forward declarations give errors + I want some explanation

[SweetOrange]

Hi I am using the find example from "C++ Gui programming with Qt" book chapter 2.
There is the file finddialog.h :

#ifndef FINDDIALOG_H
#define FINDDIALOG_H

#include <QDialog>

class QCheckBox;
class QLabel;
class QLineEdit;
class QPushButton;

class FindDialog : public QDialog
{
    Q_OBJECT

public:
    FindDialog(QWidget *parent = 0);

signals:
    void findNext(const QString &str, Qt::CaseSensitivity cs);
    void findPrevious(const QString &str, Qt::CaseSensitivity cs);

private slots:
    void findClicked();
    void enableFindButton(const QString &text);

private:
    QLabel *label;
    QLineEdit *lineEdit;
    QCheckBox *caseCheckBox;
    QCheckBox *backwardCheckBox;
    QPushButton *findButton;
    QPushButton *closeButton;
};

#endif

I get errors with the class forward declarations at beginning of code.
http://s13.postimg.org/jilgxzcsn/1_20_2016a.jpg

I have included in project file the line

QT      += widgets

finddialog.cpp:

#include <QtGui>

#include "finddialog.h"

FindDialog::FindDialog(QWidget *parent)
    : QDialog(parent)
{
    label = new QLabel(tr("Find &what:"));
    lineEdit = new QLineEdit;
    label->setBuddy(lineEdit);

    caseCheckBox = new QCheckBox(tr("Match &case"));
    backwardCheckBox = new QCheckBox(tr("Search &backward"));

    findButton = new QPushButton(tr("&Find"));
    findButton->setDefault(true);
    findButton->setEnabled(false);

    closeButton = new QPushButton(tr("Close"));

    connect(lineEdit, SIGNAL(textChanged(const QString &)),
            this, SLOT(enableFindButton(const QString &)));
    connect(findButton, SIGNAL(clicked()),
            this, SLOT(findClicked()));
    connect(closeButton, SIGNAL(clicked()),
            this, SLOT(close()));

    QHBoxLayout *topLeftLayout = new QHBoxLayout;
    topLeftLayout->addWidget(label);
    topLeftLayout->addWidget(lineEdit);

    QVBoxLayout *leftLayout = new QVBoxLayout;
    leftLayout->addLayout(topLeftLayout);
    leftLayout->addWidget(caseCheckBox);
    leftLayout->addWidget(backwardCheckBox);

    QVBoxLayout *rightLayout = new QVBoxLayout;
    rightLayout->addWidget(findButton);
    rightLayout->addWidget(closeButton);
    rightLayout->addStretch();

    QHBoxLayout *mainLayout = new QHBoxLayout;
    mainLayout->addLayout(leftLayout);
    mainLayout->addLayout(rightLayout);
    setLayout(mainLayout);

    setWindowTitle(tr("Find"));
    setFixedHeight(sizeHint().height());
}

void FindDialog::findClicked()
{
    QString text = lineEdit->text();
    Qt::CaseSensitivity cs =
            caseCheckBox->isChecked() ? Qt::CaseSensitive
                                      : Qt::CaseInsensitive;
    if (backwardCheckBox->isChecked()) {
        emit findPrevious(text, cs);
    } else {
        emit findNext(text, cs);
    }
}

void FindDialog::enableFindButton(const QString &text)
{
    findButton->setEnabled(!text.isEmpty());
}

main.cpp:

#include <QApplication>

#include "finddialog.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    FindDialog *dialog = new FindDialog;
    dialog->show();
    return app.exec();
}

find.pro:

QT      += widgets

TEMPLATE      = app
HEADERS       = finddialog.h
SOURCES       = finddialog.cpp \
                main.cpp

[mrjj]

hi
when you say
class QLabel;
its ok to use it for pointers.

QLabel *myname;

But if you start to use ANYTHING in it
like
myname->setBuddy

then a forward is NOT enough and u must include the include file for it
#include <QLabel>

So that is what incomplete class error means.

as you only tell it the name. and if u use more than name, you must give compiler the full class definition.

[ValentinMichelet]

In addition to what @mrjj said, it's a good practice to use forward declaration in your .h file, but you have to add #include <QLabel> in your .cpp for the reasons mrjj gave you. Indeed, the compiler is complaining about your .cpp file, not your .h file.

But for instance, if you don't use a pointer for your variable, or if you inherit from an other class, then you have to #include it, since the compiler needs to know extra information about the content of the class.

[SweetOrange]

@mrjj
I added the #includes at finddialog.h:

#ifndef FINDDIALOG_H
#define FINDDIALOG_H

#include <QDialog>
#include <QCheckBox>
#include <QLabel>
#include <QLineEdit>
#include <QPushButton>

class QCheckBox;
class QLabel;
class QLineEdit;
class QPushButton;

class FindDialog : public QDialog
{
    Q_OBJECT

public:
    FindDialog(QWidget *parent = 0);

signals:
    void findNext(const QString &str, Qt::CaseSensitivity cs);
    void findPrevious(const QString &str, Qt::CaseSensitivity cs);

private slots:
    void findClicked();
    void enableFindButton(const QString &text);

private:
    QLabel *label;
    QLineEdit *lineEdit;
    QCheckBox *caseCheckBox;
    QCheckBox *backwardCheckBox;
    QPushButton *findButton;
    QPushButton *closeButton;
};

#endif

Do I need the forward declarations anymore?

Also I get the errors:
http://s11.postimg.org/4izv8mff7/1_20_2016b.jpg
which I suspect that I haven't included the include files for QHBoxLayout and QVBoxLayout.
In which file do I include these?

[ValentinMichelet]

@SweetOrange

Look at the file on the right of the error message, you'll know where to #include what the compiler lacks.

[mrjj]

Hi in 99% of all cases the include has same name as class.
#include <QVBoxLayout>
and no, no need for forward when u use the real include.

[ValentinMichelet]

Maybe it's a bit technical, but here is a link on Stack Overflow explaining what a forward declaration is:
http://stackoverflow.com/questions/553682/when-can-i-use-a-forward-declaration

Basically, a forward declaration is only a "Don't worry compiler, this class exists." statement. As soon as the implementation of the class is required, the #include is mendatory.

The question of "should I use #include or forward declaration here?" is off topic tho.

[SweetOrange]

Thanks for the info.

#ifndef FINDDIALOG_H
#define FINDDIALOG_H

#include <QDialog>
#include <QLabel>
#include <QLineEdit>
#include <QCheckBox>
#include <QPushButton>

//class QCheckBox;
//class QLabel;
//class QLineEdit;
//class QPushButton;

class FindDialog : public QDialog
{
    Q_OBJECT

public:
    FindDialog(QWidget *parent = 0);

signals:
    void findNext(const QString &str, Qt::CaseSensitivity cs);
    void findPrevious(const QString &str, Qt::CaseSensitivity cs);

private slots:
    void findClicked();
    void enableFindButton(const QString &text);

private:
    QLabel *label;
    QLineEdit *lineEdit;
    QCheckBox *caseCheckBox;
    QCheckBox *backwardCheckBox;
    QPushButton *findButton;
    QPushButton *closeButton;
};

#endif

#include <QtGui>

#include "finddialog.h"
#include <QHBoxLayout>
#include <QVBoxLayout>






FindDialog::FindDialog(QWidget *parent)
    : QDialog(parent)
{
    label = new QLabel(tr("Find &what:"));
    lineEdit = new QLineEdit;
    label->setBuddy(lineEdit);

    caseCheckBox = new QCheckBox(tr("Match &case"));
    backwardCheckBox = new QCheckBox(tr("Search &backward"));

    findButton = new QPushButton(tr("&Find"));
    findButton->setDefault(true);
    findButton->setEnabled(false);

    closeButton = new QPushButton(tr("Close"));

    connect(lineEdit, SIGNAL(textChanged(const QString &)),
            this, SLOT(enableFindButton(const QString &)));
    connect(findButton, SIGNAL(clicked()),
            this, SLOT(findClicked()));
    connect(closeButton, SIGNAL(clicked()),
            this, SLOT(close()));

    QHBoxLayout *topLeftLayout = new QHBoxLayout;
    topLeftLayout->addWidget(label);
    topLeftLayout->addWidget(lineEdit);

    QVBoxLayout *leftLayout = new QVBoxLayout;
    leftLayout->addLayout(topLeftLayout);
    leftLayout->addWidget(caseCheckBox);
    leftLayout->addWidget(backwardCheckBox);

    QVBoxLayout *rightLayout = new QVBoxLayout;
    rightLayout->addWidget(findButton);
    rightLayout->addWidget(closeButton);
    rightLayout->addStretch();

    QHBoxLayout *mainLayout = new QHBoxLayout;
    mainLayout->addLayout(leftLayout);
    mainLayout->addLayout(rightLayout);
    setLayout(mainLayout);

    setWindowTitle(tr("Find"));
    setFixedHeight(sizeHint().height());
}

void FindDialog::findClicked()
{
    QString text = lineEdit->text();
    Qt::CaseSensitivity cs =
            caseCheckBox->isChecked() ? Qt::CaseSensitive
                                      : Qt::CaseInsensitive;
    if (backwardCheckBox->isChecked()) {
        emit findPrevious(text, cs);
    } else {
        emit findNext(text, cs);
    }
}

void FindDialog::enableFindButton(const QString &text)
{
    findButton->setEnabled(!text.isEmpty());
}

#include <QApplication>

#include "finddialog.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    FindDialog *dialog = new FindDialog;
    dialog->show();
    return app.exec();
}

Now works and compiles properly and runs.

Some explanations though that I found the book lacks. What does the header of the constructor means in finddialog.cpp ?

FindDialog::FindDialog(QWidget *parent)
    : QDialog(parent)

This is class inheritance as far as I understand it

[mrjj]

Hi
FindDialog::FindDialog(QWidget *parent)
: QDialog(parent) <---- here we call base class constructor with the parent.

This is need to setup the part of the class we get from the base class.

if u have
class A :B

then some of A is from B and it might need to do stuff in its constructor.
so we call it also.

[SweetOrange]

Tell me if I am wrong...

QDialog is derived from QWidget.

The FindDialog constructor is typical of Qt widget classes. The parent parameter specifies the parent widget. The default is a null pointer meaning that the dialog has no parent

when we have

FindDialog::FindDialog(QWidget *parent)
   : QDialog(parent)

we just say that QDialog is the parent of FindDialog and as mentioned above QWidget is the parent of QDialog.

[ValentinMichelet]

@SweetOrange

Just to be sure that you don't mix up "C++ parent class" and "Qt parent mechanism".

In pure C++, a class can inherit from a parent class:

class QDialog: public QWidget {}

Here the QDialog inherits from QWidget.

In Qt, there is a mechanism that makes your life easier for memory managment. You give your class a parent to let it handle the memory in destruction. If you code in C++, you may know that there is no garbage collector as there is in other languages (Java for instance). So as soon as you create an object dynamically - on the heap - using "new" operator, you are responsible for it's destruction using "delete". Qt enables you not to worry about deletion: give your object a parent, and the memory will be handle at its destruction.

FindDialog::FindDialog(QWidget *parent): QDialog(parent)

Here we call the parent constructor (QDialog) with the parent parameter and we let Qt handle destruction, as well as a provide a way to potential future inherited classes to enjoy this mechanism.

[SGaist]

Hi,

To add to my fellows, you should move:

#include <QDialog>
#include <QLabel>
#include <QLineEdit>
#include <QCheckBox>
#include <QPushButton>

in your implementation file. That's were they are needed in your case.

Note that you are using #include <QtGui>, while good for rapid prototyping it's a module wide include which means it will pull in all headers from said module thus your compilation time will increase since all files are going to be parsed.