checking for null

This may be a ridiculous question. And it may be just a C++ question. But I am totally lost here.

I would think that if I have declared an object, say a QListView, in a header file, but have not yet initialized that object, I could test for that declared but not initialized state with

Code:

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if(sequenceList == NULL)

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where sequenceList is the name of my QListView. But it is always returning false, whether initialized or not. I've also tried:

Code:

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if(sequenceList == null)

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and

Code:

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if(sequenceList == 0)

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What am I doing wrong here? Any help would be greatly appreciated.

Thanks,
Derek

Quote:

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Originally Posted by drkbkr

This may be a ridiculous question. And it may be just a C++ question. But I am totally lost here.

I would think that if I have declared an object, say a QListView, in a header file, but have not yet initialized that object, I could test for that declared but not initialized state with

Code:

---

if(sequenceList == NULL)

---

where sequenceList is the name of my QListView. But it is always returning false, whether initialized or not. I've also tried:

Code:

---

if(sequenceList == null)

---

and

Code:

---

if(sequenceList == 0)

---

What am I doing wrong here? Any help would be greatly appreciated.

Thanks,
Derek

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Depends on your compiler but pointers are rarely set to 0. You've got to do it in the constructor:

Code:

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SomeClass::SomeClass(...)
 : sequenceList(0)
{
  //other stuff
}

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which is the same as :

Code:

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SomeClass::SomeClass(...)
{
  sequenceList = 0;
  //other stuff
}

---

Quote:

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Originally Posted by fullmetalcoder

Depends on your compiler but pointers are rarely set to 0.

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It is undefined to what value they will be set. They could be zero or not there is no default value.

Quote:

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Originally Posted by fullmetalcoder

which is the same as :

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It's not exactly the same, better initialize to rather than assign the default values http://www.parashift.com/c++-faq-lit....html#faq-10.6

Strictly speaking, in C++, a null pointer is 0, not 0l, 0L, null, or NULL. It's just something to be aware of, since 0 is usually the convention, and also some people feel pretty strongly about it. :/

Bojan

NULL is simply a macro that will be replaced with 0 by the preprocessor. I think it's more clear. Pointers can be NULL. Integers can be the numeric value 0.

Quote:

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Originally Posted by Bojan

Strictly speaking, in C++, a null pointer is 0, not 0l, 0L, null, or NULL. It's just something to be aware of, since 0 is usually the convention

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Depends which books you read. I always use NULL.

And holy wars have started about less. ;)

In c++ a pointer is only 0 if you set it to 0. You can do this in the initializer list.
You would also need to do it after any delete.

Qt provides a QPointer that starts "nulled", gets "nulled" after a delete
and has a member isNull().