What are the problems addressed by a "COM-like" framework? [closed]

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Closed 8 years ago.

Often I see the GObject library compared to the COM-Windows-like solutions, In general I don't really get what kind of problems this frameworks are supposed to solve and what are the benefits originated from their adoption.

I have found small answers like "it's a wrapper to easily port C libs to other languages", but GObject also offers other features, like a signal/slot mechanism, there are other things involved, but I don't honestly get the big picture about this.

Can you explain this in plain old english ?

Answer 1

COM and similar libraries provide the ability to create and use binary object-oriented components.

The "binary" means that it's fairly language independent, accessible in e.g. C, C++, Pascal, etc.

A layer on top of COM called "Automation" provides access from scripting languages, for components that specifically support "Automation".

---

As a concrete example, the Windows API provides an Automation COM component that represents (and can create or modify) a Windows shortcut. Windows shortcuts are ".lnk" files that are somewhat like symbolic links, except that they have to be explicitly dereferenced by the application. On the computer I'm writing this on, the MingW g++ installation contains a JavaScript program to make that COM-based functionality afvailable as a simple command line command.

The COM object creation in that program:

var Shortcut = WinShell.CreateShortcut( prefix + assigned[lnkname] );

where WinShell is another Windows API COM object, created via a function provided by the script host environment.

The complete program:

>MinGW\libexec\mingw-get\shlink.js

/*
 * shlink.js
 *
 * $Id: shlink.js,v 1.1 2012/04/06 22:49:36 keithmarshall Exp $
 *
 * Invoke the Windows Scripting Host to create or modify a windows shortcut,
 * with specified parameters.
 *
 * Usage:
 *   cscript -nologo shlink.js [--option[=value] ...] target name
 *
 * Options:
 *   --verbose  report shortcut parameter assignments on stdout
 *
 *   --all-users
 *          create the shortcut in the start menu, or on the
 *          desktop available to all users; (has no effect,
 *          unless --start-menu or --desktop is specified)
 *
 *   --desktop  create the shortcut on the desktop of the current
 *          user, (or for all users, with --all-users)
 *
 *   --start-menu
 *          create the shortcut in the user's start menu, (or
 *          all users' start menu, with --all-users)
 *
 *   --arguments="argument list ..."
 *      specify arguments to be passed to the command
 *      invoked by the shortcut
 *
 *   --description="text ..."
 *      specify the "tool tip" for the shortcut
 *
 *   --hotkey="[SHIFT+][CTRL+][ALT+]key"
 *          specify a hot-key combination which may be used
 *          to invoke the shortcut command
 *
 *   --icon="d:\path\to\icon\file[,index]"
 *          specify an icon to associate with the shortcut;
 *          if unspecified, the first icon in the target file,
 *          or failing that, a system default is used
 *
 *   --show=normal|maximised|minimised
 *          specify the style of window in which the program
 *          invoked by the shortcut will start running
 *
 *   --workingdir="d:\path\to\working\directory"
 *          specify the absolute path name for the directory
 *          which will become the current working directory when
 *          the program invoked by the shortcut is started
 *
 * Parameters:
 *   target the path name to the program to invoke, or the file
 *      to be opened, when the shortcut is activated.
 *
 *   name   the path name for the shortcut file itself; if this
 *      does not end with either a ".lnk" or ".url" extension,
 *      then ".lnk" will be appended, so creating a regular
 *      file system link type shortcut.
 *
 *
 * This file is a component of mingw-get.
 *
 * Written by Keith Marshall <keithmarshall@users.sourceforge.net>
 * Copyright (C) 2012, MinGW Project
 *
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 *
 */
var cmdName = WScript.ScriptName;
var WinShell = WScript.CreateObject("WScript.Shell");

/* Properties for the shortcut may be specified as arguments
 * to command line options, each of which is represented in the
 * GNU long option form, by keywords with property associations
 * as specified by:
 */
var options = Array(
      "name",       /*  File name/path of .lnk file */
      "all-users",
      "start-menu",
      "desktop",
      "verbose",
  /* 
   *  Option            Property
   *  -------------     -------------------------
   */ 
      "arguments",  /*  Shortcut.Arguments      */
      "description",    /*  Shortcut.Description    */
      "hotkey",     /*  Shortcut.HotKey     */
      "icon",       /*  Shortcut.IconLocation   */
      "show",       /*  Shortcut.WindowStyle    */
      "target",     /*  Shortcut.TargetPath     */
      "workingdir"  /*  Shortcut.WorkingDirectory   */
    );

/* The "complain" function provides a mechanism for writing
 * diagnostic messages to stderr, and optionally aborting the
 * calling script.
 */
function complain( condition, message )
{
  WScript.StdErr.WriteLine( cmdName + ": " + message );
  if( condition < 0 )
    WScript.Quit( -condition );
  return condition;
}

/* Strip ".js" suffix from command name.
 */
var i = cmdName.lastIndexOf( ".js" );
if( (i > 0) &&  (cmdName.substr( i ) == ".js") )
  cmdName = cmdName.substr( 0, i );

/* Initialise all options to "unassigned" state.
 */
var target = -1;
var lnkname = -1;
var desktop = -1;
var allusers = -1;
var startmenu = -1;
var verbosity = -1;
var unassigned = "+++unassigned+++";
var assigned = Array( options.length );
for( var k = 0; k < assigned.length; k++ )
{
  switch( options[k] )
  {
    case "name":
      lnkname = k;
      break;

    case "desktop":
      desktop = k;
      break;

    case "start-menu":
      startmenu = k;
      break;

    case "all-users":
      allusers = k;
      break;

    case "target":
      target = k;
      break;

    case "verbose":
      verbosity = k;
  }
  assigned[k] = unassigned;
}

/* Define the prefix, which will qualify the location
 * of the shortcut; initialise it to nothing, so that by
 * default shortcuts will be located by absolute path name,
 * as specified, or relative to current working directory.
 */
var prefix = "";

var j;
function assign_option( name, value )
{
  switch( name )
  {
    case "desktop":
    case "start-menu":
    case "all-users":
    case "verbose":
      j = i;
      return "set";
  }
  return value;
}

/* Parse the command line.
 */
var argv = WScript.Arguments;
for( i = 0; i < argv.length; i++ )
{
  if( argv( i ).indexOf( "--" ) == 0 )
  {
    /* Handle arguments specifying options...
     */
    if( (optind = argv( j = i ).indexOf( "=" ) + 1) > 3 )
    {
      optnam = argv( j ).substr( 2, optind - 3 );
      optarg = argv( j ).substr( optind );
    }
    else
    {
      optnam = argv( j ).substr( 2 );
      if( ++j < argv.length )
    optarg = argv( j );
      else
    optarg = unassigned;
    }

    var matched = 0;
    for( var k = 0; k < options.length; k++ )
    {
      if( optnam == options[k] )
      {
    matched = 1;
    assigned[k] = assign_option( options[k], optarg );
    k = options.length;
      }
      else if( options[k].indexOf(optnam) == 0 )
      {
    if( ++matched > 1 )
      complain( -2, cmdName + "option '" + argv( i ) + "' is ambiguous" );
    assigned[k] = assign_option( options[k], optarg );
      }
    }
    if( matched == 1 )
    {
      i = j;
    }
    else
      complain( -2, "option '" + argv( i ) + "' is not supported" );
  }

  /* Handle non-option arguments...
   */
  else if( (target >= 0) && (assigned[target] == unassigned) )
    /*
     * ...the first of which is the target to which the
     * (shortcut) link is to refer...
     */
    assigned[target] = argv( i );

  else if( (lnkname >= 0) && (assigned[lnkname] == unassigned) )
    /*
     * ...the second is the file system path name at which
     * the link file itself is to be saved...
     */
    assigned[lnkname] = argv( i );

  else
    /* ...and any more than two is an error.
     */
    complain( -2, "too many arguments" );
}

/* Verify that mandatory arguments have been specified...
 */
if( assigned[target] == unassigned )
  /*
   * First of these, the TargetPath specification, is required;
   * diagnose and abort, if missing.
   */
  complain( -2, "missing argument: no target path specified" );

if( assigned[lnkname] == unassigned )
  /*
   * Second, the path name for the link file itself is essential;
   * again, diagnose and abort if missing.
   */
  complain( -2, "missing argument: no shortcut name specified" );

/* We have both the mandatory arguments; check that the link name
 * is properly terminated by a ".lnk" or ".url" suffix...
 */
var suffix = "";
if( (i = assigned[lnkname].length - 4) > 0 )
  suffix = assigned[lnkname].substr( i );

/* ...and append ".lnk" as default, if not already present.
 */
if( (suffix != ".lnk") && (suffix != ".url") )
  assigned[lnkname] += ".lnk";

/* Add the appropriate prefix for '--desktop' or '--start-menu' shortcuts.
 */
if( assigned[desktop] != unassigned )
{
  if( assigned[startmenu] != unassigned )
    complain( -2, "options '--desktop' and '--start-menu' are incompatible" );
  else prefix = WinShell.SpecialFolders( (assigned[allusers] == unassigned)
      ? "Desktop" : "AllUsersDesktop" ) + "\\";
}
else if( assigned[startmenu] != unassigned )
  prefix = WinShell.SpecialFolders( (assigned[allusers] == unassigned)
      ? "StartMenu" : "AllUsersStartMenu" ) + "\\";

else if( assigned[allusers] != unassigned )
  complain( -2,
      "option '--all-users' also requires '--desktop' or '--start-menu'"
    );

/* Handle verbosity...
 */
function verbose_assignment( property, value )
{
  if( assigned[verbosity] == "set" )
    WScript.Echo( cmdName + ": set " + property + " = " + value );
}

/* Initialise the shortcut entity reference.
 */
if( assigned[verbosity] == "set" )
  WScript.Echo( cmdName + ": create shortcut: " + prefix + assigned[lnkname] );

var Shortcut = WinShell.CreateShortcut( prefix + assigned[lnkname] );

for( var k = 0; k < options.length; k++ )
{
  if( assigned[k] != unassigned )
    switch( options[k] )
    {
      case "arguments":
    verbose_assignment( "Shortcut.Arguments", assigned[k] );
    Shortcut.Arguments = assigned[k];
    break;

      case "description":
    verbose_assignment( "Shortcut.Description", assigned[k] );
    Shortcut.Description = assigned[k];
    break;

      case "hotkey":
    verbose_assignment( "Shortcut.HotKey", assigned[k] );
    Shortcut.HotKey = assigned[k];
    break;

      case "icon":
    verbose_assignment( "Shortcut.IconLocation", assigned[k] );
    Shortcut.IconLocation = assigned[k];
    break;

      case "show":
    verbose_assignment( "Shortcut.WindowStyle", assigned[k] );
        var style = Array(
        "normal",     1,
        "maximised",  3,
        "maximized",  3,
        "minimised",  7,
        "minimized",  7
      );
    for( j = 0; j < style.length; j++ )
      if( style[j++].indexOf( assigned[k] ) == 0 )
      {
        Shortcut.WindowStyle = style[j];
        j = 100;
      }
    if( j < 100 )
      complain( 1, "unrecognised mode '--show=" + assigned[k] + "' ignored" );
    break;

      case "target":
    verbose_assignment( "Shortcut.TargetPath", assigned[k] );
    Shortcut.TargetPath = assigned[k];
    break;

      case "workingdir":
    verbose_assignment( "Shortcut.WorkingDirectory", assigned[k] );
    Shortcut.WorkingDirectory = assigned[k];
    }
  else if( assigned[verbosity] == "set" )
    WScript.Echo( cmdName + ": option '--" + options[k] + "' is unassigned" );
}

/* Commit the shortcut entity to disk.
 */
Shortcut.Save();

/* $RCSfile: shlink.js,v $: end of file */