Why i can not create an object ?

[JoeGreen]

@Chris-Kawa
thank you for replying,
i need the array object to be created after enter is pressed
in another words after the user enter the size of the array
and i do not want it to be resizable

[Chris Kawa]

@JoeGreen said in Why i can not create an object ?:

i need the array object to be created after enter is pressed

Ok, then create it in the function that is called when user presses enter and not in the global scope, which is executed at the start of the app.

[JoeGreen]

@Chris-Kawa
if i done that i wont be able to use the local ar object outside of that method
and i do want to use in further methods

[Chris Kawa]

@JoeGreen Make the pointer member of your class. Create an object in the method and assign it to that member, then you can use it in any other method of your class.

[JoeGreen]

@Chris-Kawa

#include "mainwindow.h"
#include "ui_mainwindow.h"
using namespace std;


class Array{
  public:
  int size;
  int length,index;

  Array(int s){
    size=s;
    index=9;
  }
};
Array *pointer; // the pointer member of class array
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_lineEdit_returnPressed() // size
{
    QString text = ui->lineEdit->text();
    ui->lineEdit->clear();
    Array ar=*new Array(text.toInt());
    pointer=&ar; // getting the address of the created object

}//these methods are invoked when the app reaches main


void MainWindow::on_lineEdit_2_returnPressed() // insert
{
    ui->label_5->setText(QString::number(pointer->size));// garbage

    ui->label_4->setText(QString::number(pointer->index));// garbage
   
}

i ve tried that but now pointer->size and pointer->index gives me garbage value

[Chris Kawa]

You've got pretty much all of it wrong.

Array *pointer;

That is not a class member. It's an uninitialized global variable. Member variables are declared in the header file, inside your class declaration.

This Array ar creates a local variable ar on the stack. This new Array creates second, unnamed variable on the heap. This =* copies the heap variable to the stack variable. This pointer=&ar takes address of a local variable ar and assigns it to a global variable. This } ends the scope. Local variable ar is destroyed. Unnamed heap variable is leaking memory and pointer points to garbage, because the local object no longer exists.

First make pointer a member of class MainWindow, like I said, and initialize it to nullptr value. Then in your method simply do:

pointer = new Array(text.toInt());

which creates a variable on the heap and assigns its address to the member.

Also remember that you have to delete that object somewhere and that this method can be called multiple times, so before you assign new value to pointer check if it already points to the previous object and if yes then delete it first.

[JoeGreen]

@Chris-Kawa
sorry but this line pointer = new Array(text.toInt());
gives me this error: Incompatible pointer types assigning to 'int *' from 'Array *'

[Chris Kawa]

@JoeGreen What type did you give to pointer and where did you declare it?

[wrosecrans]

@JoeGreen You may want to focus on following some basic C++ tutorials and learn the core language before you focus on learning Qt and making a complicated GUI. All of the Qt documentation and tutorials is going to assume a solid foundation of the basics of the language, like types and pointers.

[JoeGreen]

@Chris-Kawa
i 've made it
i v read about pointers and member class declarations
i had to put my class array declarations in a separate header file so that i could
declare an array pointer in mainWindow.h and it worked finally

thank you for your precious time