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Comment by cominous | Hacker News Reader

I have to admit, that C++ is still not the industry "Go-To" language for embedded. But if you apply modern C++ correctly, there is very few overhead compared to C and the software is much easier to maintain.

The performance of embedded MCU's are continuously rising over the years and that little overhead is buying development speed.

Not to mention smart pointers, templates and constexpr making my life easier.

The only real issue with C++ is, that as soon as you get into serious embedded applications, you have restrictions when it comes to heap usage e.g. in medical devices using the heap is forbidden. So you cant use the STL.

There is a promising embedded STL project, but it's not there yet: https://www.etlcpp.com

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Shameless plug: At EA we've open-sourced our C++ standard library implementation that focuses on games. While it's not necessarily focused on embedded development, you might be interested in checking it out - it provides a number of fixed_* containers (fixed_vector, fixed_set, etc) which can be configured to use stack allocations only, among some other things you might find interesting :)

https://github.com/electronicarts/EASTL

(In case it wasn't obvious, disclaimer: I work at EA, and frequently do work on EASTL, though I'm not the primary maintainer.)

> So you cant use the STL.

std::array is in there since C++ 11 and it doesn’t use heap.

And/or you can use STL with custom allocators that work without heap. We did something similar developing for Nintendo Wii console. There was a heap but we didn’t want to use it to avoid memory fragmentation. AFAIR we used two arenas (essentially stacks), one very small for temporary data cleaned up at the start of each frame, and a large one cleaned up when a level is unloaded.

However, I don’t have hands on experience developing firmware for medical devices, so I’m not sure it’ll work for them.

The problem for medical devices isn't so much whether custom allocators will work. The problem is whether the FDA will freak out because you're not following industry-best-practice coding guidelines.

That's really not how it works. The FDA is fundamentally concerned about two things, safety and efficacy. You need a plan to demonstrate the latter, and you need your quality system, SDP, etc. to demonstrate how you approach the former. This is about good engineering practices, not particular implementation techniques.

So you can do things many different ways. If you do say "we do this like X, which is industry standard, just like T, U, and V do" it's a simpler argument than "we do this like Y. Lots of people do X, but here is how we have demonstrated Y is better for us...". But this can be fine too, just possibly more work.

Also worth noting (a) there is no industry-wide best practices agreement (b) there is no FDA wide agreement on what should be done (different device types are reviewed by different panels (c) the FDA doesn't understand software development deeply across it's panels, but it is catching up.

Sure, that's all true. I'm looking at the "simpler argument" part.

It's especially true if you're saying "This new device is just like our previous device, with these few small changes". (I forget what that's called, but you can do a lot less paperwork if that's true.) But if you start doing memory allocations where you never did before, they're probably going to want to apply higher scrutiny to your entire software. That's... painful.

You are possibly thinking about note-to-file (which isn’t quite the process any more, but similar)

But it is worth pointing out, FDA (or other national body) isn’t doing code reviews. They are mostly interested in process , and how you follow it. The place where this really interacts s is hazard analysis, and then it’s just up to you to demonstrate your checks and balances.

Arenas/Pools/Planks have been industry best-practice for my entire career.

Also, STL is data structures + algorithms. Even if you can't use the datastructures out of the box you might be able to use the algos. Boost.Intrusive provides STL compatibe datastructures with full control of allocation (and more).

Out of curiosity, what is the rationale for not using the heap with medical devices? Resource constraints are one thing but that is not limited to medical nor is that entirely solved with preventing heap use. If it's for runtime safety to avoid raw pointers, has anyone done an analysis to determine if smart pointers (unique_ptr, shared_ptr), combined with diligent static code analysis diagnostics to avoid the kinds of issues Herb raises in the OP video, could reduce the risk to an acceptable level?

I have not worked in medical (my experience is in games) but my best guess is its more about reliability and predictability. Using a heap suffers from the fact that you can run out of memory to satisfy a malloc/new request (either due to system memory limit or due to fragmentation).

With static memory techniques you can "prove" the system has enough memory to work in all modes, i.e. device consumes 20 readings per second, keeps them in a ring buffer backed by a static fixed array, that buffer is large enough to satisfy processing rate.

Safety critical software (or even mission critical software) should not be using dynamic allocation for a few reasons.

- Fragmentation.

- Non-deterministic runtime (in the real-time cases).

- Insufficient analysis of worst-case conditions (i.e. you haven't worked out what your worst case RAM usage is, otherwise you would have statically allocated it).

IMO, the worst is the final case as it shows a lack of thoroughness in the design as a whole and brings the rest of the code into suspicion. Fragmentation can be worked around, but not the others.

   Out of curiosity, what is the rationale for not using the heap with medical devices?

Avoiding heap allocation is not at all a general constraint for medical devices. For certain types of components (think safety-critical real-time sub systems, for example) they are going to be very interested in your hazard analysis and the mitigating approaches to possible issues.

So if there is a way to say: we don't have to worry about [class of error X] because we don't ever do Y, that's a straightforward way to sort out those components. If you have a compelling tech reason to do Y, better start thinking about all the controls you'll put on it.

Think about it this way: What's the worse thing that can happen if your code causes an OOM error? If the answer includes things like "somebody dies if it happens at the wrong time", you'll want to be really careful to prove (prove, not just test out) that can't happen.

When I was using C++ for embedded, which is admittedly over 15 years ago, I had to switch off RTTI and exception handling to get compact binaries. Basically just using classes and surface language features. We did use templates but only very selectively.

Is it still possible today to pare down the compiler output like that? I imagine a lot of modern C++ just doesn't work unless everything is enabled.

Yes it is possible.

Check this talk about fitting C++17 on a C64.

CppCon 2016: Jason Turner “Rich Code for Tiny Computers: A Simple Commodore 64 Game in C++17”

https://www.youtube.com/watch?v=zBkNBP00wJE

Also be aware that embedded devices like Arduino and Cortex-M (with Mbed OS) do use C++ toolchains.

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