Basic question on new and delete

[GreyGeek]

...snip...
Where QObject really helps out is when you create a GUI. A toplevel window, say derived from QMainWindow, may contain a myriad of other subwidgets within itself (Consider a complex gui that you have created in Designer). When you intsantiate your main window widget, you may do it like this:

MyMainWindow* mw = new MyMainWindow(0);// note null parent

MyMainWindow* mw = new MyMainWindow(0);// note null parent

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Within the MyMainWindow constructor, there will be code that creates and arranges many child widgets of the main window.


When you delete the mainwindow, you would do so like this

delete mw;

delete mw;

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You don't worry about deleting all the pushbuttons, listboxes, labels etc that are within your main window. Why? because QObject takes care of it with its parent-child model.

Well, here is the main function, from my main.cpp, from my Homestead application:

int main( int argc, char * argv[] ) {
	QString strRejected = "";
	QApplication app(argc, argv);
	app.setQuitOnLastWindowClosed(false);
	dlgLogin dlg; 
	if( dlg.exec() == QDialog::Accepted ){
		QSqlDatabase hapdb = QSqlDatabase::addDatabase(DBDRIVER);
		hapdb.setHostName(DBHOST);
		hapdb.setDatabaseName(DBNAME);
		hapdb.setUserName(dlg.dui.leUserName->text());
		hapdb.setPassword(dlg.dui.leUserPassword->text());
		if ( hapdb.open() ) {
			homestead ht;
			ht.RevID = dlg.dui.leUserName->text();
			ht.show();
			app.setQuitOnLastWindowClosed(true);
			return app.exec();
		} else { 
			strRejected = QString("The Login was rejected because: %1").arg(hapdb.lastError().text()).toLatin1();
			QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
			return 1;
		}
	} else {
		strRejected = QString("User Canceled the login!").toLatin1();
		QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
		return 2;
	}
}

int main( int argc, char * argv[] ) {
	QString strRejected = "";
	QApplication app(argc, argv);
	app.setQuitOnLastWindowClosed(false);
	dlgLogin dlg; 
	if( dlg.exec() == QDialog::Accepted ){
		QSqlDatabase hapdb = QSqlDatabase::addDatabase(DBDRIVER);
		hapdb.setHostName(DBHOST);
		hapdb.setDatabaseName(DBNAME);
		hapdb.setUserName(dlg.dui.leUserName->text());
		hapdb.setPassword(dlg.dui.leUserPassword->text());
		if ( hapdb.open() ) {
			homestead ht;
			ht.RevID = dlg.dui.leUserName->text();
			ht.show();
			app.setQuitOnLastWindowClosed(true);
			return app.exec();
		} else { 
			strRejected = QString("The Login was rejected because: %1").arg(hapdb.lastError().text()).toLatin1();
			QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
			return 1;
		}
	} else {
		strRejected = QString("User Canceled the login!").toLatin1();
		QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
		return 2;
	}
}

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As I understand your comments, since "dlg" is called only once, and that call is in this function, its memory will be recovered when main goes out of scope (i.e., the app closes). And, hapdb is also instantiated only once so its memory, too, will be reclaimed when the app closes. The ht object, where Homestead really operates, is also instantiated only once and like the others, the memory it uses is returned to the OS when the app closes. So, I don't need as the last three lines of main():

delete ht;
delete hapdb;
delete dlg;

delete ht;
delete hapdb;
delete dlg;

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because none of these objects leak memory, even though dlg has a parent:

class dlgLogin : public QDialog
{
    Q_OBJECT
public:
    Ui::dlgLoginUi dui;
    dlgLogin()
    {
        dui.setupUi(this);
	dui.leUserName->setText("your revid");
	dui.leUserPassword->setText("");
        dui.leUserName->setFocus();
	dui.leUserName->selectAll();
    }
};

class dlgLogin : public QDialog
{
    Q_OBJECT
public:
    Ui::dlgLoginUi dui;
    dlgLogin()
    {
        dui.setupUi(this);
	dui.leUserName->setText("your revid");
	dui.leUserPassword->setText("");
        dui.leUserName->setFocus();
	dui.leUserName->selectAll();
    }
};

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but homestead does not...

class homestead : public QMainWindow
{
	Q_OBJECT
public:
	homestead(QWidget *parent = 0);
...

class homestead : public QMainWindow
{
	Q_OBJECT
public:
	homestead(QWidget *parent = 0);
...

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Right?

[Chicken Blood Machine]

Well, here is the main function, from my main.cpp, from my Homestead application:

int main( int argc, char * argv[] ) {
	QString strRejected = "";
	QApplication app(argc, argv);
	app.setQuitOnLastWindowClosed(false);
	dlgLogin dlg; 
	if( dlg.exec() == QDialog::Accepted ){
		QSqlDatabase hapdb = QSqlDatabase::addDatabase(DBDRIVER);
		hapdb.setHostName(DBHOST);
		hapdb.setDatabaseName(DBNAME);
		hapdb.setUserName(dlg.dui.leUserName->text());
		hapdb.setPassword(dlg.dui.leUserPassword->text());
		if ( hapdb.open() ) {
			homestead ht;
			ht.RevID = dlg.dui.leUserName->text();
			ht.show();
			app.setQuitOnLastWindowClosed(true);
			return app.exec();
		} else { 
			strRejected = QString("The Login was rejected because: %1").arg(hapdb.lastError().text()).toLatin1();
			QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
			return 1;
		}
	} else {
		strRejected = QString("User Canceled the login!").toLatin1();
		QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
		return 2;
	}
}

int main( int argc, char * argv[] ) {
	QString strRejected = "";
	QApplication app(argc, argv);
	app.setQuitOnLastWindowClosed(false);
	dlgLogin dlg; 
	if( dlg.exec() == QDialog::Accepted ){
		QSqlDatabase hapdb = QSqlDatabase::addDatabase(DBDRIVER);
		hapdb.setHostName(DBHOST);
		hapdb.setDatabaseName(DBNAME);
		hapdb.setUserName(dlg.dui.leUserName->text());
		hapdb.setPassword(dlg.dui.leUserPassword->text());
		if ( hapdb.open() ) {
			homestead ht;
			ht.RevID = dlg.dui.leUserName->text();
			ht.show();
			app.setQuitOnLastWindowClosed(true);
			return app.exec();
		} else { 
			strRejected = QString("The Login was rejected because: %1").arg(hapdb.lastError().text()).toLatin1();
			QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
			return 1;
		}
	} else {
		strRejected = QString("User Canceled the login!").toLatin1();
		QMessageBox::information(0,"Login Rejected!",strRejected,
				QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
		return 2;
	}
}

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As I understand your comments, since "dlg" is called only once, and that call is in this function, its memory will be recovered when main goes out of scope (i.e., the app closes). And, hapdb is also instantiated only once so its memory, too, will be reclaimed when the app closes. The ht object, where Homestead really operates, is also instantiated only once and like the others, the memory it uses is returned to the OS when the app closes. So, I don't need as the last three lines of main():

delete ht;
delete hapdb;
delete dlg;

delete ht;
delete hapdb;
delete dlg;

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That would not even compile. None of the variables you mentioned are pointer variables, so how could you ever call delete on them? Your code (above) as listed will not leak any memory, because all objects are created on the stack.

because none of these objects leak memory, even though dlg has a parent:

class dlgLogin : public QDialog
{
    Q_OBJECT
public:
    Ui::dlgLoginUi dui;
    dlgLogin()
    {
        dui.setupUi(this);
	dui.leUserName->setText("your revid");
	dui.leUserPassword->setText("");
        dui.leUserName->setFocus();
	dui.leUserName->selectAll();
    }
};

class dlgLogin : public QDialog
{
    Q_OBJECT
public:
    Ui::dlgLoginUi dui;
    dlgLogin()
    {
        dui.setupUi(this);
	dui.leUserName->setText("your revid");
	dui.leUserPassword->setText("");
        dui.leUserName->setFocus();
	dui.leUserName->selectAll();
    }
};

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Dialog does NOT have a parent. Look how you instantiated it

dlgLogin dlg; // <- No parent

dlgLogin dlg; // <- No parent

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It's constructor does not event have a QWidget* 'parent' parameter, so there is no way to create a dlgLogin object with a parent (even if you wanted to).

What makes you think that it has a parent?

but homestead does not...

class homestead : public QMainWindow
{
	Q_OBJECT
public:
	homestead(QWidget *parent = 0);
...

class homestead : public QMainWindow
{
	Q_OBJECT
public:
	homestead(QWidget *parent = 0);
...

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Right?

Correct (but not for the reason that you have stated). Your homestead object does not have a parent, because you declared it like this:

homestead ht;

homestead ht;

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If you wanted it to have a parent, you would have declared it like this:

...
    homestead ht(parent); // Parent is some QWidget*, declared further up

...
    homestead ht(parent); // Parent is some QWidget*, declared further up

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In both cases, the fact theat you declared your widgets with no parent is fine, because you are ensuring their destruction by declaring them on the stack (not as pointers allocated with 'new') and therefore, they are destroyed when they run out of scope.

[GreyGeek]

That would not even compile. None of the variables you mentioned are pointer variables, so how could you ever call delete on them? Your code (above) as listed will not leak any memory, because all objects are created on the stack.

That I knew, I was just supposing that if they were created by "new" would it matter if they were deleted using the delete operator because they were going out of scope anyway if, as you say, the OS will always get the memory back.

Dialog does NOT have a parent. Look how you instantiated it

dlgLogin dlg; // <- No parent

dlgLogin dlg; // <- No parent

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It's constructor does not event have a QWidget* 'parent' parameter, so there is no way to create a dlgLogin object with a parent (even if you wanted to).

What makes you think that it has a parent?

Because it inherited from QDialog?
class dlgLogin : public QDialog

An ancestor isn't a parent?
(I told you I was new to C++)

Correct (but not for the reason that you have stated). Your homestead object does not have a parent, because you declared it like this:

homestead ht;

homestead ht;

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If you wanted it to have a parent, you would have declared it like this:

...
    homestead ht(parent); // Parent is some QWidget*, declared further up

...
    homestead ht(parent); // Parent is some QWidget*, declared further up

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In both cases, the fact theat you declared your widgets with no parent is fine, because you are ensuring their destruction by declaring them on the stack (not as pointers allocated with 'new') and therefore, they are destroyed when they run out of scope.

So, using "new" as shown below means that "ui.gridMultiProp" is a parent to *model ??

QSqlQueryModel *model = new QSqlQueryModel(ui.gridMultiProp);

QSqlQueryModel *model = new QSqlQueryModel(ui.gridMultiProp);

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Pardon the dumb questions but you'll dealing with someone who has read perhaps too much and it's all jumbled up inside?
It's amazing that I can, using QT, write the app I did and that it works.

[Chicken Blood Machine]

That I knew, I was just supposing that if they were created by "new" would it matter if they were deleted using the delete operator because they were going out of scope anyway if, as you say, the OS will always get the memory back.

Ok, then no, it wouldn't matter. As a matter of good practice, I would delete them anyway though.

Because it inherited from QDialog?
class dlgLogin : public QDialog

An ancestor isn't a parent?
(I told you I was new to C++)

No, QDialog is its baseclass, not its parent. The parent is the object you pass as the QWidget* parameter (which can be NULL). 'Baseclass', 'superclass' or 'inherited class' are names for the same C++ OO constructs. 'Parent' (in this context) is a Qt construct and refers to ownership, not inheritance. Reading the comprehensive documentation of QObject will explain it more.

So, using "new" as shown below means that "ui.gridMultiProp" is a parent to *model ??

QSqlQueryModel *model = new QSqlQueryModel(ui.gridMultiProp);

QSqlQueryModel *model = new QSqlQueryModel(ui.gridMultiProp);

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That's correct.

Pardon the dumb questions but you'll dealing with someone who has read perhaps too much and it's all jumbled up inside?
It's amazing that I can, using QT, write the app I did and that it works.

It is amazing. Did you check your prog for memory leaks
Actually, it's a very bad idea to use Qt to learn C++ (not suggesting that you did it this way). It is much better to learn C++ and get confident with it (warts and all), then start using Qt to see how much easier it makes your life! The QObject parent-child model along with the references counted non-QObjects really help cut down on the amount of memory management that you have to worry about (but you still must be wary).