Traits

sbepp provides a way to access nearly all information provided by XML schema and other useful properties via traits mechanism.

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Tags

To access a trait you need a schema entity's tag. Tag is basically a path to an entity. The tag for a schema itself (for sbepp::schema_traits) is <schema_name>::schema. Tags for messages have form <schema_name>::schema::messages::<msg_name> and tags for types <schema_name>::schema::types::<type_name>. Here's an example:

<?xml version="1.0" encoding="UTF-8"?>
<!-- schema_name::schema -->
<sbe:messageSchema package="schema_name">
    <types>
        <!-- schema_name::schema::types::uint32_req -->
        <type name="uint32_req" primitiveType="uint32"/>
 
        <!-- schema_name::schema::types::numbers -->
        <enum name="numbers" encodingType="uint8">
            <!-- schema_name::schema::types::numbers::One -->
            <validValue name="One">1</validValue>
            <!-- schema_name::schema::types::numbers::Two -->
            <validValue name="Two">2</validValue>
        </enum>
 
        <!-- schema_name::schema::types::options -->
        <set name="options" encodingType="uint8">
            <!-- schema_name::schema::types::options::A -->
            <choice name="A">0</choice>
            <!-- schema_name::schema::types::options::B -->
            <choice name="B">2</choice>
        </set>
 
        <!-- schema_name::schema::types::groupSizeEncoding -->
        <composite name="groupSizeEncoding">
            <!-- schema_name::schema::types::groupSizeEncoding::blockLength -->
            <type name="blockLength" primitiveType="uint16"/>
            <!-- schema_name::schema::types::groupSizeEncoding::numInGroup -->
            <type name="numInGroup" primitiveType="uint16"/>
        </composite>
    </types>
 
    <!-- schema_name::schema::messages::msg -->
    <sbe:message name="msg" id="1">
        <!-- schema_name::schema::messages::msg::field -->
        <field name="field" id="1" type="uint32"/>
 
        <!-- schema_name::schema::messages::msg::group -->
        <group name="group" id="2">
            <!-- schema_name::schema::messages::msg::group::field -->
            <field name="field" id="1" type="uint32"/>
        </group>
 
        <!-- schema_name::schema::messages::msg::data -->
        <data name="data" id="3" type="varDataEncoding"/>
    </sbe:message>
</sbe:messageSchema>

For built-in types like sbepp::char_t, the type itself works as a tag, e.g.

// get maxValue of a built-in required `char` type
auto max_char = sbepp::type_traits<sbepp::char_t>::max_value();

Implementation-wise, each tag is an empty struct, it's possible to test tag kind using sbepp::is_message_tag and similar helpers.

See also

Tag-based accessors

sbepp::traits_tag

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Using traits

Similar to std::numeric_limits, sbepp traits are accessed like trait_name<Tag>::value():

auto schema_version = sbepp::schema_traits<schema_name::schema>::version();
auto null_value = sbepp::type_traits<
    schema_name::schema::types::optional>::null_value();

For the list of available traits see Traits list.

Note that there's no such thing as sbepp::ref_traits because <ref> fields always refer to another type. To access their properties, use traits corresponding to the referred type (e.g. sbepp::set_traits if <ref> refers to a <set>).