Replacing old-style C++ allocator with a polymorphic memory resource (PMR)

[AliAshkaniNia]

This is a basic example mimicking the original demo. We could use a more advanced resource, like unsynchronized_pool_resource, for a realistic scenario.

[AliAshkaniNia]

This is a change for issue #650 .

[fujitatomoya]

some uncrustify errors, https://build.ros2.org/job/Rpr__demos__ubuntu_jammy_amd64/235/testReport/junit/demo_nodes_cpp/uncrustify/src_topics_allocator_tutorial_cpp/

and, could you also address DCO error?

[fujitatomoya]

Alphabetical order.

Suggested change

	#include <string>
	#include <utility>
	#include <memory_resource>
	#include <memory_resource>
	#include <string>
	#include <utility>

[AliAshkaniNia]

My apologies, I should have read the ROS2 developer guides before making changes. Now, it is compliant with ROS2 style, and I have also reordered the #include statements.

[fujitatomoya]

We could use a more advanced resource, like unsynchronized_pool_resource, for a realistic scenario.

I am not sure about this, we could. but we would not want to introduce the polymorphic memory details here, instead we would like it to be simple for demonstration code.

[clalancette]

I am not sure about this, we could. but we would not want to introduce the polymorphic memory details here, instead we would like it to be simple for demonstration code.

Yeah, I agree. What we have here is easy to understand, and is the main point of this tutorial, so I think we should keep it simple.

[clalancette]

Thanks, this is much easier to understand.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows

[AliAshkaniNia]

Thanks, this is much easier to understand.

Glad I could assist!

[AliAshkaniNia]

For the unsynchronized_pool_resource stuff, I agree with you. I'm a fan of making tutorials dead simple, which is why I came up with this refactoring idea. Please correct me if I'm wrong, but on the other hand, my perception is that ROS2 comes with "batteries included"; it provides all the necessary tools and resources for creating production-ready code. I believe there's a good chance that someone interested in custom allocators might also be interested in using stack memory instead of heap. So, I thought it's worth mentioning that topic thanks to the intuitive API:

std::array<std::uint8_t, 10*1024> buffer{};
std::pmr::monotonic_buffer_resource upstream_resource(buffer.data(), buffer.size());
std::pmr::unsynchronized_pool_resource mem_resource(&upstream_resource);

auto alloc = std::make_shared<Alloc>(&mem_resource);

Again, this is just a suggestion, and overall, I agree that it might be too complicated for the tutorial. I just mentioned it for the sake of completeness.