Add const reference service callback signatures - #3269

Problem

AnySubscriptionCallback accepts const MessageT & callbacks, but AnyServiceCallback only accepts std::shared_ptr signatures. Every service callback therefore has to take shared_ptrs (the response one is copied per call) even when the handler answers immediately and never needs to extend the request/response lifetime. This is inconsistent with subscriptions.

Design

Follows the approach suggested in the issue: the request and response keep arriving as shared_ptr internally; dispatch() dereferences them for the user callback, exactly like AnySubscriptionCallback does for const MessageT &. The existing shared_ptr overloads are unchanged.

Fix

rclcpp/include/rclcpp/any_service_callback.hpp

• New variant alternatives ConstRefCallback = void (const Request &, Response &) and ConstRefWithRequestHeaderCallback = void (const rmw_request_id_t &, const Request &, Response &), appended after the existing alternatives.
• Both set() overloads get matching function_traits::same_arguments branches so std::bind results resolve like the shared_ptr shapes do.
• dispatch() calls cb(*request, *response) / cb(*request_header, *request, *response) and returns the response as before.

The new alternatives cannot be ambiguous with the existing ones in the std::variant converting assignment: a callable taking const Request & is not invocable with a shared_ptr<Request> and vice versa, so at most one std::function alternative is viable for any callback.

How tested

rclcpp/test/rclcpp/test_any_service_callback.cpp: four new cases (const-ref without/with request header, response mutation propagated to the returned shared_ptr, std::bind for both shapes). rclcpp/test/rclcpp/test_service.cpp: create_service<> with a const-ref callback, exercised end to end through a client.

Before the change, the new callback shapes fail to compile:

any_service_callback.hpp:102:17: error: no match for 'operator=' (operand types are
'std::variant<std::monostate, std::function<void(std::shared_ptr<...Request>, std::shared_ptr<...Response>)>, ...>'
and '<lambda(const ...Request&, ...Response&)>')

After the change the header compiles and all cases pass. I could not run colcon test locally (no ROS 2 workspace), so the new gtest cases were mirrored in a standalone gtest harness that compiles the unmodified any_service_callback.hpp / function_traits.hpp against stub rmw/tracetools headers: 10/10 pass with -Wall -Wextra on GCC 13, including the pre-existing callback shapes. cpplint with the ament filters reports no issues. Please rely on CI for the full build and for ament_uncrustify.

Links

• Service callback without shared pointer #3078

This change was prepared with an AI agent operated by KR-Ravindra, who reviewed and tested it.