I'm substituting a Perl sender script for your C++ program here, but this is the idea. You may need to use sysread, instead of the <> operator, there may be blocking problems in the pipes on win32 platforms, and error checking is needed. P.S. To control your C++ program, print to the STDIN filehandle.... i didn't include that feature here because I'm lazy and didn't want to add listener code into the sender script. But look at IPC3 buffer limit problem for a non-gui full control over a script.

sender_program script ( like your C++ program)

#!/usr/bin/perl
use warnings;
use strict;

$| = 1;
my $count = 0;
while(1){
 $count++;
 print "$count\n";
 warn "\tuh oh warning $count\n";
 sleep 1;
}
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and the Tk reader script

#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;
use Tk;
use Tk::ProgressBar;

my $mw = new MainWindow;
$mw->geometry("600x400");
my $tframe = $mw->Frame()->pack();

my $count = 0;
my $l1 = $tframe->Label(-text => '%',
               -bg       => 'black',
               -fg       => 'green',
               )->pack(-side => 'left');
       
my $l2 = $tframe->Label(
                 -text => $count,    
                # -text option better here, than textvariable which
                # may cause unicode display problem
                #-textvariable => \$count,  
                #may cause unicode display problem over a socket
                 
                -bg       => 'black',
               -fg       => 'green',
               -width => 3,   
           )->pack(-side => 'left');

my $p = $tframe->ProgressBar(
 -troughcolor => 'black',
    -fg       => 'lightgreen',
    -blocks   => 1,
    -width    => 20,
    -length   => 200,
    -from     => 0,
    -to       => 100,
    -variable => \$count,
)->pack(-side =>'right');

my $tout = $mw->Scrolled(
    'Text',
    -foreground => 'white',
    -background => 'black',
    -width  => 80,
    -height => 20,
                 )->pack;

$tout->tagConfigure( 'red',   -foreground => 'red' );

my $pid = open3( 0, \*OUT, \*ERR, "sender_program" );
#the 0 is for ignoring \*IN  (STDIN)
$mw->fileevent( \*OUT, 'readable', \&write_out );
$mw->fileevent( \*ERR, 'readable', \&write_err );

MainLoop;
###############################################
sub write_out {
    $count = <OUT>;
    chomp $count;
#    eval $count;   #prevents raw unicode, you may not need it
#    print "$count\n";
    $l2->configure( '-text' => $count);  # works better than textvaria
+ble
    $tout->insert( "end", "$count\n" );
    $tout->see("end");
}
############################################
sub write_err {
    my $str = <ERR>;
    $tout->insert( "end", "\t$str", 'red' );
    $tout->see("end");
}
__END__
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