I think doing this might maintain ABI compatibility. Can you give it a try?

AFAIS report complaining on code, not on ABI (which I mentioned in comment above).
I believe ABI checker checking for disappearing some symbols, but here we have new symbol which should be fine

I built both versions (with and w/o change) and checked difference in symbols in libgz-common5-profiler.so.5.7.1 library.
And difference is couple o new symbols (some of them are public(capital letter), some of them private)

# diff clear_build/lib/out.txt branch/lib/out.txt 
51a52
> b _ZGVZN2gz6common8Profiler8InstanceEvE8profiler
72a74,75
> T _ZN2gz6common8Profiler8InstanceEv
> t _ZN2gz6common8Profiler8InstanceEv.cold
116a120
> b _ZZN2gz6common8Profiler8InstanceEvE8profiler
187a192,194
> U __cxa_guard_abort@CXXABI_1.3
> U __cxa_guard_acquire@CXXABI_1.3
> U __cxa_guard_release@CXXABI_1.3

command to get symbols w/o addresses: gz-common/branch/lib# nm libgz-common5-profiler.so.5.7.1 | cut -c18- > out.txt

I will try proposed code but I have big doubts about CI result change in this case

From source perspective it's big change, before symbol was inlined in every translation unit and now implementation hidden inside

The code clearly removes the SingletonT<Profiler>::__ct [C1] ( SingletonT<Profiler>const& p1 ) symbol because it no longer uses creates a SingletonT<Profiler> object, so in that sense the ABI checker is correct. However, like you said since those symbols are inlined, it's probably not a problem. But for added safety, I'd prefer if we can get our ABI checker ✅ .

Interesting mangling, first time see this way, which platform is it? I guess it's a copy constructor for class template Singleton. Which I guess will be or private symbol, or inlined.

I pushed proposed version and got same result https://build.osrfoundation.org/job/gz_common-abichecker-any_to_any-ubuntu-jammy-amd64/133/:

Binary compatibility: 100%
Source compatibility: 99.1%

I guess it's all about hiding Instance member function from Singleton parent.

If you OK, I'd prefer to proceed with static variable. As I said before, I'd really suggest to get rid of Singleton class due to it's just making more confusion and technically trying to replace 2 line function

Okay. Thanks for trying. I was hoping we would get a green CI. But since that's not going to happen, I would suggest a slightly different approach of using the gz::utils::NeverDestroyed class https://github.com/gazebosim/gz-sim/blob/f4a3b61b333ae822d5afbd2f8cae43d9ad361004/src/ComponentFactory.cc#L22-L26

This class is designed to work well with address sanitizers and avoids the static initialization order fiasco.

Also, I totally agree that we should remove the SingletonT class. If you are interested in helping out with removing the SingletonT class from our code base, please go right ahead :) (target main branches please).

About static initialization order fiasco, it refer to objects in global scope, which initialization happens before main(). Here we have a Meyers Singleton which provide a lazy initialization on 1st access. Also it happens thread-safe from C++11. So it's not this case

https://www.modernescpp.com/index.php/c-20-static-initialization-order-fiasco/

And about NeverDestroyed (very short version). IMHO it's a very dirty hack.

1. Valgrind will see indirect memory leak if object inside it allocates memory.
2. AFAIU it was invented for cases of library loading with dlopen and unloaded before termination. In this case code of destructor of static object is unloaded and attempt to call it will lead to crash on termination (I saw similar case with destructor of exception on plugin loading mechanism in one of project where I worked)
3. I guess it can be treated as UB. Can deep dive in standard to find more accurate interpretation, but it will take a lot of time

Also, I totally agree that we should remove the SingletonT class. If you are interested in helping out with removing the SingletonT class from our code base, please go right ahead :) (target main branches please).

I guess I can, just want to close other PRs which I opened (2 in this repo, 1 in gz-sim)