+This is what I have read in the document: “Pure Functions promise not to modify state or the members of the class in any way”
(Functions | Unreal Engine 4.27 Documentation)
+But the example in the picture show that a function marked with pure can even change the member value and give no error
; Am I missing or misunderstand something ? please help
i think that promise is from the developer not the function haha
you can do it but generally shouldnt as it may cause bugs
Auran, it didn’t give error from beginning, I thought UE5 is the best? 
because you can do it and it does work its just dangerous 
the statement is from UE4.27, but now it is already UE5.3 ; but Epic Game still not fix it? 
my point is its not a bug, there are uses for it you just have to be careful
thanks for your help Auran
This line should be included in the documentation. BPs dismiss most aspects of what pure stands for. BPs pure is more like: a function with no execution pins that is more tricky to use. As mentioned by @Auran131, you must police yourself.