traits.hpp

#pragma once
 
#include <string_view>
#include <type_traits>
 
namespace NYdb {
class TType;
class TValue;
class TValueParser;
class TValueBuilder;
class TParamValueBuilder;
 
template <typename Builder>
class TValueBuilderBase;
}  // namespace NYdb
 
USERVER_NAMESPACE_BEGIN
 
namespace ydb {
 
/// Used for error reporting in ydb::ValueTraits::Parse.
struct ParseContext final {
    std::string_view column_name;
};
 
template <typename T, typename Enable = void>
struct ValueTraits {
    /// Parses an element of type `T` from `parser`. `context` may be used
    /// for diagnostic messages.
    static T Parse([[maybe_unused]] NYdb::TValueParser& parser, [[maybe_unused]] const ParseContext& context) {
        static_assert(
            !sizeof(T),
            "Cannot convert from YDB value. No ValueTraits defined for "
            "this type. userver/ydb/io/supported_types.hpp "
            "contains all known definitions"
        );
        return T{};
    }
 
    /// Writes an element of type `T` to `builder`.
    template <typename Builder>
    static void Write([[maybe_unused]] NYdb::TValueBuilderBase<Builder>& builder, [[maybe_unused]] const T& value) {
        static_assert(
            !sizeof(T),
            "Cannot convert to YDB value. No ValueTraits defined for "
            "this type. userver/ydb/io/supported_types.hpp "
            "contains all known definitions"
        );
    }
 
    /// Uses NYdb::TTypeBuilder to create a representation of `T` type.
    static NYdb::TType MakeType() = delete;
};
 
/// A shorthand for calling ydb::ValueTraits<T>::Parse.
template <typename T>
inline constexpr auto Parse = [](NYdb::TValueParser& parser, const ParseContext& context) -> T {
    // Note: using a CPO instead of a global function to avoid ADL.
    // Customization should be performed using ValueTraits.
    return ValueTraits<T>::Parse(parser, context);
};
 
/// A shorthand for calling ydb::ValueTraits<T>::Write.
inline constexpr auto Write = [](auto& builder, auto&& value) {
    // Note: using a CPO instead of a global function to avoid ADL.
    // Customization should be performed using ValueTraits.
    using RawValueType = std::remove_const_t<std::remove_reference_t<decltype(value)>>;
    using ValueType =
        std::conditional_t<std::is_convertible_v<RawValueType, std::string_view>, std::string, RawValueType>;
    ValueTraits<ValueType>::Write(builder, value);
};
 
}  // namespace ydb
 
USERVER_NAMESPACE_END