Write a C++ program that takes a user-input set of characters and uses bit manipulation techniques to generate all possible combinations of the characters. The program should output each combination as a string and display them all. The code should be well-documented and include comments explaining how the bit manipulation techniques were used. Additionally, the program should prompt the user for input and handle input validation to ensure that the user provides a set of valid characters to work with.
This prompt doesn't seem malicious.