Is this a functor?

Hello, everyone. I've read a strange declaration from Chapter 7.3, Accelera=
ted C++. The default parameter split seems a normal function. What passes i=
n the function xref via the parameter vector<string> find_words(const strin=
g&)? 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 s=
paces
        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;
}