I’m not familiar with C++ myself, but the main thing to keep in mind when working with UML is that the UML standard is language independent. So it is possible that certain specific language features can’t be fully integrated.
However, that’s only looking at this from an UML perspective. I know Visual Paradigm supports specific enhancements for certain programming languages, amongst which C++.
So I looked up the definition for the initializer list and found this example, then did some experimenting which makes me believe that this isn’t currently supported (but please keep my inexperience with C++ in mind).