Can I use the same variable to initialise two class objects?

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I have to create and initialise objects of a class called Device in my main program. But the number of objects created are based on the user input. The IDE is Qt so I have access to data structures like QLists.

What I have tried:

My idea was that in the header file I would declare a Device pointer like this
Device* temp; QList<Device*>myList;
then in the source file, I would initialise this in a for loop based on the number of inputs. Like so:

C++

for(int i = 0; i<inputVal; i++){
     temp = new Device;
     myList.append(temp);
}

I can't run the code yet as it is part of a bigger module, so I wanted to know if this is completely wrong from a theoretical perspective. I was hoping that I could reuse the variable name temp and in the end, temp would contain the pointer to the last Device object created, and to clean up the memory I would use a foreach loop on the QList.

Please advice!!

Posted 22-Nov-17 17:34pm

TheLostJedi

Updated 22-Nov-17 21:17pm

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Jochen Arndt · Accepted Answer · 2017-11-22T21:16:00

Solution 1

You can do that but it is bad style. A temporary variable should be local and not a class member:

for (int i = 0; i < inputVal; i++)
{
     Device *temp = new Device;
     myList.append(temp);
}

There is even no need to use a temporary variable:

for (int i = 0; i < inputVal; i++)
{
     myList.append(new Device());
}

To access the last item just call ~~myList.takeLast()~~ myList.last().

The cleanup function should be called from the destructor of the class that owns myList and might be called from elsewhere if required:

C++

MyClass::~MyClass()
{
    // Other cleanup here

    DeleteAllDevices();
}

void MyClass::DeleteAllDevices()
{
    while (!myList.isEmpty())
        delete myList.takeFirst();
}

Posted 22-Nov-17 21:16pm

Jochen Arndt

Updated 22-Nov-17 22:09pm

v2

Comments

CPallini 23-Nov-17 4:14am

5.

TheLostJedi 23-Nov-17 6:56am

This is exactly what I was looking for! And more! Question: If one of the Device objects gets destroyed, what will happen to the QList member? Will it become a null pointer? I have a unique ID for each of these objects, I was thinking of using a QMap<int,Device*> and a signal in the destructor of the Device class, so that I can delete the member if it gets destroyed during runtime.

Jochen Arndt 23-Nov-17 7:44am

If you delete a Device you should remove that item also from the list because
the - then invalid - pointer will be still there.

Note what takeFirst() and takeLast() are doing: They will return the pointer but remove the first/last item from the list (therefore I had to correct my answer regarding accesing the last item).

Use

delete takeAt(ndx);

to do the same for a Device by index or removeAt() to remove items from the list where the Device has been already deleted.

If you don't know the index of a Device that has been deleted already, iterate over the list items until you find the pointer and remove that item.

I suggest to manage the Devices at one place where the class containing the list would be the best place. Provide a function to delete a device by passing the pointer or the ID which then searches the list.

That avoids using a signal. If you really need a signal (can't call the delete function directly) I would choose a signal that triggers the delete.