From what I understand: when you pass by value, the function makes a local copy of the passed argument and uses that; when the function ends, it goes out of scope. When you pass by const reference, the function uses a reference to the passed argument that can't be modified. I don't understand, however, why one would choose one over the other, except in a situation where an argument needs to be modified and returned. If you had a void function where nothing is getting returned, why choose one over the other?