cpprustser/cpp_rs_reflect.h at main · cppforever1/cpprustser · GitHub

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#pragma once

// Compile-time reflection over aggregate types. This is what makes the JSON
// bridge generic: no per-type registration code is required.

#include <cstddef>
#include <type_traits>
#include <utility>

namespace cpp_rs::reflect {

namespace detail {

/// Converts to anything except `Owner`, so it never hijacks a copy/move constructor.
template <typename Owner>
struct any_field {
    template <typename U, typename = std::enable_if_t<!std::is_same_v<Owner, std::remove_cvref_t<U>>>>
    constexpr operator U() const noexcept;
};

template <typename T, typename... Probes>
consteval std::size_t count_fields() {
    if constexpr (requires { T{Probes{}..., any_field<T>{}}; }) {
        return count_fields<T, Probes..., any_field<T>>();
    } else {
        return sizeof...(Probes);
    }
}

} // namespace detail

template <typename T>
inline constexpr std::size_t field_count = detail::count_fields<std::remove_cvref_t<T>>();

template <typename T>
inline constexpr bool is_reflectable_aggregate_v =
    std::is_aggregate_v<std::remove_cvref_t<T>> && !std::is_array_v<std::remove_cvref_t<T>>;

/// Calls `visitor` with references to every member of `value`, in declaration order.
template <typename T, typename Visitor>
constexpr decltype(auto) visit_fields(T &&value, Visitor &&visitor) {
    constexpr std::size_t count = field_count<T>;
    static_assert(count <= 16, "cpp_rs reflection supports up to 16 fields per aggregate");

    if constexpr (count == 0) {
        return std::forward<Visitor>(visitor)();
    } else if constexpr (count == 1) {
        auto &&[f0] = value;
        return std::forward<Visitor>(visitor)(f0);
    } else if constexpr (count == 2) {
        auto &&[f0, f1] = value;
        return std::forward<Visitor>(visitor)(f0, f1);
    } else if constexpr (count == 3) {
        auto &&[f0, f1, f2] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2);
    } else if constexpr (count == 4) {
        auto &&[f0, f1, f2, f3] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3);
    } else if constexpr (count == 5) {
        auto &&[f0, f1, f2, f3, f4] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4);
    } else if constexpr (count == 6) {
        auto &&[f0, f1, f2, f3, f4, f5] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5);
    } else if constexpr (count == 7) {
        auto &&[f0, f1, f2, f3, f4, f5, f6] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6);
    } else if constexpr (count == 8) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7);
    } else if constexpr (count == 9) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8);
    } else if constexpr (count == 10) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9);
    } else if constexpr (count == 11) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10);
    } else if constexpr (count == 12) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11);
    } else if constexpr (count == 13) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12);
    } else if constexpr (count == 14) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12,
                                              f13);
    } else if constexpr (count == 15) {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12,
                                              f13, f14);
    } else {
        auto &&[f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15] = value;
        return std::forward<Visitor>(visitor)(f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12,
                                              f13, f14, f15);
    }
}

} // namespace cpp_rs::reflect