有效的C++项目23更喜欢非成员非朋友函数而不是成员函数

While puzzling with some facts on class design, specifically whether the functions should be members or not, I looked into Effective c++ and found Item 23, namely, Prefer non-member non-friend functions to member functions. Reading that at first hand with the web browser example made some sense, however convenience functions( named the nonmember functions like this in the book) in that example change the state of the class, don't they? While puzzling with some facts on class design,