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pointers while serializing


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#1 bollµ   Members   -  Reputation: 354

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Posted 09 February 2013 - 09:13 AM

Hi all!

 

I've been reading about serialization, and in multiple places, I've seen people take about "reinitializing pointers" on serializing classes that have pointers within them.

 

How exactly would one implement this? I mean, I don't get it. don't pointers point to locations in virtual memory? and don't these locations change every time the program is loaded? so how can someone "bake" this data?

 

could someone please explain this to me?

 

Thanks

~Bollu


a WIP 2d game engine: https://code.google.com/p/modulusengine/

English is not my first language, so do feel free to correct me :)


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#2 Ectara   Crossbones+   -  Reputation: 2968

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Posted 09 February 2013 - 09:36 AM

They likely meant to serialize it like a deep copy; directly serialize the data/class pointed to. Then, when reading it in, allocate the memory to hold the deserialized data (which would "reinitialize" the pointers), and deserialize to the newly allocated memory.



#3 frob   Moderators   -  Reputation: 21297

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Posted 09 February 2013 - 09:56 AM

It is common for serialized objects to be given a unique ID instead of an address. Basically just a number 1, 2, 3, 4 ...

When writing them out that value is used instead of the pointer. When reading them back, when it is finished, the numbers are replaced by the actual pointers on the new system -- or 're-initializing' them.
Check out my personal indie blog at bryanwagstaff.com.

#4 lawnjelly   Members   -  Reputation: 429

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Posted 09 February 2013 - 10:32 AM

You can also store the pointers as offsets from the start of a structure.

 

Then when you load this structure into memory, you can 'fixup' the pointers by adding the offset to the actual memory address of the start of the loaded structure and saving the result back, and voila, your pointers are valid again.

 

0 Start of structure
1 Pointer to Chicken (offset 3)
2 Pointer to Duck (offset 6)
3 Chicken
4 ..
5 ..
6 Duck
7 ..
8 ..

 



#5 EWClay   Members   -  Reputation: 659

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Posted 09 February 2013 - 12:14 PM

As you serialise, you can store a set of pointers. Only write the object the first time a pointer is seen. Write a unique ID for each pointer.

When deserialising, keep a map of IDs to pointers. Create the object the first time; after that get it from the map.

You can also support polymorphism if you write an ID for the type, and use a factory to create the object when deserialising.

#6 HonestDuane   Members   -  Reputation: 151

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Posted 09 February 2013 - 03:15 PM

You can also store the pointers as offsets from the start of a structure.

Then when you load this structure into memory, you can 'fixup' the pointers by adding the offset to the actual memory address of the start of the loaded structure and saving the result back, and voila, your pointers are valid again.

0 Start of structure1 Pointer to Chicken (offset 3)2 Pointer to Duck (offset 6)3 Chicken4 ..5 ..6 Duck7 ..8 ..

Or better explained in C code (And x-posted on my site)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Declare your basic struct.
typedef struct MyGameObject{
unsigned char hp;
char *name; // Doesn't matter what this is for our demo here.
} GameObject;

// Declare your game id type to make things easier later.
typedef unsigned long int GameObjectId;

// Declare the struct that contains all of the data.
typedef struct MyGameRealObject{
GameObjectId id;
GameObject gameObject;
} AllocatedGameObject;

// Function to build the allocated object.
// Param name should be allocated off the heap not the stack.
GameObject *BuildGameObject(char *name, unsigned char hp){
// Get our gameObjectId.
// Lets just Pretend the number is coming from a
// global singleton manager.
GameObjectId gameobjectIdNumber = 12345;

// Allocate enough memory for your new struct
// and a hidden game object id.
AllocatedGameObject *go = (AllocatedGameObject*)malloc(sizeof(AllocatedGameObject));

if(!go){
// barf how you like, this isn't meant to be complete.
}

go->id = gameobjectIdNumber;
go->gameObject.hp = hp;
go->gameObject.name = name;

GameObject *gameObjectToReturn = &go->gameObject;

return gameObjectToReturn;
}

// Function to get the full allocated object.
// param gameObject should be part of a AllocatedGameObject allocated off the heap.
AllocatedGameObject *GetAllocatedGameObject(GameObject *gameObject){

if(!gameObject){
return 0;
}

char *vhs = (char *)gameObject; // Makes our math easier.
AllocatedGameObject* allocated = (AllocatedGameObject*)(vhs - sizeof(GameObjectId));

return allocated;
}

// Get the gameObject's hidden id.
// param should be allocated off the heap not the stack or bad things will happen.
GameObjectId GetGameObjectId(GameObject *gameObject){
AllocatedGameObject* allocated = GetAllocatedGameObject(gameObject);

if(allocated){
return allocated->id;
}

return 0;
}

// Cleanup and destroy the allocated GameObject.
// param gameObject should be part of a AllocatedGameObject allocated off the heap.
// Per the above so should the game objects ->name.
void DestroyGameObject(GameObject *gameObject){
if(!gameObject){
return;
}

AllocatedGameObject* allocated = GetAllocatedGameObject(gameObject);

if(!allocated){
return;
}

allocated->id = 0;
allocated->gameObject.hp = 0;

if(allocated->gameObject.name){
size_t len = strlen(allocated->gameObject.name);
memset(allocated->gameObject.name, 0, len);
free(allocated->gameObject.name);
allocated->gameObject.name = NULL;
}

free(allocated);
}

int main(char* argv){
// Build game object
char *objectName = strdup("Scary Monster!");
GameObject *gameObject = BuildGameObject( objectName, 100 );
GameObjectId gameObjectId = GetGameObjectId(gameObject);

printf("\nId: %d\n", gameObjectId);
printf("Name: %s\n", gameObject->name);

// Always clean up after yourself.
DestroyGameObject(gameObject);

return 0;

}

Edited by HonestDuane, 09 February 2013 - 03:16 PM.





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