What does this block define?

Question

I'm learning about vectors in Accelerated C++ (which is C++ 98, not C++11) by Andrew Koenig and Barbara Moo. In this code...

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

...what is being defined in the block? xref or find_words? In my debugger, the call stack goes: main() > xref() > split(). find_words isn't defined elsewhere.

// find all the lines that refer to each word in the input
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;
}

Also, split looks like this:

vector<string> split(const string& s) { ... }

This could answer your question https://stackoverflow.com/a/47835838/5470596 — YSC, Mar 20 '18 at 16:02
`map >` is no longer needed, you can use `map>` now, fixed in C++11. — Drise, Mar 20 '18 at 16:04
aren't you repeating your question: https://stackoverflow.com/questions/49387638/difference-between-vectorstring-x-and-vectorstring-xconst-string-s/49387713#49387713 .... — bolov, Mar 20 '18 at 16:08
`find_words` _isn't defined elsewhere_ - but you can **see** where it's called, and you can see it really calls `split` on that line. So you know the line calling `find_words` really calls `split`. Maybe you haven't encountered function pointers before? — Useless, Mar 20 '18 at 16:11
@bolov, in https://stackoverflow.com/questions/49387638/difference-between-vectorstring-x-and-vectorstring-xconst-string-s/49387713#49387713, I was trying to understand the theory (different definitions of vector), whereas in this question, I'm trying to understand the (book's) implementation of the second one: in a map. — mellow-yellow, Mar 20 '18 at 16:16
@Useless, "function pointers" are indeed new to me. The book has not (yet) explained them. — mellow-yellow, Mar 20 '18 at 16:21
A typedef for the function pointer type would sure make that code a bit more readable. — Sean Burton, Mar 20 '18 at 17:15

score 4 · Accepted Answer · answered Mar 20 '18 at 16:05

map<string, vector<int>> xref(
    istream& in, 
    vector<string> find_words(const string&) = split
) { /* ... */ }

This defines the function named xref. According to Clockwise Spiral Rule, xref is a function:

taking as arguments:
1. a istream reference (istream& in)
2. a function: (vector<string> find_words(const string&) = split)
  - taking as argument:
    1. a string constant reference(const string&)
  - returning a vector<string>
  - whose default value is split (= split)
and returning a map<string, vector<int>>

score 0 · Answer 2 · answered Mar 20 '18 at 16:03

0

xref is the function that is defined, find_words is one of its parameters (a function taking a const string& as parameter and returning a vector<string>) and split is the default value for that parameter.

answered Mar 20 '18 at 16:03

463035818_is_not_an_ai

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score 0 · Answer 3 · answered Mar 20 '18 at 16:14

xref is being defined in the code block and returns a map of a string and a vector of ints. It is taking in parameters of an istreamby reference, a function that takes a string constant reference and returns a vector that the default value is split.

What does this block define?

3 Answers3