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Using a pointer to point to a new object every frame


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#1 AussieSpoon   Members   -  Reputation: 184

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Posted 22 December 2012 - 06:08 AM

Hello,

 

I have a Sprite class with a Vector2 pointer (member) that is used for a sprite's position. It's a pointer because I'm forward declaring it's class so it has to be a pointer.

 

So I want to take in a Vector2 (parameter) and then have the pointer point to the Vector taken in without creating a new object every frame and still have it be efficient. (Or is this the way to do it?). . But I'm unsure how to do it. All I can think of is:

 

void Sprite::SetPos(const Vector2 _position)						
{
	delete m_Position;
	m_Position = new Vector2(_position);
}	

 

 

 

But I'm pretty sure it doesn't work (or does it?)

 

Please tell me if that doesn't make sense. 

 

Thanks

 



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#2 Brother Bob   Moderators   -  Reputation: 7744

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Posted 22 December 2012 - 06:22 AM

You should definitely consider if you really want to have a single primitive thing like a vector to be a pointer. Why do you only have a forward declaration of the vector and not the full class?



#3 AussieSpoon   Members   -  Reputation: 184

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Posted 22 December 2012 - 06:43 AM

You should definitely consider if you really want to have a single primitive thing like a vector to be a pointer. Why do you only have a forward declaration of the vector and not the full class?

For cleanliness reasons (I don't want 100 lines of code before the actual class

 

and I'm trying to use forward declarations so I don't have a whole bunch of #includes (it can slow down compile time, it's just a practice thing for me though) . 



#4 AussieSpoon   Members   -  Reputation: 184

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Posted 22 December 2012 - 06:56 AM

Managed to figure it out by creating the object in the classes constructor, thanks 



#5 Trienco   Crossbones+   -  Reputation: 2039

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Posted 22 December 2012 - 07:28 AM

Forward declarations should be used if you only need a pointer or reference to minimize includes. Making every member a pointer and basically screwing up your design, just so you can use forward declarations isn't going to make your code cleaner. The consequences will be far messier than just having that include. Are you deleting the object behind the pointer in the destructor? Did you create a copy constructor and assignment operator to prevent crashes from deleting it twice (or at least make your class noncopyable)? That's a lot of extra code and effort just to be able to avoid that one include.

 

I'm also not sure how it's supposed to save you code. Whether it's "include xxx" or "class xxx", you are going to have this one line of code.


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#6 Acid-Chris   Members   -  Reputation: 440

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Posted 22 December 2012 - 09:37 AM

There's much much more wrong in that piece of code than just a plain pointer issue.

 

Besides the allocations you do every frame, there are also a lot more copy-constructor calls than you might expect.

Get rid of that; it may be just a Vec2 class but clean code doesnt hurt anyone.  use const references, or just pass the position as 2 parameters.

 

Next point is ... why do you want to use a pointer anyway? Sprite classes usually store their position for collision detection or rendering or whatever.

And you do realize that pointers take up memory as well, right?  Depending on the OS, the size is different. On Windows it's 4 byte if i remember correctly.

So basically you win nothing using your method.

 

best regards



#7 FlyingDutchman   Members   -  Reputation: 207

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Posted 22 December 2012 - 09:59 AM

There's much much more wrong in that piece of code than just a plain pointer issue.

 

Besides the allocations you do every frame, there are also a lot more copy-constructor calls than you might expect.

Get rid of that; it may be just a Vec2 class but clean code doesnt hurt anyone.  use const references, or just pass the position as 2 parameters.

 

Next point is ... why do you want to use a pointer anyway? Sprite classes usually store their position for collision detection or rendering or whatever.

And you do realize that pointers take up memory as well, right?  Depending on the OS, the size is different. On Windows it's 4 byte if i remember correctly.

So basically you win nothing using your method.

 

best regards

 

true. And talking about efficiency. Malloc/new are quite expensive in terms of computation cost (if you compare it to just decreasing the stack pointer) and also lead to heap fragmentation. 


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#8 FLeBlanc   Crossbones+   -  Reputation: 3081

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Posted 22 December 2012 - 10:15 AM

You should definitely consider if you really want to have a single primitive thing like a vector to be a pointer. Why do you only have a forward declaration of the vector and not the full class?

For cleanliness reasons (I don't want 100 lines of code before the actual class

 

and I'm trying to use forward declarations so I don't have a whole bunch of #includes (it can slow down compile time, it's just a practice thing for me though) . 

 

Trading run-time performance away for better compile-tile performance is just a bad optimization to make. Bad, bad, bad. Don't do that. 



#9 SiCrane   Moderators   -  Reputation: 9337

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Posted 22 December 2012 - 10:22 AM

Note that you can still use assignment on an object that you have a pointer to (assuming the assignment operator wasn't made non-public or otherwise inaccessible). In your case you can do *m_Position = _position; which would update the value without requiring a new allocation.

#10 MarkS   Prime Members   -  Reputation: 875

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Posted 22 December 2012 - 12:59 PM

I'm lost...
//vector2.hpp
#ifndef VECTOR2_HPP
#define VECTOR2_HPP

class vector2{
//stuff...
};

#endif
//sprite.hpp
#ifndef SPRITE_HPP
#define SPRITE_HPP

#include "vector2.hpp"

class sprite{
private:
vector2 m_position;
};

#endif

What am I missing here?

Edited by MarkS, 22 December 2012 - 01:05 PM.


#11 iMalc   Crossbones+   -  Reputation: 2250

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Posted 22 December 2012 - 07:31 PM

I'm trying to use forward declarations so I don't have a whole bunch of #includes (it can slow down compile time, it's just a practice thing for me though) .
Sounds to me like you've had enough practice on how to do it, and now you could use some practice with when it's appropariate to use it.
This is not really an appropriate place to do it, and you certainly shouldn't be deleting and newing up a new one each time. At the very least, you'd use copy-assignment.

Edited by iMalc, 22 December 2012 - 07:32 PM.

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#12 AussieSpoon   Members   -  Reputation: 184

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Posted 22 December 2012 - 07:57 PM

Ok so based on what everyone is saying I'm just making things harder for myself (which is obvious). So I will just put the all the required #includes in the Sprite.h

 

The only question I have is how do I know when I should use forward declartions and when is it ok to #include in a header? (I know you can do the later whenever but I have heard people say try to avoid it) 

 

Thanks for all the replies. 


Edited by AussieSpoon, 22 December 2012 - 07:58 PM.


#13 Ectara   Crossbones+   -  Reputation: 2743

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Posted 23 December 2012 - 01:01 AM

I use forward declaration when it will only ever need a pointer to the object (like passing a pointer reference, or writing a container or a linked list), the core functionality is privately implemented with structure that the user never sees (thus can't include the definition), or for circular references.

Most of these apply to coding in C, where there isn't always a better way to do it, but the first one must be stressed: forward declaration is doable if you only ever need a pointer to the object. Don't decide to only use a pointer just to use forward declaration, because that's backwards. Always judge whether or not you need the full definition or not based on how the interface will be designed; don't design the interface for one or the other.



#14 Aardvajk   Crossbones+   -  Reputation: 5257

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Posted 23 December 2012 - 03:12 PM

I take the view that if I only need to use pointers or references in the including header, I use a forward reference. As soon as I find myself naturally needing anything more than a pointer or reference, I include the file unless I have a very good reason not to (circular dependancy etc).

Sometimes if the first case is true except for also needing an enumeration type or something else defined in the header, I'll consider splitting the header out.

Compile time is a serious consideration for me at work as a full rebuild of our source takes about 20 minutes for me, which is a long time to be paid to twiddle my thumbs and check email. However, even bearing this in mind, I'd not compromise code design or runtime efficiency in order to improve compile time with the abuse of forward declarations.

Edited by Aardvajk, 23 December 2012 - 03:13 PM.


#15 iMalc   Crossbones+   -  Reputation: 2250

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Posted 23 December 2012 - 07:26 PM

If your SetPos function were to take _position as a const reference then as long as you keep the actual function implementation to the cpp file, then you can get away with just a forward declaration in the header file.
Oh, except that you would not be able to have m_Position as a member of that class - duh um ignore that.

Edited by iMalc, 23 December 2012 - 07:28 PM.

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#16 SiCrane   Moderators   -  Reputation: 9337

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Posted 23 December 2012 - 07:35 PM

Actually, it's legal to declare a function that takes an object parameter by value with just a forward declaration. It's just not legal to call it or define it with only the forward declaration.

#17 Khatharr   Crossbones+   -  Reputation: 2771

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Posted 23 December 2012 - 11:02 PM

Actually, it's legal to declare a function that takes an object parameter by value with just a forward declaration. It's just not legal to call it or define it with only the forward declaration.

 

It was a joyous day when I figured that one out, my friend.


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