Clearing QLineEdit leads to segmentation fault and crash

Shoaib Muhammad

Hi All,

This may be an easy solution that I just don't understand so please bear with me as I am a complete newbie. I have a month to create a GUI and I learn things on the go.

So to start, I wanted to clear my input fields (named StrtEdit and StopEdit) upon pressing the PrcBtn QPushButton. However, the App crashes when I do it.

Here is my header.h:

#pragma once

#include <QWidget>
#include <QPainter>
#include <QApplication>
#include <QPushButton>
#include <QLabel>
#include <QIntValidator>
#include <Qt>
#include <QtDebug>
#include <QLineEdit>
#include <QString>
#include <QFont>
#include <QGraphicsScene>



class CGui : public QWidget {
    
  Q_OBJECT

  public:
    CGui(QWidget *parent = 0);

    QLineEdit *StrtEdit;
    QLineEdit *StopEdit;

  private slots:

    void clickProcess();
    void clickDatacap();

  private:
    QLabel *rltxt;
    QLabel *imtxt;

    QLabel *strttxt;
    QLabel *stoptxt;

  protected:
    void paintEvent(QPaintEvent *e);
    void drawRectangles();
    void drawGraphs();
};

And here is header.cpp

#include "mainheader.h"

CGui::CGui(QWidget *parent)
    : QWidget(parent) {

  QPushButton *PrcBtn = new QPushButton("PROCESS", this);
  PrcBtn->setFont(font1);
  PrcBtn->setStyleSheet("background-color:rgb(65,182,230); border:none; color:white;");
  PrcBtn->setGeometry(405, 195, 193, 60);

  QLineEdit *StrtEdit = new QLineEdit("",this);
  StrtEdit->setFont(font2);
  StrtEdit->setAlignment(Qt::AlignLeft);
  StrtEdit->setGeometry(555, 140, 240, 50);

  QLineEdit *StopEdit = new QLineEdit("",this);
  StopEdit->setAlignment(Qt::AlignLeft);
  StopEdit->setFont(font2);
  StopEdit->setGeometry(555, 85, 240, 50);

  connect(PrcBtn, &QPushButton::clicked, this, &CGui::clickProcess);
}

void CGui::clickProcess(){
  qDebug() << "Process Button Clicked";
  this->StrtEdit->setText("");
  this->StopEdit->setText("");
}

finally, here is main.cpp

#include "mainheader.h"

int main(int argc, char *argv[]) {
    
  QApplication app(argc, argv);  
    
  CGui window;

  window.resize(800, 445);
  window.setWindowTitle("Complex Impedance Analyzer");
  window.show();

  return app.exec();
}

I have omitted the parts of the code that are unrelated and that is why some objects in the header are unused.

What could be the problem?

Kind regards,
Shoaib

chris_rookie

I thought your class CGui should extends QMainWindow

dheerendra

QLineEdit *StrtEdit are local variable. Looks like you have same member variables. replace QLineEdit *StrtEdit with *StrtEdit. it should work.

Shoaib Muhammad

@chris_rookie said in Clearing QLineEdit leads to segmentation fault and crash:

I thought your class CGui should extends QMainWindow

I'm not sure I follow. I don't know CGui extends QMainWindow.

By the way I have been following tutorials here since I am using qt version 5.6.1 with an AM4379 Development Kit on a Ubuntu Linux 14.04 Operating System.

Shoaib Muhammad

@dheerendra said in Clearing QLineEdit leads to segmentation fault and crash:

QLineEdit *StrtEdit are local variable. Looks like you have same member variables. replace QLineEdit *StrtEdit with *StrtEdit. it should work.

Hi dheerendra,
You are absolutely right. To be precise I changed it to StrtEdit (without asterisk) in my header.cpp file. I would love to learn more about this. Will you be able to give a brief explanation or link explaining what member variables and local variables are? Thank you very much.

Kind regards,
Shoaib

jsulm

@Shoaib-Muhammad Member variables are variables defined in a class, like:

class A
{
private:
    int aVar; // aVar is a member of class A
};

Local variables are variables defined in a local scope. Local scope can be inside function or method for example:

class A
{
private:
    int aVar; // aVar is a member of class A
    void someMethod()
    {
        int aVar = 0; // This is a local variable which only exists inside someMethod and
                                 is NOT the same as aVar above!
        // aVar = 0; //If you do it like this then you access the member variable
    }
};

The problem with your code is exactly this: you define local variables with exactly same names as your member variables.

Shoaib Muhammad

@jsulm said in Clearing QLineEdit leads to segmentation fault and crash:

@Shoaib-Muhammad Member variables are variables defined in a class, like:
class A
{
private:
    int aVar; // aVar is a member of class A
};
Local variables are variables defined in a local scope. Local scope can be inside function or method for example:
class A
{
private:
    int aVar; // aVar is a member of class A
    void someMethod()
    {
        int aVar = 0; // This is a local variable which only exists inside someMethod and
                                 is NOT the same as aVar above!
        // aVar = 0; //If you do it like this then you access the member variable
    }
};
The problem with your code is exactly this: you define local variables with exactly same names as your member variables.

Thank you for the concise and clear explanation. It does make sense now that you mention it.