Struct QoSSystemDefaults 

pub struct QoSSystemDefaults {
    pub reliability: QoSReliabilityPolicy,
    pub durability: QoSDurabilityPolicy,
    pub history: QoSHistoryPolicy,
    pub depth: u32,
}

What ONE backend resolves the SYSTEM_DEFAULT sentinel to.

issue 0829. rmw_qos_profile_system_default is an absence, and the RMW fills it — which means the answer is per backend and there is no constant this crate could bake. Each backend declares its own QoSSystemDefaults and applies it with QoSProfile::resolve_system_default at its create entry, before anything is derived from the QoS.

That ordering is load-bearing on the zenoh path: the profile is serialised into the liveliness-token keyexpr that a ROS rmw_zenoh_cpp peer parses out of the graph (nros-rmw-zenoh/src/keyexpr.rs). Upstream resolves in best_available_qos before the entity and its token exist, so its tokens never carry a sentinel; ours must not either, or we advertise 0:0:0,0 to peers as though it were a policy.

The values a backend picks should mirror the corresponding upstream RMW, not the raw middleware default — interop with a ROS peer is the requirement, and the two differ. Leaving dds_qset_reliability unset gives Cyclone’s own reader default of BEST_EFFORT (ddsi_plist.c:3470), where rmw_cyclonedds_cpp deliberately picks RELIABLE.

Fields

reliability: QoSReliabilityPolicy

Concrete reliability. Must not itself be SystemDefault.

durability: QoSDurabilityPolicy

Concrete durability. Must not itself be SystemDefault.

history: QoSHistoryPolicy

Concrete history. Must not itself be SystemDefault.

depth: u32

Concrete queue depth. 0 means the backend genuinely has no answer and defers further down the stack — the XRCE case, where the client encodes exactly that (optional_history_depth = false) and the Agent’s DDS layer resolves it.

Trait Implementations

impl Clone for QoSSystemDefaults

fn clone(&self) -> QoSSystemDefaults

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for QoSSystemDefaults

impl Debug for QoSSystemDefaults

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for QoSSystemDefaults

impl PartialEq for QoSSystemDefaults

fn eq(&self, other: &QoSSystemDefaults) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl StructuralPartialEq for QoSSystemDefaults