Header file tagged_variant.hpp

namespace eosio
{
    template <name::raw N, typename T>
    struct tagged_type;

    enum tagged_variant_options;

    template <eosio::tagged_variant_options Options, typename ... NamedTypes>
    struct tagged_variant;
}

A tagged_variant is a type-safe union; it may contain one value from a set of types. Each type is tagged with a name; this name appears in the JSON representation and optionally in the binary serialization.

Example:

using token_query_request = eosio::tagged_variant<                                //
    eosio::serialize_tag_as_name,                                                 //
    eosio::tagged_type<"transfer"_n, token_transfer_request>,                     //
    eosio::tagged_type<"bal.mult.acc"_n, balances_for_multiple_accounts_request>, //
    eosio::tagged_type<"bal.mult.tok"_n, balances_for_multiple_tokens_request>>;  //

The JSON representation takes one of the following forms:

["transfer", { ...token_transfer_request properties... }]
["bal.mult.acc", { ...balances_for_multiple_accounts_request properties... }]
["bal.mult.tok", { ...balances_for_multiple_tokens_request properties... }]

Since this example uses the eosio::serialize_tag_as_name flag, the binary serialization for instances of this type contains:

• 8-byte eosio::name containing “transfer”, “bal.mult.acc”, or “bal.mult.tok”

• bytes for token_transfer_request, balances_for_multiple_accounts_request, or balances_for_multiple_tokens_request

If the serialize_tag_as_index was used instead, the binary serialization would contain:

• 1-byte eosio::unsigned_int containing 0, 1, or 2

• bytes for token_transfer_request, balances_for_multiple_accounts_request, or balances_for_multiple_tokens_request

The 1-byte eosio::unsigned_int format is more compact, and is compatible with ABI 1.1’s variant serialization. The 8-byte name format provides an easier path to upgrading the format of binary data in the future.

Struct eosio::tagged_type

template <name::raw N, typename T>
struct tagged_type
{
    static constexpr eosio::name const name = eosio::name{N};

    using type = T;
};

Pairs a name with a type

Enumeration eosio::tagged_variant_options

enum tagged_variant_options
{
    serialize_tag_as_name = 1,
    serialize_tag_as_index = 2
};

Options for tagged_variant

Struct eosio::tagged_variant

template <eosio::tagged_variant_options Options, typename ... NamedTypes>
struct tagged_variant
{
    std::variant<typename NamedTypes::type...> value;

    static constexpr eosio::name const keys[] = {NamedTypes::name...};
};

Type-safe union