#BlueprintImplementableEvent
You can create BlueprintImplementableEvents in C++ to tell BP / UMG what to do
They in turn can call BlueprintCallable C++ functions to ask C++ to do stuff
My Wiki on BlueprintImplementableEvents
#Wiki by Bn_Green
You can also check out this tutorial that talks about UMG within C++ itself
https://wiki.unrealengine.com/UMG,_Referencing_UMG_Widgets_in_Code
:)
Rama