constexpr-sql/include/ra/projection.hpp at master · mkitzan/constexpr-sql · GitHub

45 lines (35 loc) · 879 Bytes

#pragma once

#include "ra/operation.hpp"

#include "sql/row.hpp"

namespace ra

{

template <typename Output, typename Input>

class projection : public ra::unary<Input>

{

using input_type = typename ra::unary<Input>::input_type;

public:

using output_type = Output;

static auto&& next()

{

fold<output_type>(output_row, Input::next());

return std::move(output_row);

}

private:

template <typename Dest>

static inline constexpr void fold(Dest& dest, input_type const& src)

{

if constexpr (Dest::depth == 0)

{

return;

}

else

{

dest.head() = sql::get<Dest::column::name>(src);

fold<typename Dest::next>(dest.tail(), src);

}

static output_type output_row;

};

template <typename Output, typename Input>

typename projection<Output, Input>::output_type projection<Output, Input>::output_row{};

} // namespace ra

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