Pixmap updating QLabel

I am new to Qt as of yesterday, and I was hoping someone would be able to help me with a problem. I am writing a program to act like XKCD, but I can't get pixmap to update label. Here is a section, I will attach the full files in case they would help.

Code:

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int val = 1;
XKCD::XKCD(QWidget *parent)
    : QWidget(parent)
{
  QString number;
  QString png = ".png";
  QString pics = "pics/";
  number.setNum(val);
  number.prepend(pics);
  number.append(png);
 
  QPixmap *original=new QPixmap(QString(number));
  QPixmap image(original->scaled ( 1300,700, Qt::KeepAspectRatio, Qt::SmoothTransformation ));
  QLabel *label = new QLabel(this);
  label->setPixmap(image);
  int pixwidth = image.width();
  int labelwidth = (WIDTH - pixwidth) / 2;
  label->setGeometry(labelwidth, SIZEY, pixwidth, 700);
}
 
void XKCD::OnPrev()
{
  val--;
  if (val < 1) {val = 0;}
  QString number;
  QString png = ".png";
  QString pics = "pics/";
  number = number.setNum(val);
  number = number.prepend(pics);
  number = number.append(png);
  QPixmap *original=new QPixmap(QString(number));
  QPixmap image= (original->scaled ( 1300, 600, Qt::KeepAspectRatio, Qt::SmoothTransformation ));
  QLabel *label = new QLabel(this);
  label->setPixmap(image);
  int pixwidth = image.width();
  int labelwidth = (WIDTH - pixwidth) / 2;
  label->setGeometry(labelwidth, SIZEY, pixwidth, 700);
  /* Debugging purposes */ std::cout << "Val is now: "<< val << ".  The image path is now: " << number.toStdString() << "." << std::endl;
}

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Hopefully someone can help me, I haven't found anything in documentation yet. Thanks for looking at my problem.

Attached are my project files.
Attachment 5080
Attachment 5081
Attachment 5082

Edit: If it would be possible or easier to do using QImage, I am certainly open to that if someone could help me with it. My attempt to switch from QPixmap to QImage gave me a mile long set of errors that I couldn't find a workaround for.

If

Code:

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QLabel *label;

---

in the header file is any indication, I'm fairly certain that you want to change

Code:

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QLabel *label = new QLabel(this);

---

to

Code:

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label = new QLabel(this);

---

in your constructor. If this is the case, then you should consider carefully why you are re-instantiating the label variable in onPrev().

Thanks for the reply.
A constructor is? Like I said, I'm new to Qt. I think it is the parent widget, but I'm not sure about that.
As to re-instantiating the variable in Prev, I thought they only last for a single function. Maybe I'm mistaken here, but isn't the parent widget one function and Prev, next, and random are different functions? Or is C++ just messing me up here?

Making the change doesn't seem to change any functionality though =/

onPrev and onNext still won't change the image when they are clicked. Is there a buffering feature for label?

When you declare a variable in a header file, that variable is considered to be an integral part of any object instantiated from your class. Thus, it persists until the object is destroyed. These so-called "instance variables" are generally initialized in the constructor, a method with the same name as your class. When you say "QLabel *label = new QLabel()", you are in fact calling the default constructor (i.e., no parameters) of the QLabel class.

In C++ you can create a variable on the stack or on the heap. When you say "type *var = new type();", you are putting the variable on the heap, and even if you do this inside a method, the variable will persist. On the other hand, if you say "type var;" then you are putting the variable on the stack. If you do this inside a brace-delimited scope (such as a method), the variable will not persist or be visible outside that scope unless you take a pointer to it first. This conforms to the idea of a local variable.

I see, thanks. With a *, it lasts the whole program. Without, it is just a local variable. So i need to call the *label at some point, but can just call label afterwards.
Do you have any idea why the image wouldn't update though? The debugging line I put in works properly, it is like label(pixmap) is buffered and won't output anything even when I tell it to.
Perhaps making the other variables I am relying on heap variables would help, I'll try that.

What do YOU get from running the following code?

Code:

---

label = new QLabel(this);
QPixmap pmap(QString("C:\\path\\to\\file.jpg")); // windows machine, hence escapes
if (pmap.isNull())
    label->setText("Null pixmap!");
else
    label->setPixmap(pmap);

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At first, I got null pixmap, then I remembered the windows directory delimiters and I got my image in the label. Anyways, yeah, you might have a null pixmap for whatever reason.

When I run that, replacing the image obviously, it displays the image I want it to.

If I had a null pixmap in one of the lower functions (onPrev, etc), would it display a null or just continue to show the same thing. If it continues to show the same thing, that is my problem.

Edit: A little testing with and pixmap.isNull() has determined that my image isn't null in any of my functions. It just refuses to update to the new filepath.

Quote:

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Originally Posted by Matt

If I had a null pixmap in one of the lower functions (onPrev, etc), would it display a null or just continue to show the same thing.

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It would display absolutely nothing at all, or in the case of my example code, "Null Pixmap!" (tested). A null pixmap has no content, but QLabel::setPixMap() wipes out the label's previous pixmap.

Code:

---

QString number;
  QString png = ".png";
  QString pics = "pics/";
  number = number.setNum(val);
  number = number.prepend(pics);
  number = number.append(png);
  QPixmap *original=new QPixmap(QString(number));
  QPixmap image= (original->scaled ( 1300, 600, Qt::KeepAspectRatio, Qt::SmoothTransformation ));
  label = new QLabel(this);
  label->setPixmap(image);

---

So in this code, label should be cleared and replaced with the new value of image, which is clearly not happening. Could the problem be in QPixmap *original? Removing the * from that variable makes it refuse to compile, but could the * be preventing the value of original from updating? If so, is there a way to get rid of the * safely without giving errors about converting QPixmap* to QPixmap?

How about I just show you what worked for me?

First, here is the header file:

Code:

---

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QtGui/QWidget>
 
class QLabel;
class QPushButton;
 
class MainWindow : public QWidget
{
    Q_OBJECT
 
public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
public slots:
    void updatePixmap();
 
private:
    QPixmap pmap1, pmap2;
    QLabel *label;
    QPushButton *button;
    int numUpdates;
};
 
#endif // MAINWINDOW_H

---

Now, here is the .cpp file:

Code:

---

#include "mainwindow.h"
#include <QLabel>
#include <QPushButton>
#include <QVBoxLayout>
 
MainWindow::MainWindow(QWidget *parent)
    : QWidget(parent), numUpdates(0)
{
    pmap1 = QPixmap("C:\\path\\to\\image1.jpg");
    pmap2 = QPixmap("C:\\path\\to\\image2.jpg");
    label = new QLabel();
    label->setPixmap(pmap1);
 
    button = new QPushButton("Update!");
    connect(button, SIGNAL(clicked()), this, SLOT(updatePixmap()));
 
    QVBoxLayout *layout = new QVBoxLayout(this);
    layout->addWidget(label);
    layout->addWidget(button);
    layout->setSizeConstraint(QLayout::SetFixedSize);
    setLayout(layout);
}
 
void MainWindow::updatePixmap()
{
    ++numUpdates % 2 ? label->setPixmap(pmap2) : label->setPixmap(pmap1);
}
 
MainWindow::~MainWindow()
{
 
}

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update: images now toggle back and forth instead of being a one-shot deal :p

Quote:

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label should be cleared and replaced with the new value of image, which is clearly not happening.

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But but but you are destroying the previous label entirely...

What happens if you comment out line 9?

If I comment out line 9, I get an error. But that line is only for resizing the window, it doesn't affect anything else.

I am reading over your first post trying to integrate it into my system.

Edit: Thank you very much, that did it. When I removed the code that destroyed label, it solved the problem. I will definitely remember this in the future.