Couldn't use QPainter in a function.

Shoaib Muhammad

Hi All,

I am a beginner QT/C++ Programmer tasked to create an application to graph values.

The goal is to draw a graph (using QPainter) based on values from an array, upon pressing a button, and once that same button is pressed again, the graph is redrawn but with empty values (erasing the previous plot).
I receive errors when using an 'emit' signal command.

The following is the way my code is structured:

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>
#include <QVector>
#include <QDesktopWidget>



class CGui : public QWidget {
    
  Q_OBJECT

  public:
    CGui(QWidget *parent = 0);


  private slots:

    void clickCnts();
    void clickSngl();


  private:
    QLabel *rltxt;
    QLabel *imtxt;

    QLabel *strttxt;
    QLabel *stoptxt;

    QLabel *lG1yaxis1;
    QLabel *lG1yaxis2;
    QLabel *lG1yaxis3;
    QLabel *lG1yaxis4;
    QLabel *lG1yaxis5;

    QLabel *lG1xaxis1;
    QLabel *lG1xaxis2;
    QLabel *lG1xaxis3;

    QLabel *lG2yaxis1;
    QLabel *lG2yaxis2;
    QLabel *lG2yaxis3;
    QLabel *lG2yaxis4;
    QLabel *lG2yaxis5;

    QLabel *lG2xaxis1;
    QLabel *lG2xaxis2;
    QLabel *lG2xaxis3;
    QLabel *lG2xaxis4;

    QLabel *lG2zaxis1;
    QLabel *lG2zaxis2;
    QLabel *lG2zaxis3;

    QLineEdit *StrtEdit;
    QLineEdit *StopEdit;

    QPushButton *CntsBtn;
    QPushButton *SnglBtn;

  protected:
    void paintEvent(QPaintEvent *e);
    void drawPlot(QPainter *painter);
    void drawEmptyGraphs(QPainter *painter);

};

header.cpp

/* Code Removed Here initializing arrays and variables, as well as #includes */

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

  CntsBtn = new QPushButton("CONTINUOUS", this);
  CntsBtn->setFont(font1);
  CntsBtn->setStyleSheet("background-color:rgb(38,208,124); border:none; color:white;");
  CntsBtn->setGeometry(scrW*0.50625, scrH*0.3125, scrW*0.24125, scrH*0.1041666667);

/* Additional Code removed here for Initializing Axis labels */

  connect(CntsBtn, &QPushButton::clicked, this, &CGui::clickCnts);
}

void CGui::paintEvent(QPaintEvent *e) {
    
  Q_UNUSED(e);
  
  QPainter painter(this);
  
  painter.setRenderHint(QPainter::Antialiasing);
  drawPlot(&painter);
  drawEmptyGraphs(&painter);
}

void CGui::drawEmptyGraphs(QPainter *gpaintTool) {

/* EMPTY GRAPHS DRAWN HERE */

}

void CGui::drawPlot(QPainter *paintTool) {
  /* LINES ON GRAPH PLOTTED HERE */
}

void CGui::clickCnts(){
  if (StartFlag == 0)
  {
       StartFlag = 1;
    qDebug() << "CGui: Processing Started...";
    CntsBtn->setText("STOP");
       /* Axis Values replaced here based on min and max*/
       emit drawPlot(); //this line is an error
  }
  else
  {
    StartFlag = 0;
    qDebug() << "CGui: Processing Stopped!";
    CntsBtn->setText("CONTINUOUS");
    emit drawEmptyGraphs(); //this line is an error
  }
}

I am pretty sure that those emit lines are where the problems is. I tried creating a new QPainter object in the function, but the QPainter may only be used in QPaintEvent. What could be a solution?

Thank you for your time in reading all this.

Kind regards,
Shoaib

jsulm

@Shoaib-Muhammad said in Couldn't use QPainter in a function.:

emit drawEmptyGraphs(); //this line is an error

drawEmptyGraphs() is not declared as signal!
Also, signals are not implemented by you as developer, you only declare them! See https://doc.qt.io/qt-5/signalsandslots.html
But I don't know why you want to have signal here at all: why not just call drawEmptyGraphs() as a normal method?
If you want to trigger a paintEvent then call update() on that widget.

Shoaib Muhammad

Hi jsulm,

Thank you for the reply.

I tried just calling drawEmptyGraphs(); but I had an error stating a slot and signal mismatch.

Could you please provide an example I could base on? I am a bit lost right now.

Kind regards,
Shoaib

jsulm

@Shoaib-Muhammad said in Couldn't use QPainter in a function.:

I tried just calling drawEmptyGraphs(); but I had an error stating a slot and signal mismatch.

Please show your code. Calling a method does not have anything to do with signals/slots.

void CGui::clickCnts(){
  if (StartFlag == 0)
  {
    StartFlag = 1;
    qDebug() << "CGui: Processing Started...";
    CntsBtn->setText("STOP");
    /* Axis Values replaced here based on min and max*/
    drawPlot();
  }
  else
  {
    StartFlag = 0;
    qDebug() << "CGui: Processing Stopped!";
    CntsBtn->setText("CONTINUOUS");
    drawEmptyGraphs();
  }
}