Thread: Keep track of elements inside STL vector

Hi
I would like to know what is the best way (if there is one) to keep track of an element inside an STL vector.
Consider I have some big object 3D models, that take up a lot of space each, alligned into an STL vector and each of those models may have one or more smaller elements that refer to this model, holding proprieties such as mass, position, orientation and so on.
How do I make sure the smaller propriety elements always have a valid reference/pointer/index to a model in the models vector? Is iteration through the vector, looking for a specific marker always necessary?
Thanks,
Krones

Originally Posted by King Mir

Your discription is a little unclear. Please elaborate so we can give more specific advice

The following should help to clarify. it's not actual code, but it shows what I'm trying to figure out.

Code:

struct Entity{
3DModel *model;
}

std::vector<3DModel> models;

void load_model(Model &m){
models.push_back(m);
}
void load_entity(Entity *e, const char *modelName){
e->model = find_model(&models, modelName);
}
void draw_entity(Entity *e){
draw_model(e->model);
}

void function_a(){
3DModel mA;
Entity a;

load_model(mA);
load_entity(&a, mA.name);
draw_entity(&a);
}

void function_b(){
3DModel mB1, mB2;
Entity b;

load_model(mB1);
load_entity(&b, mB1.name);
load_model(mB2); //makes b.model invalid if reallocation occurs
draw_entity(&b);
}

I am looking for a way to keep something like b.model valid, even if I add more members to the models vector.

Originally Posted by dwks

You could just declare b to be of type Entity&, i.e., a reference. A pointer would work as well.

Even if b was a pointer, is b->model still valid, even if I change the vector models? Because it points to a region in the memory that may have been moved when the push_back was called.

You probably shouldn't be storing 3DModel objects in the vector. If it is big, it is probably expensive to copy, and that's what vector will do with it.

Instead, I'd hold pointers. Then, you'd never have to "find" the model if you already had a pointer to it, and updates to it in one place would affect all places automatically.

There are several ways to store pointers in the vector. You could use boost's ptr_vector, or you could have a vector of shared_ptrs. If you don't like either of those solutions you could use raw pointers, although I would only do that if there is a clear place to create and destroy the models.

so a std::vector<3DModel*> would work, and I just got adding pointers to it after I load the models? Then ofc, reminding to clean it all up after I'm done with them.