Question

This is an implementation of the Tic-tac-toe game.

We didn't put much effort in making a clever algorithm so it's not a challenge to play against the computer. Instead, study the source code to see how you can make reusable components such as the TicTacGameBoard widget.

---

Header file:

/****************************************************************************
** $Id:  qt/tictac.h   3.0.5   edited Oct 12 2001 $
**
** Copyright (C) 1992-2000 Trolltech AS.  All rights reserved.
**
** This file is part of an example program for Qt.  This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/

#ifndef TICTAC_H
#define TICTAC_H


#include <qpushbutton.h>
#include <qptrvector.h>

class QComboBox;
class QLabel;


// --------------------------------------------------------------------------
// TicTacButton implements a single tic-tac-toe button
//

class TicTacButton : public QPushButton
{
    Q_OBJECT
public:
    TicTacButton( QWidget *parent );
    enum Type { Blank, Circle, Cross };
    Type        type() const            { return t; }
    void        setType( Type type )    { t = type; repaint(); }
    QSizePolicy sizePolicy() const
    { return QSizePolicy( QSizePolicy::Preferred, QSizePolicy::Preferred ); }
    QSize sizeHint() const { return QSize( 32, 32 ); }
    QSize minimumSizeHint() const { return QSize( 10, 10 ); }
protected:
    void        drawButtonLabel( QPainter * );
private:
    Type t;
};

// Using template vector to make vector-class of TicTacButton.
// This vector is used by the TicTacGameBoard class defined below.

typedef QPtrVector<TicTacButton>        TicTacButtons;
typedef QMemArray<int>          TicTacArray;


// --------------------------------------------------------------------------
// TicTacGameBoard implements the tic-tac-toe game board.
// TicTacGameBoard is a composite widget that contains N x N TicTacButtons.
// N is specified in the constructor.
//

class TicTacGameBoard : public QWidget
{
    Q_OBJECT
public:
    TicTacGameBoard( int n, QWidget *parent=0, const char *name=0 );
   ~TicTacGameBoard();
    enum        State { Init, HumansTurn, HumanWon, ComputerWon, NobodyWon };
    State       state() const           { return st; }
    void        computerStarts( bool v );
    void        newGame();
signals:
    void        finished();                     // game finished
private slots:
    void        buttonClicked();
private:
    void        setState( State state ) { st = state; }
    void        updateButtons();
    int checkBoard( TicTacArray * );
    void        computerMove();
    State       st;
    int         nBoard;
    bool        comp_starts;
    TicTacArray *btArray;
    TicTacButtons *buttons;
};


// --------------------------------------------------------------------------
// TicTacToe implements the complete game.
// TicTacToe is a composite widget that contains a TicTacGameBoard and
// two push buttons for starting the game and quitting.
//

class TicTacToe : public QWidget
{
    Q_OBJECT
public:
    TicTacToe( int boardSize=3, QWidget *parent=0, const char *name=0 );
private slots:
    void        newGameClicked();
    void        gameOver();
private:
    void        newState();
    QComboBox   *whoStarts;
    QPushButton *newGame;
    QPushButton *quit;
    QLabel      *message;
    TicTacGameBoard *board;
};


#endif // TICTAC_H

---

Implementation:

/****************************************************************************
** $Id:  qt/tictac.cpp   3.0.5   edited Nov 14 2001 $
**
** Copyright (C) 1992-2000 Trolltech AS.  All rights reserved.
**
** This file is part of an example program for Qt.  This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/

#include "tictac.h"
#include <qapplication.h>
#include <qpainter.h>
#include <qdrawutil.h>
#include <qcombobox.h>
#include <qcheckbox.h>
#include <qlabel.h>
#include <qlayout.h>
#include <stdlib.h>                             // rand() function
#include <qdatetime.h>                          // seed for rand()


//***************************************************************************
//* TicTacButton member functions
//***************************************************************************

// --------------------------------------------------------------------------
// Creates a TicTacButton
//

TicTacButton::TicTacButton( QWidget *parent ) : QPushButton( parent )
{
    t = Blank;                                  // initial type
}

// --------------------------------------------------------------------------
// Paints TicTacButton
//

void TicTacButton::drawButtonLabel( QPainter *p )
{
    QRect r = rect();
    p->setPen( QPen( white,2 ) );               // set fat pen
    if ( t == Circle ) {
        p->drawEllipse( r.left()+4, r.top()+4, r.width()-8, r.height()-8 );
    } else if ( t == Cross ) {                  // draw cross
        p->drawLine( r.topLeft()   +QPoint(4,4), r.bottomRight()-QPoint(4,4));
        p->drawLine( r.bottomLeft()+QPoint(4,-4),r.topRight()   -QPoint(4,-4));
    }
}


//***************************************************************************
//* TicTacGameBoard member functions
//***************************************************************************

// --------------------------------------------------------------------------
// Creates a game board with N x N buttons and connects the "clicked()"
// signal of all buttons to the "buttonClicked()" slot.
//

TicTacGameBoard::TicTacGameBoard( int n, QWidget *parent, const char *name )
    : QWidget( parent, name )
{
    st = Init;                                  // initial state
    nBoard = n;
    n *= n;                                     // make square
    comp_starts = FALSE;                        // human starts
    buttons = new TicTacButtons(n);             // create real buttons
    btArray = new TicTacArray(n);               // create button model
    QGridLayout * grid = new QGridLayout( this, nBoard, nBoard, 4 );
    QPalette p( blue );
    for ( int i=0; i<n; i++ ) {                 // create and connect buttons
        TicTacButton *ttb = new TicTacButton( this );
        ttb->setPalette( p );
        ttb->setEnabled( FALSE );
        connect( ttb, SIGNAL(clicked()), SLOT(buttonClicked()) );
        grid->addWidget( ttb, i%nBoard, i/nBoard );
        buttons->insert( i, ttb );
        btArray->at(i) = TicTacButton::Blank;   // initial button type
    }
    QTime t = QTime::currentTime();             // set random seed
    srand( t.hour()*12+t.minute()*60+t.second()*60 );
}

TicTacGameBoard::~TicTacGameBoard()
{
    delete buttons;
    delete btArray;
}


// --------------------------------------------------------------------------
// TicTacGameBoard::computerStarts( bool v )
//
// Computer starts if v=TRUE. The human starts by default.
//

void TicTacGameBoard::computerStarts( bool v )
{
    comp_starts = v;
}


// --------------------------------------------------------------------------
// TicTacGameBoard::newGame()
//
// Clears the game board and prepares for a new game
//

void TicTacGameBoard::newGame()
{
    st = HumansTurn;
    for ( int i=0; i<nBoard*nBoard; i++ )
        btArray->at(i) = TicTacButton::Blank;
    if ( comp_starts )
        computerMove();
    else
        updateButtons();
}


// --------------------------------------------------------------------------
// TicTacGameBoard::buttonClicked()             - SLOT
//
// This slot is activated when a TicTacButton emits the signal "clicked()",
// i.e. the user has clicked on a TicTacButton.
//

void TicTacGameBoard::buttonClicked()
{
    if ( st != HumansTurn )                     // not ready
        return;
    int i = buttons->findRef( (TicTacButton*)sender() );
    TicTacButton *b = buttons->at(i);           // get piece that was pressed
    if ( b->type() == TicTacButton::Blank ) {   // empty piece?
        btArray->at(i) = TicTacButton::Circle;
        updateButtons();
        if ( checkBoard( btArray ) == 0 )       // not a winning move?
            computerMove();
        int s = checkBoard( btArray );
        if ( s ) {                              // any winners yet?
            st = s == TicTacButton::Circle ? HumanWon : ComputerWon;
            emit finished();
        }
    }
}


// --------------------------------------------------------------------------
// TicTacGameBoard::updateButtons()
//
// Updates all buttons that have changed state
//

void TicTacGameBoard::updateButtons()
{
    for ( int i=0; i<nBoard*nBoard; i++ ) {
        if ( buttons->at(i)->type() != btArray->at(i) )
            buttons->at(i)->setType( (TicTacButton::Type)btArray->at(i) );
        buttons->at(i)->setEnabled( buttons->at(i)->type() ==
                                    TicTacButton::Blank );
    }
}


// --------------------------------------------------------------------------
// TicTacGameBoard::checkBoard()
//
// Checks if one of the players won the game, works for any board size.
//
// Returns:
//  - TicTacButton::Cross  if the player with X buttons won
//  - TicTacButton::Circle if the player with O buttons won
//  - Zero (0) if there is no winner yet
//

int TicTacGameBoard::checkBoard( TicTacArray *a )
{
    int  t = 0;
    int  row, col;
    bool won = FALSE;
    for ( row=0; row<nBoard && !won; row++ ) {  // check horizontal
        t = a->at(row*nBoard);
        if ( t == TicTacButton::Blank )
            continue;
        col = 1;
        while ( col<nBoard && a->at(row*nBoard+col) == t )
            col++;
        if ( col == nBoard )
            won = TRUE;
    }
    for ( col=0; col<nBoard && !won; col++ ) {  // check vertical
        t = a->at(col);
        if ( t == TicTacButton::Blank )
            continue;
        row = 1;
        while ( row<nBoard && a->at(row*nBoard+col) == t )
            row++;
        if ( row == nBoard )
            won = TRUE;
    }
    if ( !won ) {                               // check diagonal top left
        t = a->at(0);                           //   to bottom right
        if ( t != TicTacButton::Blank ) {
            int i = 1;
            while ( i<nBoard && a->at(i*nBoard+i) == t )
                i++;
            if ( i == nBoard )
                won = TRUE;
        }
    }
    if ( !won ) {                               // check diagonal bottom left
        int j = nBoard-1;                       //   to top right
        int i = 0;
        t = a->at(i+j*nBoard);
        if ( t != TicTacButton::Blank ) {
            i++; j--;
            while ( i<nBoard && a->at(i+j*nBoard) == t ) {
                i++; j--;
            }
            if ( i == nBoard )
                won = TRUE;
        }
    }
    if ( !won )                                 // no winner
        t = 0;
    return t;
}


// --------------------------------------------------------------------------
// TicTacGameBoard::computerMove()
//
// Puts a piece on the game board. Very, very simple.
//

void TicTacGameBoard::computerMove()
{
    int numButtons = nBoard*nBoard;
    int *altv = new int[numButtons];            // buttons alternatives
    int altc = 0;
    int stopHuman = -1;
    TicTacArray a = btArray->copy();
    int i;
    for ( i=0; i<numButtons; i++ ) {            // try all positions
        if ( a[i] != TicTacButton::Blank )      // already a piece there
            continue;
        a[i] = TicTacButton::Cross;             // test if computer wins
        if ( checkBoard(&a) == a[i] ) {         // computer will win
            st = ComputerWon;
            stopHuman = -1;
            break;
        }
        a[i] = TicTacButton::Circle;            // test if human wins
        if ( checkBoard(&a) == a[i] ) {         // oops...
            stopHuman = i;                      // remember position
            a[i] = TicTacButton::Blank;         // restore button
            continue;                           // computer still might win
        }
        a[i] = TicTacButton::Blank;             // restore button
        altv[altc++] = i;                       // remember alternative
    }
    if ( stopHuman >= 0 )                       // must stop human from winning
        a[stopHuman] = TicTacButton::Cross;
    else if ( i == numButtons ) {               // tried all alternatives
        if ( altc > 0 )                         // set random piece
            a[altv[rand()%(altc--)]] = TicTacButton::Cross;
        if ( altc == 0 ) {                      // no more blanks
            st = NobodyWon;
            emit finished();
        }
    }
    *btArray = a;                               // update model
    updateButtons();                            // update buttons
    delete[] altv;
}


//***************************************************************************
//* TicTacToe member functions
//***************************************************************************

// --------------------------------------------------------------------------
// Creates a game widget with a game board and two push buttons, and connects
// signals of child widgets to slots.
//

TicTacToe::TicTacToe( int boardSize, QWidget *parent, const char *name )
    : QWidget( parent, name )
{
    QVBoxLayout * l = new QVBoxLayout( this, 6 );

    // Create a message label

    message = new QLabel( this );
    message->setFrameStyle( QFrame::WinPanel | QFrame::Sunken );
    message->setAlignment( AlignCenter );
    l->addWidget( message );

    // Create the game board and connect the signal finished() to this
    // gameOver() slot

    board = new TicTacGameBoard( boardSize, this );
    connect( board, SIGNAL(finished()), SLOT(gameOver()) );
    l->addWidget( board );

    // Create a horizontal frame line

    QFrame *line = new QFrame( this );
    line->setFrameStyle( QFrame::HLine | QFrame::Sunken );
    l->addWidget( line );

    // Create the combo box for deciding who should start, and
    // connect its clicked() signals to the buttonClicked() slot

    whoStarts = new QComboBox( this );
    whoStarts->insertItem( "Computer starts" );
    whoStarts->insertItem( "Human starts" );
    l->addWidget( whoStarts );

    // Create the push buttons and connect their clicked() signals
    // to this right slots.

    newGame = new QPushButton( "Play!", this );
    connect( newGame, SIGNAL(clicked()), SLOT(newGameClicked()) );
    quit = new QPushButton( "Quit", this );
    connect( quit, SIGNAL(clicked()), qApp, SLOT(quit()) );
    QHBoxLayout * b = new QHBoxLayout;
    l->addLayout( b );
    b->addWidget( newGame );
    b->addWidget( quit );

    newState();
}


// --------------------------------------------------------------------------
// TicTacToe::newGameClicked()                  - SLOT
//
// This slot is activated when the new game button is clicked.
//

void TicTacToe::newGameClicked()
{
    board->computerStarts( whoStarts->currentItem() == 0 );
    board->newGame();
    newState();
}


// --------------------------------------------------------------------------
// TicTacToe::gameOver()                        - SLOT
//
// This slot is activated when the TicTacGameBoard emits the signal
// "finished()", i.e. when a player has won or when it is a draw.
//

void TicTacToe::gameOver()
{
    newState();                                 // update text box
}


// --------------------------------------------------------------------------
// Updates the message to reflect a new state.
//

void TicTacToe::newState()
{
    static const char *msg[] = {                // TicTacGameBoard::State texts
        "Click Play to start", "Make your move",
        "You won!", "Computer won!", "It's a draw" };
    message->setText( msg[board->state()] );
    return;
}

---

Main:

/****************************************************************************
** $Id:  qt/main.cpp   3.0.5   edited Oct 12 2001 $
**
** Copyright (C) 1992-2000 Trolltech AS.  All rights reserved.
**
** This file is part of an example program for Qt.  This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/

#include <qapplication.h>
#include <stdlib.h>
#include "tictac.h"


int main( int argc, char **argv )
{
    QApplication a( argc, argv );
    int n = 3;
    if ( argc == 2 )                            // get board size n
        n = atoi(argv[1]);
    if ( n < 3 || n > 10 ) {                    // out of range
        qWarning( "%s: Board size must be from 3x3 to 10x10", argv[0] );
        return 1;
    }
    TicTacToe ttt( n );                         // create game
    a.setMainWidget( &ttt );
    ttt.setCaption("Qt Example - TicTac");
    ttt.show();                                 // show widget
    return a.exec();                            // go
}

See also Examples.