for declaration of struct QToolButton

[saman_artorious]

why do I receive error when I do not #include for example QToolButton , but rather go for its forward class declaration?

#ifndef PLAYVIDEO_H
#define PLAYVIDEO_H
 
#include <QWidget>
 
 
class QToolButton;
class QGridLayout;
class QHBoxLayout;
class QVBoxLayout;
 
 
class PlayVideo : public QWidget
{
    Q_OBJECT
public:
    PlayVideo(QWidget *parent = 0);
 
private:
    void createButtons();
    void createControls();
 
 
    QToolButton *playButton;
    QToolButton *pauseButton;
    QToolButton *stopButton;
 
    QGridLayout *controlsLayout;
    QHBoxLayout *buttonsLayout;
    QVBoxLayout *mainLayout;
 
};
 
#endif // PLAYVIDEO_H

#ifndef PLAYVIDEO_H
#define PLAYVIDEO_H

#include <QWidget>


class QToolButton;
class QGridLayout;
class QHBoxLayout;
class QVBoxLayout;


class PlayVideo : public QWidget
{
    Q_OBJECT
public:
    PlayVideo(QWidget *parent = 0);

private:
    void createButtons();
    void createControls();


    QToolButton *playButton;
    QToolButton *pauseButton;
    QToolButton *stopButton;

    QGridLayout *controlsLayout;
    QHBoxLayout *buttonsLayout;
    QVBoxLayout *mainLayout;

};

#endif // PLAYVIDEO_H

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error: forward declaration of ‘struct QToolButton’