C++ Insights

Source:

#include <iostream>
#include <ostream>
#include <tuple>

template <typename TupleT, std::size_t... Is>
std::ostream& printTupleImp(std::ostream& os, const TupleT& tp, std::index_sequence<Is...>) {
	size_t index = 0;
    auto printElem = [&index, &os](const auto& x) {
		if (index++ > 0) 
            os << ", ";
		os << x;
	};

os << "(";
    (printElem(std::get<Is>(tp)), ...);
    os << ")";
    return os;
}

template <typename TupleT, std::size_t TupSize = std::tuple_size_v<TupleT>>
std::ostream& printTuple(std::ostream& os, const TupleT& tp) {
    return printTupleImp(os, tp, std::make_index_sequence<TupSize>{});    
}

// works:
template <typename TupleT, std::size_t TupSize = std::tuple_size<TupleT>::value>
// doesn't work:
//template <typename TupleT, std::size_t TupSize = std::tuple_size_v<TupleT>>
std::ostream& operator <<(std::ostream& os, const TupleT& tp) {
    return printTupleImp(os, tp, std::make_index_sequence<TupSize>{}); 
}

int main() {
    std::tuple tp { 10, 20, "hello"};
    std::cout << tp << '\n';
}

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