C++ Nifty Counter idiom; why?

Question

I recently came across the Nifty Counter Idiom. My understanding is that this is used to implement globals in the standard library like cout, cerr, etc. Since the experts have c

Answer 1

The static local/Meyer's singleton + static global reference (your solution) is nearly equivalent to the nifty counter.

The differences are as follows:

1. No .cpp file is required in your solution.

2. Technically the static Steam& exists in every compilation unit; the object being referred to does not. As there is no way to detect this in the current version of C++, under as-if this goes away. But some implementations might actually create that reference instead of eliding it.

3. Someone could call getStream() prior to the static Stream& being created; this would cause difficulty in destruction order (with the stream being destroyed later than expected). This can be avoided by making that against the rules.

4. The standard is mandated to make creating the static Stream local in the inline getStream thread safe. Detecting that this is not going to happen is challenging for the compiler, so some redundant thread-safety overhead may exist in your solution. The nifty counter does not support thread safety explicitly; this is considered safe as it runs at static initialization time, prior to when threads are expected.

5. The call to getStream() must occur in each and every compilation unit. Only if it is proven that it cannot do anything may it be optimized out, which is difficult. The nifty counter has a similar cost, but the operation may or may not be be simpler to optimize out or in runtime cost. (Determining this will require inspecting resulting assembly output on a variety of compilers)

6. "magic statics" (statics locals without race conditions) where introduced in C++11. There could be other issues prior to C++11 magic statics with your code; the only one I can think of is someone calling getStream() directly in another thread during static initialization, which (as mentioned above) should be banned in general.

7. Outside the realm of the standard, your version will automatically and magically create a new singleton in each dynamicly linked chunk of code (DLL, .so, etc). The nifty counter will only create the singleton in the cpp file. This may give the library writer tighter control over accidentally spawning new singletons; they can stick it into the dynamic library, instead of spawning duplicates.

Avoiding having more than one singleton is sometimes important.