An arithmetic_mean Function For Various Type Arbitrary Nested Iterable Implementation in C++
Instead of simply printing output, it would be better to have self-checking unit tests, so you can fail your build as soon as you break something, rather than having to notice a change in the output.
You probably want to include some tests with very large (positive and negative) values to make sure you get accurate results with these, and don't fall foul of integer overflow (which will cause gross inaccuracy with unsigned types, and is Undefined Behaviour with signed integer types).
It's disappointing that there are no tests using primitive types other than int.