){...}(a,)
and this , leads compilation error.
Thus we split the first and second arguments of FOO into the captured and the passed ones respectively as follows.
Then we can use a generic lambda in the macro even if __VA_ARGS__ is empty.
[a](){...}(__VA_ARGS__)
The size of __VA_ARGS__ can be evaluated at compile-time as constexpr auto N. Then we can use if constexpr to separate function calls.
We can also apply if statement with initializer which is introduced from C++17 for if(a).
Then the proposed macro is as follows.
This also works for you.
DEMO
#include <tuple>
#define FOO(a, ...) \
if(const bool a_ = (a); a_) \
[a_](auto&&... args) \
{ \
const auto t = std::make_tuple(std::forward<decltype(args)>(args)...); \
constexpr auto N = std::tuple_size<decltype(t)>::value; \
\
if constexpr( N==0 ) { \
return foo(a_); \
} \
else { \
return foo(a_, std::get<0>(t)); \
} \
}(__VA_ARGS__)

Answer 2

Based on this answer, I could solve the problem:
#define PRIVATE_CONCAT(a, b) a ## b
#define CONCAT(a, b) PRIVATE_CONCAT(a, b)
#define GET_100TH( \
_01, _02, _03, _04, _05, _06, _07, _08, _09, _10, \
_11, _12, _13, _14, _15, _16, _17, _18, _19, _20, \
_21, _22, _23, _24, _25, _26, _27, _28, _29, _30, \
_31, _32, _33, _34, _35, _36, _37, _38, _39, _40, \
_41, _42, _43, _44, _45, _46, _47, _48, _49, _50, \
_51, _52, _53, _54, _55, _56, _57, _58, _59, _60, \
_61, _62, _63, _64, _65, _66, _67, _68, _69, _70, \
_71, _72, _73, _74, _75, _76, _77, _78, _79, _80, \
_81, _82, _83, _84, _85, _86, _87, _88, _89, _90, \
_91, _92, _93, _94, _95, _96, _97, _98, _99, PAR, \
...) PAR
#define HAS_PARAMETER(...) GET_100TH(placeholder, ##__VA_ARGS__, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
1, 1, 1, 1, 1, 1, 1, 1, 0)
#define FIRST_PARAMETER_WITH_PREPENDED_COMMA0(...)
#define FIRST_PARAMETER_WITH_PREPENDED_COMMA1(a, ...) , a
#define FIRST_PARAMETER_WITH_PREPENDED_COMMA(...) CONCAT(FIRST_PARAMETER_WITH_PREPENDED_COMMA, HAS_PARAMETER(__VA_ARGS__))(__VA_ARGS__)
#define FOO(a, ...) if (a) foo(a FIRST_PARAMETER_WITH_PREPENDED_COMMA(__VA_ARGS__))