Generators in C++ revisited.

Previous version of generators has a design problem – it required some special stop value. That is the same kind of problem as with iterators in C++ – they require special end() value. But in some cases it is not even possible to choose such a value.

So is this new version:


// generator/continuation for C++
// author: Andrew Fedoniouk @ terrainformatica.com
// idea borrowed from: "coroutines in C" Simon Tatham,
//                     http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html

  //++ coroutine, generator, continuation for C++

  struct _generator
  {
    int _line;
    _generator():_line(-1) {}
  };

  #define $generator(NAME) struct NAME : public _generator

  #define $emit(T) bool operator()(T& _rv) { \
                      if(_line < 0) { _line=0;}\
                      switch(_line) { case 0:;

  #define $stop  } _line = 0; return false; }

  #define $yield(V)     \
          do {\
              _line=__LINE__;\
              _rv = (V); return true; case __LINE__:;\
          } while (0)

  //-- coroutine, generator, continuation for C++

Implementation of typical iterator practically is the same an in previous version except of $stop is being used without any parameter now:

#include "generator.h"

$generator(descent)
{
   int i;
   $emit(int) // will emit int values. Start of body of the generator.
      for (i = 10; i > 0; i–)
         $yield(i); // a.k.a. yield in Python, 
                      // returns next number in [1..10], reversed.
   $stop; // stop, end of sequence. End of body of the generator.
};

But its usage is a bit different and is close to JavaScript for(var in sequence) statement:

int main(int argc, char* argv[])
{
  descent gen;
  for(int n; gen(n);) // "get next" generator invocation
    printf("next number is %d\n", n);
  return 0;
}

And here is version of the generator that supports restart (recursive call) - needed for walking through tree alike (recursive) data structures. It uses allocations on the heap that I think is overkill for such constructions.

Generators in C++

As we know iterators in C++ is a good but not perfect abstraction. Concept of foreach() (D, Python, Ruby, etc.) appears as more generic solution. At least foreach() does not require artificial iterator::end() to be defined for the collection.

Abstraction foreach() can be imagined as some function/object that is returning next value of collection/sequence each time it is getting invoked. Such functions are known as generators.

Here is my version of generator() for the C++. It is based on the bright idea of Simon Tatham – “coroutines in C“.

First of all sample, here is a declaration of our generator function in C++:

#include "generator.h"

generator(descent) 
{
   int i;
   emit(int) // will emit int values. Start of body of the generator.
      for (i = 10; i > 0; i--)
         yield(i); // a.k.a. yield in Python - return next number in [1..10], reversed.
   stop(0); // stop, end of sequence. End of body of the generator.
};

Having declared such generator we can use it as:

int main(int argc, char* argv[])
{
  descent gen;
  do
    printf("next number is %d\n", gen());
  while (gen);
  return 0;
}

That is pretty simple and should be intuitively clear, no?

Here is full source of the generator.h file:

// generator/continuation for C++
// author: Andrew Fedoniouk @ terrainformatica.com
// idea borrowed from: "coroutines in C" Simon Tatham, 
//                     http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html

#ifndef __generator_h_
#define __generator_h_

struct _generator
{
  int _line;
  _generator():_line(-1) {}
  operator bool() const { return _line != 0; }
};

#define generator(NAME) struct NAME : public _generator

#define emit(T)     T operator()() { \
                     if(_line < 0) { _line=0;}\
                     switch(_line) { case 0:;

#define stop(V)     } _line = 0; return (V); }

#define yield(V)     \
        do {\
            _line=__LINE__;\
            return (V); case __LINE__:;\
        } while (0)

#endif // __generator_h_

Enjoy!

JavaScript. Private members (instance variables).

As you know JavaScript has no concept of private members in objects. Objects there are “racks of properties” that anyone can change.

Here is a simple way of making “objects” in JS that have private instance variables. Such variables can be changed only by [public] methods you provide.

Consider following code:

  function CreateMyObject()
  {
    var m_first = 0;
    var m_second = 0;

    var envelope = // our public interface
    {
        getFirst: function() { return m_first; },
        setFirst: function(v) { m_first = v; },
        getSecond: function() { return m_second; },
        setSecond: function(v) { m_second = v; }
    };    
    return envelope;
  }  
  
  var f = CreateMyObject();
  
  var n = f.getFirst(); // calling our methods 

Here we have CreateMyObject() that behaves as a constructor of our object. It returns envelope object that has four methods. Only these methods can update or retrieve state of m_first and m_second variables. There are literally no other ways to access them from anywhere else other than set of methods provided.

Under the hood: In this example each interface method that we’ve created is a closure in fact: such methods have access to environment of outer function (CreateMyObject in this case) so they can access variables in this environment. Making a closure can be a memory expensive operation so avoid their use when you expect many instances of such objects.

Sciter. Working with persistent data (database)

Here is an example of minimalistic application working with DB (persistent data) in Sciter.

Screenshot of DB form.

This sample is using three files:

  1. simple-db-form.htm – main file of our application;
  2. db.tis – open or create database;
  3. form.tis – behavior that handles showing/saving data from list of input elements on the form.

db.tis – open or create database

This file contains single procedure that will open or create new storage if does not exist:

function openDatabase(pathname = "records.db")
{
  var ndb = Storage.open(pathname);
  if(!ndb.root)
  { 
    // new db, initialize structure
    ndb.root = 
      { // root of our db is an object having single field 'records'
        records: ndb.createIndex(#integer) // main index, int -> item
      };
  }
  return ndb;
}

Structure of our storage is simple – its root node contains single field records of type Index. In our example this index maps integer – number of the record to the record itself – object that may have arbitrary structure.

form.tis – showing/saving data from input elements on a form

// Form handler class
type Form: Behavior
{
  function show(rec)
  {
    if(this.shown) this.save(); // save previously shown record.
    this.shown = rec;
    function load_value(el)
    {
      var name = el.attributes#name;
      el.value = rec[symbol(name)];
    }
    this.select(load_value, "[name]"); 
    // call load_value for each element having "name" defined.
    // De facto this means that form content defines structure of the record.
  }
  function save()
  {
    var shown = this.shown;
    function store_value(el)
    {
      var name = el.attributes#name;
      shown[symbol(name)] = el.value;
    }
    this.select(store_value, "[name]"); 
    // call store_value for each element having "name" defined.
  }
}

This behavior class has two methods:

  • show(rec) – show fields of object rec in fields of the form and
  • save() – stores content of input elements of the form in fields of last shown rec object

simple-db-form.htm – main file of our application

And last part is our main file that defines UI layout and assembles all parts together:

<html>
<head>
  <style>
    form#record 
    { 
      prototype:Form; 
    }
    ...
  </style>  
  <script type="text/tiscript">
    include "form.tis";
    include "db.tis";
    
    var form = self.select("form#record");
    var rec_no = self.select("#rec-no");
    
    var db = openDatabase();
    var no = 0;
    
    function gotoRecNo( n )
    {
      no = n;
      if( no < 0 ) no = 0;
      else if( no >= db.root.records.length ) no = db.root.records.length - 1;
      form.show( db.root.records[no] );
      rec_no.text = String.printf("%d of %d", no, db.root.records.length); rec_no.update();
    }

    function gotoNewRecord() 
    { 
      // create new record 
      var newidx = db.root.records.length;
      db.root.records[ newidx ] = { first:0, second:"", third:"<html><\/html>" };
      gotoRecNo(newidx); 
    }

    // handlers of navigatonal buttons
    function self #first .onClick() { gotoRecNo(0); }
    function self #prev  .onClick() { gotoRecNo(no - 1); }
    function self #next  .onClick() { gotoRecNo(no + 1); }
    function self #last  .onClick() { gotoRecNo(db.root.records.length - 1); }
    function self #new   .onClick() { gotoNewRecord(); }
    
    function view.onLoad()   { if(db.root.records.length) gotoRecNo(0); 
                                        else gotoNewRecord(); }
    function view.onUnload() { form.save(); db.close(); }
    
  </script>  
<head>
<body>
  <h1>Simple DB form</h1>
  <form #record>
    <table>  
      <tr><td>First (number)</td><td><input name="first" type="number"/></td></tr>
      <tr><td>Second (text)</td><td><input name="second" type="text"/></td></tr>
      <tr><td>Third (richtext(html))</td><td><richtext name="third" /></td></tr>
    </table>
  </form>
  <div #nav>
    <button #first title="to first record">|<</button>
    <button #prev title="to prev record"><</button>
    <button #next title="to next record">></button>
    <button #last title="to last record">>|</button>
    <button #new title="create new record">new</button>
    record: <span #rec-no>?</span>
  </div>
</body>
</html>

Done. We have simple form that allows us to create records with text, numeric and richtext (html) fields.


Code above is not the best one from architectural point of view – its goal is to show basic principles, not more.
You can find this sample in [Sciter SDK]/samples/ideas/db/ folder.