Re: Is this a functor?

Hello, everyone. I've read a strange declaration from Chapter 7.3,
Accelerated C++. The default parameter split seems a normal function.
What passes in the function xref via the parameter vector<string>
find_words(const string&)? Is it a functor? Or function pointer? Or
anything else?

Declaration:

map<string, vector<int> > xref(istream& in, vector<string>
find_words(const string&) = split)

Implementation:

map<string, vector<int> > xref(istream& in, vector<string>
find_words(const string&) = split) { string line; int line_number =
0; map<string, vector<int> > ret;

// read the next line while (getline(in, line)) { ++line_number;

// break the input line into words vector<string> words =
find_words(line);

// remember that each word occurs on the current line for
(vector<string>::const_iterator it = words.begin(); it !=
words.end(); ++it) ret[*it].push_back(line_number); } return ret; }

split.h

#ifndef GUARD_split_h #define GUARD_split_h

#include<vector> #include<string> std::vector<std::string>
split(const std::string&);

#endif

split.cpp

#include<cctype> #include<string> #include<vector>

#include "split.h"

using std::vector; using std::string;

using std::isspace;

vector<string> split(const string& s) { vector<string> ret;
typedef string::size_type string_size; string_size i = 0;

// invariant: we have processed characters `['original value of `i',
`i)' while (i != s.size()) { // ignore leading blanks // invariant:
characters in range `['original `i', current `i)' are all spaces
while (i != s.size()&& isspace(s[i])) ++i;

// find end of next word string_size j = i; // invariant: none of the
characters in range `['original `j', current `j)' is a space while (j
!= s.size()&& !isspace(s[j])) ++j;

// if we found some nonwhitespace characters if (i != j) { // copy
from `s' starting at `i' and taking `j' `\-' `i' chars
ret.push_back(s.substr(i, j - i)); i = j; }

} return ret; }