QLabel isn't displayed if not a member - c++

Question

I have a class which displays QLabel with some text in QGridLayout. When QLabel is a class member, everything is ok, but when it is not, it isn't displayed in grid.
QLabel is not a member and is created in the constructor.
class Account : public QWidget
{
private:
//QLabel lab;
QGridLayout * grid;
public:
Account(QWidget * parent=0);
public slots:
void spend(int);
void update();
};
.cpp
#include <QLabel>
#include <QLineEdit>
#include <QTextStream>
#include <QFileInfo>
#include <vector>
#include <QGridLayout>
#include <iostream>
Account::Account(QWidget * parent) : QWidget(parent)
{
grid=new QGridLayout(this);
QLabel lab;
lab.setText("RFD");
grid->addWidget(&lab,0,0); //is not displayed
}
QLabel is a member:
class Account : public QWidget
{
private:
QLabel lab;
QGridLayout * grid;
public:
Account(QWidget * parent=0);
public slots:
void spend(int);
void update();
};
.cpp
#include <QLabel>
#include <QLineEdit>
#include <QTextStream>
#include <QFileInfo>
#include <vector>
#include <QGridLayout>
#include <iostream>
Account::Account(QWidget * parent) : QWidget(parent)
{
grid=new QGridLayout(this);
lab.setText("RFD");
grid->addWidget(&lab,0,0); //is displayed
}
WHy is it so?

Answer 1

Because when you declare QLabel in your constructor it is only a local variable, therefore it falls out-of-scope at the end of the function (see my comments below):
{
grid=new QGridLayout(this);
QLabel lab; // created here
lab.setText("RFD");
grid->addWidget(&lab,0,0); //is not displayed
} // destroyed here as the variable is out of scope
note
If you want to create it in your constructor then make it a pointer and new it - you will still need to keep track of this pointer, either as a member variable or pass the pointer to some other function / class to take care of it... (i.e. to delete it when you are finished with it...
Further - addWidget() takes a pointer, so it might handle the destruction of it itself - so you could probably do:
{
grid=new QGridLayout(this);
QLabel *pLab = new QLabel(this);
lab.setText("RFD");
grid->addWidget(pLab,0,0); //is not displayed
}

Answer 2

QLabel lab; is a local variable that lives until the end of the function and then gets automatically destroyed, so the label you want displayed doesn't actually exist anymore when you want to display it.
Furthermore Qt has a broken unconventional memory model that requires you to new up the QLabel and then pass an owning raw pointer to the function which Qt will automatically delete later when the parent gets removed. If you delete it manually you get a double delete (one from you and one from Qt) which usually results in a crash. Manually deleteing an object will automatically detach it from the parent, so you don't get a double delete.
The way Qt wants you to do it is
QLabel *lab = new QLabel;
lab->setText("RFD");
grid->addWidget(lab,0,0);
While this looks like a memory leak, it isn't, due to Qt's memory model.