CODEWITHSUNDEEP
c++arraysvector
patatahooligan
patatahooligan

Reputation: 3321

C++ implementation of an array of vectors

Background: I want to implement a 3-d collision detection algorithm and would like to fragment the search space into cubes so that I only check for collisions when objects are close enough.

Actual Question: I was thinking of using an array of vectors to store pointers to the objects I am going to iterate over. For example box[0][0][0] would be a vector holding pointers to the objects in one corner of the simulation space. Regardless of whether this is an optimal solution, I am wondering how c++ handles arrays of vectors. Would the array hold pointers to the vectors so that their subsequent re-allocation have no effect on the validity of the array, or would the vectors be created inside the array and then moved out, causing undefined behavior?

Similar questions did not have answers specific to this implementation detail. Sorry if this is actually answered elsewhere and I missed it.

Upvotes: 2

Views: 159

Answers (2)

Pandrei
Pandrei

Reputation: 4951

In STL a vector is an implementation of a dynamic array ( an array that can be resided on the fly). This essentially means that the array is dynamically allocated, and the user gets a pointer to the array on the heap. When more space is needed, a new array is allocated (usually double of it's previous size), the contents of the old one is copied over and the old array freed. And that is how data consistency is handled.

Now, when you have a array of vectors, statically allocated like the question shows, you have in memory (stack, or .data section, depending where you declare this array) an or 3 vector objects, allocated one after the other in memory, each one will hold a pointer to the an array allocated on the heap.

I hope this answers your question.

Upvotes: 1

Lemming
Lemming

Reputation: 4205

An STL vector holds a pointer to a heap buffer that contains the actual data. This allows the vector to resize the buffer on demand without invalidating the vector object itself. (See documentation of vector)

So, to answer your question. An array of vectors will not become invalid if one of the vectors needs to be resized. An array of pointers to vectors would also not become invalid if one of the vectors needs to be resized.

Upvotes: 1

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