Effective C++ Item 23 Prefer non-member non-friend functions to member functions

Question

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 no

Answer 1

The motivation is simple: maintain a consistent syntax. As the class evolves or is used, various non-member convenience functions will appear; you don't want to modify the class interface to add something like toUpper to a string class, for example. (In the case of std::string, of course, you can't.) Scott's worry is that when this happens, you end up with inconsistent syntax:

s.insert( "abc" );
toUpper( s );

By only using free functions, declaring them friend as needed, all functions have the same syntax. The alternative would be to modify the class definition each time you add a convenience function.

I'm not entirely convinced. If a class is well designed, it has a basic functionality, it's clear to the user which functions are part of that basic functionality, and which are additional convenience functions (if any such exist). Globally, string is sort of a special case, because it is designed to be used to solve many different problems; I can't imagine this being the case for many classes.