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.