How to check pointer object isExist before?

MimCimm · 31 Dec 2020, 08:13

Hello Everyone!
Are there any way to control pointer object created before.
Specifically:

---x.h---

MyClass *myClassObject;

---x.cpp---

void methodX(){
//Here is line that I want to check that myClasssObject is created or not?
delete myClasssObject;
myClasssObject = new MyClassObject();
...
}

Because I trigger methodX() with QTimer periyodically so that I need o know is it created before if created then I will detele old one.
I should do do that otherwise every trigger increase thread count in OS.

Any Help ,I will be gratified..

JonB · 31 Dec 2020, 08:19

@MimCimm
I don't fully understand. You cannot check that a pointer is valid/points to something valid safely in C++. You can, however, initialize it yourself. So initialize it to nullptr, and if you delete it reset it to nullptr so that you can test that in the future.

sierdzio · 31 Dec 2020, 08:30

A pointer is just an integer - if you make sure to set it to nullptr when it's not created, you can simply check with if:

if (myClassObject) {
  delete myClassObject;
  myClassObject = nullptr;
}

Alternatively, you can use QPointer and it's isNull() method - if your MyClass is a subclass of QObject.

MimCimm · 31 Dec 2020, 08:20

@JonB If I initialize with nullptr than I am losing old created object before delete it. But old thread still countinue to work when I check OS from linux terminal.

jeremy_k · wrote on 31 Dec 2020, 08:20

Deleting a null pointer is a safe operation. There's no need to check for it.

sierdzio · 31 Dec 2020, 08:47

@MimCimm said in How to check pointer object isExist before?:

@JonB If I initialize with nullptr than I am losing old created object before delete it. But old thread still countinue to work when I check OS from linux terminal.

You definitely need to initialize with nullptr - in constructor or (IMO better) in the header:

MyClass *myClassObject = nullptr;

Otherwise it is very, very easy to crash your app.

JonB · wrote on 31 Dec 2020, 08:22

@MimCimm said in How to check pointer object isExist before?:

If I initialize with nullptr than I am losing old created object before delete it.

That's not "initializing", that's just "overwriting", of course I was not suggesting that. Just make sure it's nullptr before you start, and reset it to nullptr any time you delete it (unless you immediately set it to something else). I still don't understand what your issue is.

MimCimm · 31 Dec 2020, 08:51

@sierdzio I tried with QPointer but also QPointer stops running if I try to set myObject into QPoiter object before new ;

QPointer<MyClassObject>qPointObject(myClassObject); //it throws error in this line
qDebug()<< qPointObject.isNull();
myClasssObject = new MyClassObject();

I know It throws because I am trying to set before using new (that means create) but That is exactly thing ,I want to check.
If I control after new line I am losing the old thread.

Bonnie · wrote on 31 Dec 2020, 08:30

---x.h---
MyClass *myClassObject;

Is that a global pointer or a class member?
If it is a global pointer, it shouldn't be put in a header file...Otherwise you'll get problems when you include this header file in multiple other files...

MimCimm · wrote on 31 Dec 2020, 08:47

This post is deleted!

sierdzio · wrote on 31 Dec 2020, 08:51

@MimCimm said in How to check pointer object isExist before?:

@sierdzio I tried with QPointer but also QPointer stops running if I try to set myObject into QPoiter object before new ;
QPointer<MyClassObject>qPointObject(myClassObject); //it throws error in this line
qDebug()<< qPointObject.isNull();
myClasssObject = new MyClassObject();
I know It throws because I am trying to set before using new (that means create) but That is exactly thing ,I want to check.
If I control after new line I am losing the old thread.

I meant to use it in your header:

QPointer<MyClass> myClassObject;