C++ help needed :(

[The Teej]

Hey all, had this issue for a little while now and I can't seem to fix it. Basically, in a class called Level, I have a get method which returns a particular pointer in a pointer array (the particular pointer is defined by a number that's passed when the method is called). I have another pointer array in another class called Game, and I use a for loop to replicate the pointer array in the Level Class (by using the accessor method).

Now, the problem I'm having is that when I try to call upon the update method on one of the pointers (it's a pointer to a class) a run time error appears saying there is an unhandled exception coming from the pointer array in the game class. I check to see if the pointer isn't null first, and it's passing that check, so assumably there's a pointer there.
Any ideas?

 

[The Teej]

Hmm, I think I know the issue - it's not cloning the pointer properly. I've created a new method to try and handle the cloning on the Level Class side:

Level.cpp

//clone enemy pointer
void
CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObject * pPassedEnemyPointer_)
{
ÂÂÂÂpPassedEnemyPointer_ = m_pEnemies[iArrayPos_];
}

and then basically pass the pointer I want to be used for the cloning:

Game.cpp
CODEÂÂÂÂ//pass enemy pointers from Level class
ÂÂÂÂfor (int i = 0; i < MAXENEMIES; i++)
ÂÂÂÂ{
ÂÂÂÂÂÂÂÂm_pLevel->CloneLevelEnemyPointer(i, m_pLevelEnemies);
ÂÂÂÂ}

But I still get the same error. Is there a different method to cloning pointer arrays across classes which I am not aware of?

 

[CockroachMan]

Your cloning function is wrong.. you need to pass a pointer to the pointer!

Like this:

//clone enemy pointer
void
CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObject ** pPassedEnemyPointer_)
{
ÂÂÂÂ*pPassedEnemyPointer_ = m_pEnemies[iArrayPos_];
}

CODEÂÂÂÂ//pass enemy pointers from Level class
ÂÂÂÂfor (int i = 0; i < MAXENEMIES; i++)
ÂÂÂÂ{
ÂÂÂÂÂÂÂÂm_pLevel->CloneLevelEnemyPointer(i, &m_pLevelEnemies );
ÂÂÂÂ}

That should do the trick

When a function is executed, a local copy of the arguments are created, so the way you were doing it, you were copying the pointer to a local variable.. it's complicated..

 

[The Teej]

Thanks! There are a couple of questions, though..

What's the difference between ** and *, and what's up with the & in front of the pointer? I'm still a C++ noob really, so a lot of the syntax is pretty alien to me, hehe.

edit: also, what's with the * infront of the pPassedEnemyPointer pointer?

EDIT: I'm still getting an access violation

 

[Issac]

how about changing * for a &?

I don't know though... just remember something with calling functions with &'s as parameters....
(doin some c++ lab assignments right now)

 

[The Teej]

how about changing * for a &?

I don't know though... just remember something with calling functions with &'s as parameters....
(doin some c++ lab assignments right now)

Change it where, though?

 

[Issac]

CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObject& pPassedEnemyPointer_)

Edit: Though.. I'm completely unsure about that :S

Edit2:
an example in the lab assignment i'm doing:

void swap(Diver_info& a, Diver_info& b);
...
...
void swap(Diver_info& a, Diver_info& b){
ÂÂÂÂ Diver_info temp;
ÂÂÂÂ temp = a;
ÂÂÂÂ a = b;
ÂÂÂÂ b = temp;
}

and with this... the struct Diver_info's a and b objects actually gets changed without returning anything.. much like pointers... :)

 

[CockroachMan]

Thanks! There are a couple of questions, though..

What's the difference between ** and *, and what's up with the & in front of the pointer? I'm still a C++ noob really, so a lot of the syntax is pretty alien to me, hehe.

edit: also, what's with the * infront of the pPassedEnemyPointer pointer?

EDIT: I'm still getting an access violation

* is a pointer, so ** is a pointer to a pointer!
If you want to access the contents of a pointer, you also use *

like:

int *a; // pointer to an integer

*a = 5;

printf("%d", a); //prints the address of a, something like 4244F123
printf("%d",*a); //prints 5

Now if you have a variable that's not a pointer and want to get it's address, you use &.

The code above can be writen as:
CODEint a; //integer variable

a = 5;

printf("%d",&a); //prints the address of a, something like 4244F123
printf("%d",a); //prints 5

I think that the & on functions is basically the same thing as using *, not sure now, it works only on C++.. I'm more used to regular old C :/

Have you tried debugging your code to see where is it crashing?

 

[The Teej]

Hold on, in that case, wouldn't

CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObject ** pPassedEnemyPointer_)
{
ÂÂÂÂ*pPassedEnemyPointer_ = m_pEnemies[iArrayPos_];
}

actually make pPassedEnemyPointer the address of m_pEnemies[iArrayPos_], and not the actual pointer?

 

[CockroachMan]

Hold on, in that case, wouldn't

CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObject ** pPassedEnemyPointer_)
{
ÂÂÂÂ*pPassedEnemyPointer_ = m_pEnemies[iArrayPos_];
}

actually make pPassedEnemyPointer the address of m_pEnemies[iArrayPos_], and not the actual pointer?

Nope, that would be &m_pEnemies[iArrayPos_].. unless the m_pEnemies is wrong.. what is the type of it?

Pointers are very confuse to understand at first.. try making a simple program, make some weird pointers, try switching their values and see if the results are what you were expecting.. that should help you understand them better

 

[The Teej]

This isn't my first time with pointers, I've used static and dynamic pointers many times, I've just never attempted to use them across classes before.

m_pEnemies is type CObject *.

 

[CockroachMan]

This isn't my first time with pointers, I've used static and dynamic pointers many times, I've just never attempted to use them across classes before.

m_pEnemies is type CObject *.

CObject *m_pEnemies?
or
CObject *m_pEnemies[]?

The first one is just an array of CObjects.. not pointers.

 

[The Teej]

It's the first one, as it's storing objects, not pointers.

 

[CockroachMan]

Sorry.. made some tests using classes now..

This should work:

CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObject * pPassedEnemyPointer_)
{
ÂÂÂÂ*pPassedEnemyPointer_ = m_pEnemies[iArrayPos_];
}
...
m_pLevel->CloneLevelEnemyPointer(i, &m_pLevelEnemies[i] );
...

You could also use:

CLevel::CloneLevelEnemyPointer(int iArrayPos_, CObjectÂÂ&pPassedEnemyPointer_)
{
ÂÂÂÂpPassedEnemyPointer_ = m_pEnemies[iArrayPos_];
}
...
m_pLevel->CloneLevelEnemyPointer(i, m_pLevelEnemies[i] );
...

Mixing pointers and classes is confuse.. since you're using C++, you should try using STL classes like Vector and List instead of using pointers

 

[The Teej]

Are you sure that works? I get these errors:

Level.cpp
error C2679: binary '=' : no operator found which takes a right-hand operand of type 'CObject *' (or there is no acceptable conversion)

Game.cpp
error C2664: 'CLevel::CloneLevelEnemyPointer' : cannot convert parameter 2 from 'CObject **__w64 ' to 'CObject *'

And, we haven't been taught Vector and List in class, so I have no idea how to use them. Guh, so much for my brilliant idea of a decent level engine, I can't figure out some trivial pointer issue -.-

 

[The Teej]

Hurr hurr hurr.

Your original suggestion, Cockroach man, was 100% right. The issue was... that I wasn't calling the methods in the constructor! Also, I wasn't initialising my pointers to 0. So, I was always cloning garbage pointers!

And thus, the problem is SOLVED.