Can someone explain this code from C++ GUI Programming with Qt 4?

[deejross]

I am reading the C++ GUI Programming with Qt4 2nd Edition book trying to learn Qt. I am completely new to Qt and C++. I have experience with C#, Java, VB, Python, and PHP, but C++ is just so new right now that I'm having trouble figuring out how stuff works. The worst part is I don't know how to Google something like this. Anyways, to my question...

I'm on Chapter 5, Creating Custom Widgets where you subclass QSpinBox in order to create HexSpinBox. This is the code I am referring to:

#ifndef HEXSPINBOX_H
#define HEXSPINBOX_H
 
#include <QSpinBox>
 
class QRegExpValidator;
 
class HexSpinBox : public QSpinBox
{
    Q_OBJECT
public:
    HexSpinBox(QWidget *parent = 0);
 
protected:
    QValidator::State validate(QString &text, int &pos) const;
    int valueFromText(const QString &text) const;
    QString textFromValue(int value) const;
 
private:
    QRegExpValidator *validator;
};
 
#endif

#ifndef HEXSPINBOX_H
#define HEXSPINBOX_H

#include <QSpinBox>

class QRegExpValidator;

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

protected:
    QValidator::State validate(QString &text, int &pos) const;
    int valueFromText(const QString &text) const;
    QString textFromValue(int value) const;

private:
    QRegExpValidator *validator;
};

#endif

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My question is about the line

class QRegExpValidator;

class QRegExpValidator;

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What does this line do? Why am I not doing something like a

#include <QRegExpValidator>

#include <QRegExpValidator>

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instead? This is probably a trivial thing for someone with more experience, so I'm sorry if this is a stupid question. I'm sure I'll have several more simple questions as I keep reading.