Explain the rationale for the type of each of find_char’s parameters In particular, why is s a reference to const but occurs is a plain reference

The function prototype is

 string::size_type find_char(const string &s,
                                char c,
                                string::size_type &occurs)

`s` and `occurs` are both referecnes to avoid copy.

`s` is `const` because it isn't changed inside function and a string literal can be used here. `occurs` is plain reference beacuse it is used to pass information (changed inside function).

`c` is nonreference because copy a `char` is very cheap. It's fine to make it a `const` reference but not plain reference, because we don't want to accidentally change `c` inside function, and we may want to pass a `char` literal to the function.

If `occurs` is made a reference to `const`, then we cannot get how many times the character `c` occurred in string `s`.