pgxx_variadic_select.cc

#include <iostream>
#include <pqxx/pqxx>
#include <string>
#include <string>

template<size_t I, class It, class T, class Tuple>
std::enable_if_t<(I == std::tuple_size<Tuple>::value - 1)> assign(It it, It end, T& v, Tuple& tp)
{
	if (it == end)
		return;
	v = it->template as<T>();
}

template<size_t I, class It, class T, class Tuple>
std::enable_if_t<(I < std::tuple_size<Tuple>::value - 1)> assign(It it, It end, T& v, Tuple& tp)
{
	if (it == end)
		return;
	v = it->template as<T>();
	assign<I+1>(++it, end, std::get<I+1>(tp), tp);
}

template<class Query, class... Types>
void exec(std::string conn_str, Query query, std::vector<std::tuple<Types...>>& vs)
{
	pqxx::connection C(conn_str);
	pqxx::nontransaction N(C);
	auto r = N.exec(query);

	vs.resize(r.size());

	for (const auto& row : r)
	{
		auto& tp = vs[row.rownumber()];
		assign<0>(row.begin(), row.end(), std::get<0>(tp), tp);
	}
}

template<class Query, class... Types>
void exec(std::string conn_str, Query query, std::tuple<Types...>& tp)
{
	pqxx::connection C(conn_str);
	pqxx::nontransaction N(C);
	auto r = N.exec(query);

	for (const auto& row : r)
	{
		assign<0>(row.begin(), row.end(), std::get<0>(tp), tp);
	}
}

template<class Query, class T>
void exec(std::string conn_str, Query query, T& v)
{
	pqxx::connection C(conn_str);
	pqxx::nontransaction N(C);
	auto r = N.exec(query);

	for (const auto& row : r)
	{
		if (row.size() > 0)
		{
			v = row[0].template as<T>();
			break;
		}
	}

	C.disconnect();
}

int main()
{
	const char *conn_str = "host=localhost port=8812 user=admin password=quest dbname=qdb";
	std::vector<std::tuple<std::string, int, double>> vs;

	exec(conn_str, "SELECT DISTINCT coin, trades FROM market", vs);
	for (auto& v : vs)
		std::cout << std::get<0>(v) << " | " << std::get<1>(v) << std::endl;

	int x;
	exec(conn_str, "SELECT count() from market", x);
	std::cout << "count() = " << x << std::endl;
}