Solution to combining REP(i, n) and FOR(i, a, b)

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portable version

#include <tuple>
#include <cassert>
using namespace std;
#define __numargs(...) tuple_size<decltype(make_tuple(__VA_ARGS__))>::value
#define __argi(i, ...) get<i>(make_tuple(__VA_ARGS__))
#define rep(i, ...) for (int i = ((__numargs(__VA_ARGS__) > 1) ? __argi(0, __VA_ARGS__) : 0); i < __argi(__numargs(__VA_ARGS__) - 1, __VA_ARGS__); ++i)
#define repe(i, ...) for (int i = ((__numargs(__VA_ARGS__) > 1) ? __argi(0, __VA_ARGS__) : 0); i <= __argi(__numargs(__VA_ARGS__) - 1, __VA_ARGS__); ++i)
#define rrep(i, ...) for (int i = __argi(0, __VA_ARGS__) - 1; i >= ((__numargs(__VA_ARGS__) > 1) ? __argi(__numargs(__VA_ARGS__) - 1, __VA_ARGS__) : 0); --i)
#define rrepe(i, ...) for (int i = __argi(0, __VA_ARGS__); i >= ((__numargs(__VA_ARGS__) > 1) ? __argi(__numargs(__VA_ARGS__) - 1, __VA_ARGS__) : 0); --i)
#define repi(...) rep(i, __VA_ARGS__)

int main() {
    int s = 0;
    repi(5) s += i;
    rep(i, -4, 5) s += i * 100;
    repe(i, -4, 5) s += i * 10000;
    rrep(i, 5, -4) s += i * 1000000;
    rrepe(i, 5, -4) s += i * 100000000;
    assert(s == 500050010);
    return 0;
}

#include <tuple>
using namespace std;
#define numargs(args...) tuple_size<decltype(make_tuple(args))>::value
#define argi(i, args...) get<i>(make_tuple(args))
#define rep(i, args...) for (int i = ((numargs(args) > 1) ? argi(0, args) : 0); i < argi(numargs(args) - 1, args); ++i)
#define repi(args...) rep(i, args)

First, let argv := make_tuple(args), to get argc out of args, we use argc := tuple_size<decltype(argv)>::value, according to link. Second, we use (argc - 1 > 0) ? get<1>(argv) : 0 as the base, and get<(argc - 1)>(argv) as the end.

The code below is tested among G++11, G++14, G++17, Clang++17. For all users including MSVC, use ... instead of args... and __VA_ARGS__ instead of args.

#include <tuple>
#include <cassert>
using namespace std;
#define numargs(args...) tuple_size<decltype(make_tuple(args))>::value
#define argi(i, args...) get<i>(make_tuple(args))
#define rep(i, args...) for (int i = ((numargs(args) > 1) ? argi(0, args) : 0); i < argi(numargs(args) - 1, args); ++i)
#define repi(args...) rep(i, args)

int main() {
    int s = 0;
    repi(5) s += i;
    rep(i, -4, 5) s += i;
    assert(s == 10);
    return 0;
}

By the way, I'm using #define pb(args...) push_back(args) instead of #define pb push_back