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Tip of the Day
Language: C++
Expertise: Beginner
May 1, 1997

A problem with a class Screen

Question:
I found on the Web a simple class Screen as follows:
 /*------------------------------------------------
 * s c r e e n . h
 *
 * A class to manage an ANSI text screen. Allows moving to
 * an x, y location and clearing the screen.
 * 
 * Copyright 1996 by Interface Technologies, Inc.  * All Rights Reserved.
 *----------------------------------------------  */

#ifndef __SCREEN_H
#define __SCREEN_H

#include 


class Screen
{
public:
    Screen( )
    {
    }

    void GotoXY( int x, int y )
    {
        cout << "\033[" << y
            << ";" << x << "H";
    }

    void CharXY( int x, int y, char c)
    {
        GotoXY( x, y );
        cout << c;
    }

    void Clear()
    {
        cout << "\033[2J";
    }
};

#endif
Then I wrote a little test:
#include "screen.h"

void main()
{

     Screen s;
     s.CharXY(10,40,'A');
}
It runs (on MS-DOS), but 'A' is displayed on the very first position of the screen.

Can you explain the implementation of GotoXY() and why the class does not work?

Answer:
As the comment in the class says, it manages ANSI screens, so it does not work on the DOS platform.

ANSI terminals understand special chars which when printed to them, cause the terminal to treat it as a command rather than print it out. This program uses these special chars to achieve the same. There is a file in DOS called ANSI.SYS that you can load from the config.sys file by saying device=ansi.sys, and that makes the shell understand ANSI control chars.

Also, your argument to CharXY may need to be in range with the terminal. Remember, the first argument is the column and the second is the row.

Hope that helps.

DevX Pro
 
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