Trying to implement an iterator without an explicit container

After my recent question, I am trying to implement my own contrived example.

I have a basic structure in place but even after reading this, which is probably the best tutorial I've seen, I'm still very confused. I think I should probably convert the Chapter._text into a stream and for the increment operator do something like

string p = "";
string line;
while ( getline(stream, line) ) {
    p += line;
}
return *p;

but I'm not sure which of the "boilerplate" typedefs to use and how to put all these things together. Thanks much for your help

#include <stdio.h>
#include <stdlib.h>
#include <vector>
#include <iostream>
#include <fstream>
#include <string>

using namespace st开发者_如何学运维d;

class Paragraph {
public:
  string _text;

  Paragraph (string text) {
    _text = text;
  }
};

class Chapter {
public:
  string _text;

  /*  // I guess here I should do something like:
  class Iterator : public iterator<input_iterator_tag, Paragraph> {

  }
  // OR should I be somehow delegating from istream_iterator ? */

  Chapter (string txt_file) {
    string line;

    ifstream infile(txt_file.c_str());
    if (!infile.is_open()) {
      cout << "Error opening file " << txt_file << endl;
      exit(0);
    }
    while ( getline(infile, line) ) {
      _text += (line + "\n");
    }
  }

};

int main(int argc, char** argv) {
  Chapter c(argv[1]);

  // want to do something like:
  // for (<Paragraph>::iterator pIt = c.begin(); pIt != c.end(); pIt++) {
  //    Paragraph p(*pIt);
  //    // Do something interesting with p
  // }

  return 0;
}

As you weren't planning on a chapter loading at a time (merely a paragraph), and your paragraph is empty, I think this might be best done with a single paragraph_iterator class

class paragraph_iterator : 
public std::iterator<std::input_iterator_tag, std::string>
{
    std::shared_ptr<std::ifstream> _infile; //current file
    std::string _text; //current paragraph
    paragraph_iterator(const paragraph_iterator &b); //not defined, so no copy
    paragraph_iterator &operator=(const paragraph_iterator &b); //not defined, so no copy
    // don't allow copies, because streams aren't copiable. 
    // make sure to always pass by reference
    // unfortunately, this means no stl algorithms either

public:
    paragraph_iterator(string txt_file) :_infile(txt_file.c_str()) {
        if (!infile.is_open())
            throw std::exception("Error opening file ");
        std::getline(_infile, _text);
    }
    paragraph_iterator() {}
    paragraph_iterator &operator++() {
        std::getline(_infile, _text);
        return *this;
    }
    // normally you'd want operator++(int) as well, but that requires making a copy
    // and std::ifstream isn't copiable.
    bool operator==(const paragraph_iterator &b) const {
        if (_infile.bad() == b._infile.bad())
            return true; //all end() and uninitialized iterators equal
        // so we can use paragraph_iterator() as end()
        return false; //since they all are seperate strings, never equal
    }
    bool operator!=(const paragraph_iterator &b) const {return !operator==(b);}
    const std::string &operator*() const { return _text;}
};

int main() {
    paragraph_iterator iter("myfile.txt");
    while(iter != paragraph_iterator()) {
         // dostuff with *iter
    }

}

the stream is encapsulated in the iterator, so that if we have two iterators to the same file, both will get every line. If you have a seperate Chapter class with two iterators, you may run into "threading" problems. This is pretty bare code, and completely untested. I'm sure there's a way to do it with copiable iterators, but far trickier.

In general, an iterator class implementation is closely tied with the data structure it iterates over. Otherwise, we'd just have a few generic iterator classes.