new-component

name	new-component
description	Scaffold a new ECS component header under include/gz/sim/components/, wire its CMake listing, and (if needed) register it with ComponentFactory for serialization. Trigger when the user asks to add a new component or a per-entity data field.

Adding a new component

Components in gz-sim are header-only wrappers around the Component<> template. They live in include/gz/sim/components/<Name>.hh.

Decide first

1. What's the data type? A plain struct, a math::Vector3d, an std::string, a msgs::*? Components don't have to be POD but they must be copyable.
2. Does it need serialization? If users should be able to log/replay the component or read it from SDF, you need to give it a Serializer<T> and register it with ComponentFactory.
3. Is there a Cmd / Reset variant? If this component represents physical state that physics writes, you probably also want <Name>Cmd.hh (user → physics) and <Name>Reset.hh (snapshot for Reset()).

Template (no custom serializer)

// include/gz/sim/components/Foo.hh
#ifndef GZ_SIM_COMPONENTS_FOO_HH_
#define GZ_SIM_COMPONENTS_FOO_HH_

#include <gz/sim/components/Component.hh>
#include <gz/sim/components/Factory.hh>
#include <gz/sim/config.hh>

namespace gz::sim
{
inline namespace GZ_SIM_VERSION_NAMESPACE {
namespace components
{
  /// \brief One-line description of what this component represents and
  ///        which entities it gets attached to.
  using Foo = Component<double, class FooTag>;
  GZ_SIM_REGISTER_COMPONENT("gz_sim_components.Foo", Foo)
}
}
}

#endif

Key points:

• The unique tag class FooTag is what prevents two components with the same data type from collapsing into one — never skip it.
• GZ_SIM_REGISTER_COMPONENT(name, type) is required for any component that needs to round-trip through ComponentFactory.

Template (custom data type + serializer)

#ifndef GZ_SIM_COMPONENTS_BAR_HH_
#define GZ_SIM_COMPONENTS_BAR_HH_

#include <gz/msgs/double.pb.h>
#include <gz/sim/components/Component.hh>
#include <gz/sim/components/Factory.hh>
#include <gz/sim/components/Serialization.hh>
#include <gz/sim/config.hh>

namespace gz::sim
{
inline namespace GZ_SIM_VERSION_NAMESPACE {
namespace components
{
  using BarSerializer = serializers::MsgsDoubleSerializer;
  using Bar = Component<msgs::Double, class BarTag, BarSerializer>;
  GZ_SIM_REGISTER_COMPONENT("gz_sim_components.Bar", Bar)
}
}
}

#endif

For brand-new data types, define Serializer<T> with static std::ostream &Serialize(...) and static std::istream &Deserialize(...) in include/gz/sim/components/Serialization.hh style and reference it from the component.

CMake plumbing

Add the new header to include/gz/sim/components/CMakeLists.txt so it gets installed:

set (component_headers
  Actor.hh
  ...
  Foo.hh        # <-- new
  ...
)

Headers are picked up automatically by the install logic — just keep the list alphabetical to match existing style.

Tests

• Add a small Foo_TEST.cc next to the existing component tests under src/components/ if there's non-trivial behaviour (e.g. operator== or a custom serializer). Trivial typedef components don't need a dedicated test.
• If the component is logged/replayed, add a round-trip test under test/integration/log_system.cc patterns.

Migration note

If the component is part of the public API, append a one-line entry to Migration.md under the next-release section so downstream users notice.

Common mistakes

• Reusing an existing tag class — two components with the same tag are the same component.
• Forgetting the GZ_SIM_REGISTER_COMPONENT macro for a serialized type.
• Adding mutation logic into the component header — components are data, behaviour belongs in a system.