struct_subsets.hpp

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#pragma once
 
/// @file userver/utils/struct_subsets.hpp
/// @brief Utilities for creating a struct with a subset of fields of the
/// original struct, with conversions between them.
 
#include <type_traits>
#include <utility>
 
#include <boost/preprocessor/empty.hpp>
#include <boost/preprocessor/seq/for_each.hpp>
 
#include <userver/utils/impl/boost_variadic_to_seq.hpp>
#include <userver/utils/impl/internal_tag.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils::impl {
 
struct RequireSemicolon;
 
struct NonMovable final {
    constexpr explicit NonMovable(InternalTag) noexcept {}
};
 
template <typename T>
constexpr auto IsDefinedAndAggregate() -> decltype(static_cast<void>(sizeof(T)), false) {
    return std::is_aggregate_v<T>;
}
 
template <typename /*T*/, typename... Args>
constexpr auto IsDefinedAndAggregate(Args...) -> bool {
    return false;
}
 
}  // namespace utils::impl
 
USERVER_NAMESPACE_END
 
/// @cond
 
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_IMPL_STRUCT_MAP(r, data, elem) std::forward<OtherDeps>(other).elem,
 
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_IMPL_MAKE_FROM_SUPERSET(Self, ...)
    template <typename OtherDeps>
    static Self MakeFromSupersetImpl(OtherDeps&& other, USERVER_NAMESPACE::utils::impl::InternalTag) {
        return {BOOST_PP_SEQ_FOR_EACH(
            USERVER_IMPL_STRUCT_MAP, BOOST_PP_EMPTY(), USERVER_IMPL_VARIADIC_TO_SEQ(__VA_ARGS__)
        )};
    }
 
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_IMPL_STRUCT_SUBSET_MAP(r, data, elem)
    /* NOLINTNEXTLINE(bugprone-macro-parentheses) */
    decltype(data::elem) elem;
 
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_IMPL_STRUCT_SUBSET_REF_MAP(r, data, elem)
    /* NOLINTNEXTLINE(bugprone-macro-parentheses) */
    std::add_const_t<decltype(data::elem)>& elem;
 
/// @endcond
 
/// @brief Should be invoked inside a manually defined "root" struct to enable
/// conversions from it to subset structs created by
/// @ref USERVER_DEFINE_STRUCT_SUBSET and @ref USERVER_DEFINE_STRUCT_SUBSET_REF.
///
/// @hideinitializer
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_ALLOW_CONVERSIONS_TO_SUBSET()
    template <
        typename Other,
        std::enable_if_t<USERVER_NAMESPACE::utils::impl::IsDefinedAndAggregate<Other>(), int> Enable = 0>
    /*implicit*/ operator Other() const& {
        return Other::MakeFromSupersetImpl(*this, USERVER_NAMESPACE::utils::impl::InternalTag{});
    }
 
    template <
        typename Other,
        std::enable_if_t<USERVER_NAMESPACE::utils::impl::IsDefinedAndAggregate<Other>(), int> Enable = 0>
    /*implicit*/ operator Other()&& {
        return Other::MakeFromSupersetImpl(std::move(*this), USERVER_NAMESPACE::utils::impl::InternalTag{});
    }
 
    friend struct USERVER_NAMESPACE::utils::impl::RequireSemicolon
 
/// @brief Defines a struct containing a subset of data members
/// from @a OriginalDependencies.
///
/// Implicit conversions (by copy and by move) are allowed from any superset
/// struct to the @a SubsetStruct, as long as the names of the data members
/// match, and the superset struct is either defined using
/// @ref USERVER_DEFINE_STRUCT_SUBSET or @ref USERVER_DEFINE_STRUCT_SUBSET_REF,
/// or it contains @ref USERVER_ALLOW_CONVERSIONS_TO_SUBSET.
///
/// Usage example:
/// @snippet utils/struct_subsets_test.cpp  deps definitions
/// @snippet utils/struct_subsets_test.cpp  deps usage
///
/// @param SubsetStruct the name of the subset struct to define
/// @param OriginalStruct the name of the superset struct, including its
/// namespace if needed
/// @param ... names of the data members to copy
///
/// @hideinitializer
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_DEFINE_STRUCT_SUBSET(SubsetStruct, OriginalStruct, ...)
    struct SubsetStruct {
        BOOST_PP_SEQ_FOR_EACH(
            USERVER_IMPL_STRUCT_SUBSET_MAP,
            OriginalStruct,
            USERVER_IMPL_VARIADIC_TO_SEQ(__VA_ARGS__)
        )
 
        USERVER_IMPL_MAKE_FROM_SUPERSET(SubsetStruct, __VA_ARGS__)
 
        USERVER_ALLOW_CONVERSIONS_TO_SUBSET();
    }
 
/// @brief Defines a struct containing a subset of data members
/// from @a OriginalDependencies. Appends `const&` to types of all non-reference
/// data members.
///
/// Implicit conversions (by copy and by move) are allowed from any superset
/// struct to the @a SubsetStruct, as long as the names of the data members
/// match, and the superset struct is either defined using
/// @ref USERVER_DEFINE_STRUCT_SUBSET or @ref USERVER_DEFINE_STRUCT_SUBSET_REF,
/// or it contains @ref USERVER_ALLOW_CONVERSIONS_TO_SUBSET.
///
/// `*Ref` structs can be used for parameters of utility functions to avoid
/// copying non-reference data members.
///
/// Usage example:
/// @snippet utils/struct_subsets_test.cpp  ref definitions
/// @snippet utils/struct_subsets_test.cpp  ref usage
///
/// @param SubsetStructRef the name of the subset struct to define, it should
/// typically contain `*Ref` suffix to underline that it needs the original
/// backing struct to work
/// @param OriginalStruct the name of the superset struct, including its
/// namespace if needed
/// @param ... names of the data members to copy
///
/// @hideinitializer
// NOLINTNEXTLINE(cppcoreguidelines-macro-usage)
#define USERVER_DEFINE_STRUCT_SUBSET_REF(SubsetStructRef, OriginalStruct, ...)
    struct SubsetStructRef {
        BOOST_PP_SEQ_FOR_EACH(
            USERVER_IMPL_STRUCT_SUBSET_REF_MAP,
            OriginalStruct,
            USERVER_IMPL_VARIADIC_TO_SEQ(__VA_ARGS__)
        )
 
        /* Protects against copying into async functions */
        USERVER_NAMESPACE::utils::impl::NonMovable _impl_non_movable{USERVER_NAMESPACE::utils::impl::InternalTag{}};
 
        USERVER_IMPL_MAKE_FROM_SUPERSET(SubsetStructRef, __VA_ARGS__)
 
        USERVER_ALLOW_CONVERSIONS_TO_SUBSET();
    }