develop my own signal scope with graphicsView / how to clear memory from deleted object in c++?

M_Hu

I want to develop my own signal scope in C++. So I use qt for this purpose. I add a graphicsView and a push Button in ui. At connected method to push button I update the graphicsView(finally I'll pass this method to a thread). Each time that I press push button, Despite the deleting pointer the usage of memory is increase. How should I control this?

I check memory usage in vs15 diagnostic tool.

c++ header file:

#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_QtGuiApplication.h"
#include <QGraphicsScene>
#include <QGraphicsPathItem>
class QtGuiApplication : public QMainWindow
{
Q_OBJECT

public:
QtGuiApplication(QWidget *parent = Q_NULLPTR);

private:
Ui::QtGuiApplicationClass ui;

QGraphicsScene* scene = new QGraphicsScene();
QPolygon pol;
QGraphicsPathItem* pathItem = new QGraphicsPathItem();
int index_ = 0;            // graph offset
QPen* myPen = new QPen(Qt::red, 2);

private slots:  
void btn_Display_clicked();

};
c++ source file:

#include "QtGuiApplication.h"

#include <math.h> /* sin */

#define pi 3.14159265

QtGuiApplication::QtGuiApplication(QWidget *parent)
: QMainWindow(parent)
{
ui.setupUi(this);
ui.graphicsView->setScene(scene);
ui.graphicsView->show();
connect(ui.btn_Display, SIGNAL(clicked()), this, SLOT(btn_Display_clicked()));
}

void QtGuiApplication::btn_Display_clicked()
{
scene->removeItem(pathItem);
QPoint pos;

double x_amp_, y_amp_;
int x_pix_, y_pix_;
double phi_ = 0;
double freq_ = 5;
for (int i = 0; i<800; i++)
{
    y_amp_ = 100 * sin(2 * pi*freq_*i / 811 + phi_) + 20 * index_;
    x_amp_ = i;
    x_pix_ = x_amp_;
    y_pix_ = y_amp_;
    pos.setX(x_pix_);
    pos.setY(y_pix_);
    pol.append(pos);
}
QPainterPath* myPath = new QPainterPath();
(*myPath).addPolygon(pol);
pathItem = scene->addPath(*myPath, *myPen);
delete myPath;
pol.clear();
index_ = (index_ + 1) % 20; // just for sense of change in graph

}

jsulm

@M_Hu said in develop my own signal scope with graphicsView / how to clear memory from deleted object in c++?:

scene->removeItem(pathItem);

add this as removeItem() does not delete the item as stated in the documentation:

delete pathItem;

(*myPath).addPolygon(pol);

why not simply

myPath->addPolygon(pol);

?
There is really no need to allocate myPath on the heap:

QPainterPath myPath;
myPath.addPolygon(pol);
pathItem = scene->addPath(myPath, *myPen);

You use way too much pointers... For example: why is myPen a pointer?

M_Hu

@jsulm
if pathItem deleted then I'll lose pointer so How can I remove previous drawn path?! I want to erase previous path and plot new path at this method.
in future I'll pass this method on a thread. so I think as far as I can use pointer an don't create new object.

jsulm

@M_Hu Don't you get a new one here:

pathItem = scene->addPath(*myPath, *myPen);

?
Read this: http://doc.qt.io/qt-5/qgraphicsscene.html#addPath
"Creates and adds a path item to the scene, and returns the item pointer."
So, each time you call addPath you create a NEW item, which YOU have to delete later when you remove it from the scene.

"if pathItem deleted then I'll lose pointer" - you don't need that pointer as you delete the old item and then create new one as described above.

M_Hu

@jsulm
thank you. I did not care about that