Allocator tutorial: provide get_rcl_allocator() for the pmr allocator by mini-1235 · Pull Request #7296 · ros2/ros2_documentation

mini-1235 · 2026-10-12T06:58:54Z

Description

Updates the Allocator Template Tutorial to match the pmr demo in ros2/demos (demo_nodes_cpp/src/topics/allocator_tutorial_pmr.cpp), which now gives its allocator a get_rcl_allocator() member.

Without that member, rclcpp falls back to a generic C++-to-rcl allocator conversion that passes a wrong size to deallocate (see ros2/rclcpp#3058). That fallback is deprecated, so the tutorial's code produced this warning when built against current Rolling:

rclcpp/publisher_options.hpp:137:35: warning: 'get_rcl_allocator<char, std::pmr::polymorphic_allocator<char>, nullptr>' is deprecated: Conversion of C++ allocators to C style is not valid, as the size on deallocatecan not be determined. This will be remove in future versions of ros. To suppress this warningdefine the method 'rcl_allocator_t get_rcl_allocator()' on your allocator [-Wdeprecated-declarations]

The same warning is raised from subscription_options.hpp and message_memory_strategy.hpp.

Adds a "Providing an allocator for rcl" section. rclcpp is built on rcl, which also allocates memory, so rclcpp needs a C-style rcl_allocator_t to send those allocations to the custom allocator too. The section explains why the callbacks cannot just forward to the C++ allocator: rcl's deallocate does not pass the block size, but std::pmr::memory_resource::deallocate needs it.
Shows the full PmrAllocator implementation from the demo: a thin subclass of std::pmr::polymorphic_allocator whose get_rcl_allocator() stores each block's size in a header in front of it. The callbacks are noexcept and return nullptr on failure, like malloc.
Changes the example's Alloc alias from std::pmr::polymorphic_allocator<void> to PmrAllocator<void>.

Fixes # (issue)

Did you use Generative AI?

Yes. This PR description was AI-generated with Claude Code (Claude Opus 5.5), and Claude Code (Claude Opus 5.5) also helped write the documentation changes.

Additional Information

Companion change in ros2/demos: ros2/demos#817

Checked locally with the repository's CI targets: make test (doc8), make lint (sphinx-lint), make spellcheck (codespell) and make html (Sphinx with warnings treated as errors).

🤖 Generated with Claude Code

rclcpp falls back to a deprecated C++-to-rcl allocator conversion when the allocator has no get_rcl_allocator() member; that conversion passes a wrong size to deallocate (see ros2/rclcpp#3058). Explain why rcl needs its own allocator, show a std::pmr::polymorphic_allocator wrapper that provides one by storing the block size in a header, and use it in the example main. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Maurice <mauricepurnawan@gmail.com>

Replace the elided allocate/deallocate bodies with the complete code from demo_nodes_cpp's allocator_tutorial_pmr.cpp, including reallocate_block, zero_allocate_block and the required includes, and drop the note about the fallback conversion. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Maurice <mauricepurnawan@gmail.com>

fujitatomoya

lgtm

fujitatomoya · 2026-10-14T00:18:21Z

@3di-for-openrobotics @keithkirkwood-3di @kacperbojakowski-3di @sarahfarnborough-3di

sorry, gotta need to ask some help here.

#6981 really make it difficult to complete the action and merge the PR. it does not really tell what went wrong and how to fix though, can you have a look?

kacperbojakowski-3di · 2026-10-15T15:35:03Z

@3di-for-openrobotics @keithkirkwood-3di @kacperbojakowski-3di @sarahfarnborough-3di

sorry, gotta need to ask some help here.

#6981 really make it difficult to complete the action and merge the PR. it does not really tell what went wrong and how to fix though, can you have a look?

@fujitatomoya Hi! Yes, the solution causes PRs to fail, here is a hotfix to address that: #7294

…al-get-rcl-allocator Signed-off-by: Maurice <mauricepurnawan@gmail.com>

kscottz

@mini-1235 appreciate the help! This is really helpful content but the grammar and code need some work to make this complex topic a bit more accessible to the reader.

I made some inline suggestions, but an easier solution would be to just pipe the change through an LLM to tighten up the language and add comments to most of the code.

kscottz · 2026-10-19T19:56:03Z

+
+For ``std::pmr``, a small wrapper around ``std::pmr::polymorphic_allocator`` can store the size of each block in a header in front of it:
+
+.. code-block:: c++


CHORE: this is fairly complex code. Can you please get your favorite LLM to generate line-by-line comments to walk the user through the process?

I asked an LLM to do that, but the result felt a bit overwhelming to read, so I decided to keep only the key comments in 46236c3. Let me know what you think!

Co-authored-by: Katherine Scott <katherineAScott@gmail.com> Signed-off-by: Maurice Alexander Purnawan <mauricepurnawan@gmail.com>

Signed-off-by: Maurice <mauricepurnawan@gmail.com>

github-actions

Documentation enhancements

This pull request is missing configured documentation enhancements or uses values outside the configured vocabulary (see tools/enhance.yaml).

`source/ROS-Framework/client-libraries/Working-with-Client-Libraries/Allocator-Template-Tutorial.rst`

Invalid .. meta:: values: experience (optional, unknown advanced; allowed: beginner, intermediate, expert; capitalisation is ignored)

mini-1235 and others added 2 commits October 12, 2026 14:56

mini-1235 force-pushed the allocator-tutorial-get-rcl-allocator branch from 75d4866 to 9c6683c Compare October 12, 2026 06:59

fujitatomoya approved these changes Oct 14, 2026

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skyegalaxy assigned kscottz Oct 15, 2026

Merge remote-tracking branch 'upstream/rolling' into allocator-tutori…

f3129fd

…al-get-rcl-allocator Signed-off-by: Maurice <mauricepurnawan@gmail.com>

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kscottz requested changes Oct 19, 2026

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Apply batched suggestions from code review

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Co-authored-by: Katherine Scott <katherineAScott@gmail.com> Signed-off-by: Maurice Alexander Purnawan <mauricepurnawan@gmail.com>

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Clarify allocator tutorial code and fix sentence lint

46236c3

Signed-off-by: Maurice <mauricepurnawan@gmail.com>

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Merge branch 'rolling' into allocator-tutorial-get-rcl-allocator

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		For ``std::pmr``, a small wrapper around ``std::pmr::polymorphic_allocator`` can store the size of each block in a header in front of it:

		.. code-block:: c++

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Documentation enhancements

source/ROS-Framework/client-libraries/Working-with-Client-Libraries/Allocator-Template-Tutorial.rst

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mini-1235 commented Oct 12, 2026 •

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`source/ROS-Framework/client-libraries/Working-with-Client-Libraries/Allocator-Template-Tutorial.rst`