Qt read acces violation at:0x0, flags=0x0 (first chance)

c++ qt pointers null

10,420

If vakjes[rij][kolom] is NULL, then trying to dereference that pointer -> results in undefined behavior (usually: crash).

Just change the code to:

bool Spelbord::positionIsEmpty(int rij, int kolom) { 
  if (vakjes[rij][kolom] == nullptr) return true;
  if (vakjes[rij][kolom]->geefLetter()==nullptr) return true; 

  return false; 
}

10,420

Cédric Vandelaer

I study computer science at the UHasselt university in Belgium.

Updated on June 04, 2022

Comments

Cédric Vandelaer about 2 years
I have a very simple piece of code as follows:
```
Letter* Vakje::geefLetter() const
{
    return m_letter;
}
```
The class Vakje gets initialized with a NULL-pointer for m_letter, but I'm still getting a read access violation error, could someone please help?

this is the code that calls the function:
```
bool Spelbord::positionIsEmpty(int rij, int kolom) { 
    if (vakjes[rij][kolom]->geefLetter()==nullptr) return true;
    else return false; 
}
```
- Mats Petersson almost 11 years
  
  Exactly what else do you expect? Accessing a pointer that is NULL is undefined behaviour, which is most cases means "crash the program" [not ALWAYS the case for UB, but in this sort of case, quite typical].
- Cédric Vandelaer almost 11 years
  
  but I'm doing a check to see if it's NULL :S
- R. Martinho Fernandes almost 11 years
  
  I see no check. Were we supposed to guess that some of your code somewhere has a null-check? What else are we supposed to guess?
- Prasanth Kumar almost 11 years
  
  Which code shows the read access violation error? You are not showing client code.
- Prasanth Kumar almost 11 years
  
  There is nothing here. You can't talk about checks and violations without showing them.
- Bart almost 11 years
  
  Show us the surrounding code. I.e, what's calling it and what's using the result.
- Frank Osterfeld almost 11 years
  
  The containing Vakje object might be invalid/accessed via dangling pointer. Try with a debugger/and or Valgrind (if on Linux/OS X).
- Mat almost 11 years
  
  Are you calling geefLetter on a null Vakje?
- Cédric Vandelaer almost 11 years
  
  this is the code: bool Spelbord::positionIsEmpty(int rij, int kolom) { if (vakjes[rij][kolom]->geefLetter()==nullptr) return true; else return false; }
- Bart almost 11 years
  
  Edit your question to contain the relevant code please. Don't hide it within comments. And are you sure that the "table" has non-null Vakje-s?
- Prasanth Kumar almost 11 years
  
  @Cédric Please ask a complete question, editing it to include all the relevant code.
- Prasanth Kumar almost 11 years
  
  I read this line if (vakjes[rij][kolom]->geefLetter()==nullptr)... as "Hello, I am a native 2D array and I am going to crash because you are going to call me with values of rij and kolom out of range. The question is not if I am going to crash, but when".
- Cédric Vandelaer almost 11 years
  
  the rij and kolom values can't go out of range :S
- Bart almost 11 years
  
  So answer this: what are the dimensions? When it crashes, what does it crash on (the indices)? Might it be that the Vakje is null? Did you check that? Fire up a debugger and let us know. (OT: might I advice you to start programming in English and not Dutch. You'll thank me in the future).
- Cédric Vandelaer almost 11 years
  
  Vakje was null, sorry for the stupid question, thanks :)
- Prasanth Kumar almost 11 years
  
  @Cédric Out of range, null element, value not initialized... whatever.