the invent of :: make coder to code more , and it is use to tell the sub-function is under which parent function.

may i ask do this legal in c++? :

sell::sub_sell::sub_sell_sell

Given the proper declarations appear before that point, yes. What constitutes a proper declaration depends on what sell, sub_sell, and sub_sell_sell are supposed to be, which I can't tell just from your example.