#include <iostream>

template <typename... T>
void foo(int a, int b) {
    auto sep = [first=true]() mutable { return first? (first = false, "") : ", "; };

    std::cout << "foo(";
    if (a) std::cout << sep() << "a:" << a;
    if (b) std::cout << sep() << "b:" << b;
    std::cout << ")n";
}

int main() {
    foo(0,0);
    foo(0,1);
    foo(2,3);
    foo(4,0);
}

template <typename T = bool, T initial = true, T fallback = false>
struct once {
    mutable T flag = initial;

    operator T() const {
        T retval = fallback;
        std::swap(retval, flag);
        return retval;
    }
};

auto sep = [first=once{}]{ return first? "" : ", "; };

auto sep = one_off("", ", ");

template <typename T = char const*>
struct one_off {
    mutable once<> flag;

    T first, other;
    one_off(T first, T other) : first(std::move(first)), other(std::move(other)) {}

    operator T() const { return flag? first : other; }
};

void foo(int a, int b) {
    auto sep = one_off("", ", ");

    std::cout << "foo(";
    if (a) std::cout << sep << "a:" << a;
    if (b) std::cout << sep << "b:" << b;
    std::cout << ")n";
}

void foo(std::vector<int> const& v) {
    auto sep = [n=0u]() mutable {
        if (n++)
            return n%5==1?"n" : ", ";
        return "";
    };

    for (auto& el : v)
        std::cout << sep() << std::setw(5) << el;
    std::cout << "n";
}