What does this statement mean?

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Ship *player = static_cast<Ship *>(a == player_ ? a : (b == player_ ? b : 0));

C++

I know what static_cast is but I don't know what's done with the conditional operators.

First-chance exception at 0x00DDD250 in AsteroidsTest_2012.exe: 0xC0000005: Access violation reading location 0xFEEEFEF6.

I just need hints about what might be wrong. Not direct answers. I'll find the answer on my own. I just need the hint.

Btw the crash takes me here: -

void Collision::DoCollisions(Game *game) const
{
	for (ColliderList::const_iterator colliderAIt = colliders_.begin(), end = colliders_.end();
		colliderAIt != end;
		++colliderAIt)
	{
		ColliderList::const_iterator colliderBIt = colliderAIt;
		for (++colliderBIt; colliderBIt != end; ++colliderBIt)
		{
			Collider *colliderA = *colliderAIt;
			Collider *colliderB = *colliderBIt;
			if (CollisionTest(colliderA, colliderB))
			{
				game->DoCollision(colliderA->entity, colliderB->entity);
			}
		}
	}
}

What I have tried:

Tried rectifying the problem but I land on this code

Posted 19-Sep-18 0:19am

Siddhant Bhati

Updated 19-Sep-18 0:26am

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OriginalGriff · Accepted Answer · 2018-09-19T00:27:00

Solution 1

Look at it: it's just a nested conditional.

a == player_ ? a : (b == player_ ? b : 0)

If a == player_ then it returns a.
Otherwise if b == player_ then it returns b.
If both tests fail it returns 0

Think of it as an "in place if":

if (a == player_)
   return a;
else if (b == player_)
   return b;
return 0;

Posted 19-Sep-18 0:27am

OriginalGriff

Comments

Siddhant Bhati 19-Sep-18 6:54am

Oh thanks man.

Can I ask you more doubts about this code? I just want you to give me hints about what I should go for.

So this is an asteroid type game and when the player or bullet collides with the asteroid the game crashes and takes me here :-

void Collision::DoCollisions(Game *game) const
{
for (ColliderList::const_iterator colliderAIt = colliders_.begin(), end = colliders_.end();
colliderAIt != end;
++colliderAIt)
{
ColliderList::const_iterator colliderBIt = colliderAIt;
for (++colliderBIt; colliderBIt != end; ++colliderBIt)
{
Collider *colliderA = *colliderAIt;
Collider *colliderB = *colliderBIt;
if (CollisionTest(colliderA, colliderB))
{
game->DoCollision(colliderA->entity, colliderB->entity);
}
}
}
}

First-chance exception at 0x00CFD250 in AsteroidsTest_2012.exe: 0xC0000005: Access violation reading location 0xFEEEFEF6.

OriginalGriff 19-Sep-18 7:05am

You really should ask that as a new question, but at a guess - and I can't run your code so that's all it can be - one of your pointers isn't right.

Use the debugger to examine what is happening while you code runs and that should help you find out what pointer is bad, and that should help you find why it's bad!

Siddhant Bhati 19-Sep-18 8:59am

Hey Thanks,

i think in this for (++colliderBIt; colliderBIt != end; ++colliderBIt)
{
Collider *colliderA = *colliderAIt;
Collider *colliderB = *colliderBIt;
if (CollisionTest(colliderA, colliderB))
{
game->DoCollision(colliderA->entity, colliderB->entity);
}
}

ColliderA wouldn't be receiving values or getting any memory allocated. Right? or Everytime it might be getting the same value of ColliderAIt.

This might be a problem?

OriginalGriff 19-Sep-18 10:00am

Check with the debugger - it will show you exactly what is in what variable, and it's the only way to be sure.
You assign a value to colliderA (from colliderAlt) but I have no way of knowing what the heck that contains! :laugh:

Seriously, you need to use the debugger - it's the easiest way to find out instead of guessing.

Siddhant Bhati 20-Sep-18 1:33am

It says unable to read memory for radius, entity and enabled. Possibly they are empty that's why

OriginalGriff 20-Sep-18 3:42am

So look at why. Set breakpoints where they should be being set and see what happens.

Siddhant Bhati 25-Sep-18 2:35am

I tried debugging it but I failed to understand what the issue is -:

void Collision::DoCollisions(Game *game) const
{
for (ColliderList::const_iterator colliderAIt = colliders_.begin(), end = colliders_.end();
colliderAIt != end;
++colliderAIt)
{
ColliderList::const_iterator colliderBIt = colliderAIt;
for (++colliderBIt; colliderBIt != end; ++colliderBIt)
{
Collider *colliderA = *colliderAIt;
Collider *colliderB = *colliderBIt;
if (CollisionTest(colliderA, colliderB))
{
game->DoCollision(colliderA->entity, colliderB->entity);
}
}
}
}

this is Collision.cpp

void Game::DoCollision(GameEntity *a, GameEntity *b)
{
Ship *player = static_cast<Ship *>(a == player_ ? a : (b == player_ ? b : 0));
Bullet *bullet = static_cast<Bullet *>(IsBullet(a) ? a : (IsBullet(b) ? b : 0));
Asteroid *asteroid = static_cast<Asteroid *>(IsAsteroid(a) ? a : (IsAsteroid(b) ? b : 0));

if (player && asteroid)
{
AsteroidHit(asteroid);
DeletePlayer();
}

if (bullet && asteroid)
{
AsteroidHit(asteroid);
DeleteBullet(bullet);
}
}

this is Game.cpp

bool Collision::CollisionTest(Collider *a, Collider *b)
{
if (a->enabled == false)
return false;
if (b->enabled == false)
return false;

D3DXVECTOR3 diff;
D3DXVec3Subtract(&diff, &a->position, &b->position);
float distance = D3DXVec3Length(&diff);
if (distance < (a->radius + b->radius))
{
return true;
}

return false;
}

this is collisionTest in Collision.cpp

OriginalGriff 25-Sep-18 2:54am

We can't run that code under the same conditions that you do, so we can't duplicate your problem.

That's why you have a debugger - so you can find out exactly what "is empty" and then use the debugger to trace back through your code to find out why. But that needs you code running and having the problem - and the only person who can have that is you!

So what - exactly - did you do with the debugger, and what - exactly - did it show?

Siddhant Bhati 25-Sep-18 3:35am

Oh I just used the debugger and the debugger shows that it is unable to read memory of rotation angle, size of the asteroid and the rotation of the main player too. I think there's some issue with it.

OriginalGriff 25-Sep-18 3:39am

How did you use the debugger?
Where did you put a breakpoint?
What - exactly - did it show you?
What was in the various variables?

It's sounding like you have no idea what the debugger is, much less how to use it! :laugh:

Siddhant Bhati 25-Sep-18 5:06am

I used the breakpoints where the colliders get initialized. For eg. Asteroid *asteroid = static_cast<Asteroid *>(IsAsteroid(a) ? a : (IsAsteroid(b) ? b : 0)); here
when I look at the locals it gives correct values of a and b but when I look at the components of the Player and Asteroid, it shows that it is unable to read the memory of angle, angular speed and size. Same for the player

OriginalGriff 25-Sep-18 5:16am

Put the breakpoint at the top of the method, and step through the instructions.
You need to look at what exactly come in as A and B, and what they end up setting player, bullet, and asteroid as. Because if you start looking at your code, it doesn't look at angle, speed, or size! So either that isn't the code throwing the error, or you are assuming something which isn't necessarily true. Don't assume; check!

Siddhant Bhati 25-Sep-18 5:32am

In A Asteroid is getting initialized and in B bullet is getting initialized even when I am not shooting.

Also, I have this method too
void Game::AsteroidHit(Asteroid *asteroid)
{
int oldSize = asteroid->GetSize();
if (oldSize > 1)
{
int smallerSize = oldSize -1;
D3DXVECTOR3 position = asteroid->GetPosition();
SpawnAsteroidAt(position, smallerSize);
SpawnAsteroidAt(position, smallerSize);
}
DeleteAsteroid(asteroid);
}

are you sure Size wouldn't affect it because it was unable to read the size.

OriginalGriff 25-Sep-18 5:46am

And why do you think that is?
Have you checked to find out where the objects you pass to that method are coming from?

Since they are - it seems - the wrong objects, you need to know what they are supposed to be!

Siddhant Bhati 25-Sep-18 6:03am

A and B should be either asteroid or player till I am not shooting any bullet. yes I have checked where they are coming from- from Collision.cpp

From here -:
void Collision::DoCollisions(Game *game) const
{
for (ColliderList::const_iterator colliderAIt = colliders_.begin(), end = colliders_.end();
colliderAIt != end;
++colliderAIt)
{
ColliderList::const_iterator colliderBIt = colliderAIt;
for (++colliderBIt; colliderBIt != end; ++colliderBIt)
{
Collider *colliderA = *colliderAIt;
Collider *colliderB = *colliderBIt;
if (CollisionTest(colliderA, colliderB))
{
game->DoCollision(colliderA->entity, colliderB->entity);
}
}
}
}

OriginalGriff 25-Sep-18 6:23am

And what does the debugger show there are there?

Come on, this is going to take ages if I have to tell you to check each variable all the way back... :laugh:

Siddhant Bhati 25-Sep-18 6:27am

Haha, it's showing A as Asteroid and B as bullet when the game starts. Is this what you are asking right?

OriginalGriff 25-Sep-18 6:46am

So how is your bullet getting where it shouldn't?

Come on - follow the code while it runs and it should show up somewhere...

Siddhant Bhati 25-Sep-18 6:54am

Can I reach out to you personally? I mean mail or anything?

OriginalGriff 25-Sep-18 7:01am

No, sorry!