Electronic – Is C++ suitable for embedded systems


A common question, here and elsewhere. Is C++ suitable for embedded systems?

Microcontrollers? RTOSes? Toasters? Embedded PCs?

Is OOP useful on microcontrollers?

Does C++ remove the programmer too far from the hardware to be efficient?

Should Arduino's C++ (with no dynamic memory management, templates, exceptions) be considered as "real C++"?

(Hopefully, this wiki will serve as a place to contain this potential holy war)

Best Answer

Yes, C++ is still useful in embedded systems. As everyone else has said, it still depends on the system itself, like an 8-bit uC would probably be a no-no in my book even though there is a compiler out there and some people do it (shudder). There's still an advantage to using C++ even when you scale it down to something like "C+" even in a 8-bit micro world. What do I mean by "C+"? I mean don't use new/delete, avoid exceptions, avoid virtual classes with inheritance, possibly avoid inheritance all together, be very careful with templates, use inline functions instead of macros, and use const variables instead of #defines.

I've been working both in C and C++ in embedded systems for well over a decade now, and some of my youthful enthusiasm for C++ has definitely worn off due to some real world problems that shake one's naivete. I have seen the worst of C++ in an embedded systems which I would like to refer to as "CS programmers gone wild in an EE world." In fact, that is something I'm working on with my client to improve this one codebase they have among others.

The danger of C++ is because it's a very very powerful tool much like a two-edged sword that can cut both your arm and leg off if not educated and disciplined properly in it's language and general programming itself. C is more like a single-edged sword, but still just as sharp. With C++ it's too easy to get very high-levels of abstraction and create obfuscated interfaces that become meaningless in the long-term, and that's partly due to C++ flexibility in solving the same problem with many different language features(templates, OOP, procedural, RTTI, OOP+templates, overloading, inlining).

I finished a two 4-hour seminars on Embedded Software in C++ by the C++ guru, Scott Meyers. He pointed out some things about templates that I never considered before and how much more they can help creating safety-critical code. The jist of it is, you can't have dead code in software that has to meet stringent safety-critical code requirements. Templates can help you accomplish this, since the compiler only creates the code it needs when instantiating templates. However, one must become more thoroughly educated in their use to design correctly for this feature which is harder to accomplish in C because linkers don't always optimize dead code. He also demonstrated a feature of templates that could only be accomplished in C++ and would have kept the Mars Climate Observer from crashing had NASA implemented a similar system to protect units of measurement in the calculations.

Scott Meyers is a very big proponent on templates and judicious use of inlining, and I must say I'm still skeptical on being gung ho about templates. I tend to shy away from them, even though he says they should only be applied where they become the best tool. He also makes the point that C++ gives you the tools to make really good interfaces that are easy to use right and make it hard to use wrong. Again, that's the hard part. One must come to a level of mastery in C++ before you can know how to apply these features in most efficient way to be the best design solution.

The same goes for OOP. In the embedded world, you must familiarize yourself with what kind of code the compiler is going to spit out to know if you can handle the run-time costs of run-time polymorphism. You need to be willing to make measurements as well to prove your design is going to meet your deadline requirements. Is that new InterruptManager class going to make my interrupt latency too long? There are other forms of polymorphism that may fit your problem better such as link-time polymorphism which C can do as well, but C++ can do through the Pimpl design pattern (Opaque pointer).

I say that all to say, that C++ has its place in the embedded world. You can hate it all you want, but it's not going away. It can be written in a very efficient manner, but it's harder to learn how to do it correctly than with C. It can sometimes work better than C at solving a problem and sometimes expressing a better interface, but again, you've got to educate yourself and not be afraid to learn how.