Shall I make every function static, which do not touch the member variable?

Question

What is the Design and Performance impact for making all functions static which do not touch the member variable of the class?

Answer 1

You should actually consider making them non-static free functions, as explained in detail in this question. This question is also very interesting.

In a nutshell, these question explain that you should prefer non-friend non-member functions whenever possible (meaning when they do not access non-public members).