There are a total of 8 = 2^3 function objects in this family. (1) Member functions taking no arguments vs member functions taking one argument. (2) Call through pointer vs call through reference. (3) Const vs non-const member function.

All of this complexity is in the function objects themselves. You can ignore it by using the helper function mem_fun and mem_fun_ref, which create whichever type of adaptor is appropriate.