Reference:
https://www.youtube.com/watch?v=XJzs4kC9d-Y
https://github.com/rollbear/columnist
https://godbolt.org/z/a8P7oTjoh
#include <functional>
#include <iostream>
#include <tuple>
#include <vector>
template<typename... Ts>
class Table {
public:
using row = std::tuple<Ts&...>;
row operator[](size_t i) {
auto access = [&]<size_t... Is>(std::index_sequence<Is...>) {
return row{std::get<Is>(data_)[i]...};
};
return std::invoke(access, indexes);
}
Table() {
init(indexes);
}
private:
static constexpr std::index_sequence_for<Ts...> indexes{};
std::tuple<std::vector<Ts>...> data_;
private:
template<size_t... I>
constexpr void init(std::index_sequence<I...>) {
((std::get<I>(data_).push_back({42}),...));
}
};
struct Data1 {
int value;
};
struct Data2 {
int value;
};
int main() {
Table<Data1, Data2> table;
auto row = table[0];
std::cout << std::get<0>(row).value << '\n'; // print 42
}; |
Iterator
Use sentinal pattern.struct sentinel {}; // or using sentinal = void;
struct iterator {
using value_type = row;
using difference_type = ssize_t;
value_type operator*() const {
return std::invoke([&]<size_t... Is>(std::index_sequence<Is...>) {
return value_type{std::get<Is>(t->data_)[offset]...};
}, t->indexes);
}
iterator& operator++() { ++offset; return *this;}
iterator operator++(int) { auto copy = *this; ++*this; return copy;}
bool operator==(const iterator&) const = default;
bool operator==(sentinel) const { return offset == t->size(); }
table* t;
size_t offset;
};template<typename... Ts>
class Table {
public:
friend class iterator;
iterator begin() { return {this, 0}; }
sentinel end() { return {}; }
...
};https://en.cppreference.com/w/cpp/utility/integer_sequence
template<typename...>
struct Table;
template<typename, typename>
struct Row;
template <typename... Ts, size_t... Cs>
struct Row<Table<Ts...>, std::index_sequence<Cs...>> {
using row_id = typename Table<Ts...>::row_id;
// Indirection to make std::index_sequence<0, 2, 4> -> I==1 means std::get<2>(...)
template<size_t I>
friend auto& get(const Row& r) {
static constexpr std::array columns{Cs...};
return std::get<columns[I]>(r.t->data_)[r.offset];
}
...
Table<Ts...>* t;
size_t offset;
};
template<typename Table, size_t... Cs>
struct std::tuple_size<Row<Table, std::index_sequence<Cs...>>>
: std::integral_constant<size_t, sizeof...(Cs)> {};
template <size_t I, typename... Ts, size_t... Cs>
struct std::tuple_element<I, Row<Table<Ts...>, std::index_sequence<Cs...>>> {
static constexpr std::array indexes = { Cs... };
using type = Ts...[indexes[I]]&;
};template<size_t... Is, typename Table, size_t... Cs>
auto select(const Row<Table, std::index_sequence<Cs...>>& r) {
static constexpr size_t columns[] { Cs... };
return Row<Table, std::index_sequence<columns[Is]...>>(r);
}
drop_if(values, [](auto r){ auto [x,z] = select<0, 2>(r); return x < z; });template<typename R, size_t... Cs>
struct range_selector {
using r_iterator = decltype(std::declval<R&>().begin());
struct iterator : r_iterator {
using difference_type = ssize_t;
using value_type = decltype(select<Cs...>(*std::declval<r_iterator>()));
iterator(const r_iterator& i) : r_iterator(i) {}
auto operator*() const {
const r_iterator& i = *this;
return select<Cs...>(*i);
}
};
iterator begin() { return iterator{ r.begin() }; }
auto end() { return r.end(); }
R& r;
};template<size_t... Cs>
struct range_selector_maker {
template<typename R>
friend range_selector<R, Cs...> operator|(R& r, range_selector_maker) {
return { r };
}
};
template<size_t... Cs>
range_selector_maker<Cs...> select() { return {}; }for (auto [x, d] : values | select<0, 3>()) {
std::println("d={} x={}", d, x);
}template<typename>
inline constexpr bool row_type_v = false;
template<typename T, typename Cs>
inline constexpr bool row_type_v<Row<T, Cs>> = true;
template<typename T>
concept row_type = row_type_v<T>;template<size_t... Cs, typename F>
auto select(F&& f)
requires (! row_type<std::remove_cvref_t<F>>)
{
return [f = std::forward<F>(f)]<typename T, typename Is>(row<T, Is> r) {
return f(select<Cs...>(r));
};
}template<typename F>
auto apply(F&& f) {
return [f = std::forward<F>(f)]<row_type R>(R r) {
return std::invoke([&]<size_t... Cs>(std::index_sequence<Cs...>) {
return f(get<Cs>(r)...);
}, std::make_index_sequence<std::tuple_size_v<R>>{});
};
}drop_if(values, select<0, 2>(apply([](auto x, auto z) { return x < z; })));drop_if(values, select<0, 2>(apply(std::less{})));
