Quad tree recursive retrieve

Question

I am creating a quadtree and I get in some trouble with the retrieve function. This function gets down to the nodes that store objects and puts the objects in a vector called relevantObjects. After this, it returns the relevantObjects vector. But, when it tries to do this, I see in the debugger that the vector gets wiped of its elements (goes from 4 to 0).

I don't see where I am wrong.

std::vector<PTR> Tree::retrieveObjects(PTR p, std::vector<PTR> relevantObjects) {

int quadrant = getQuadrant(p);

if (quadrant != -1 && nodes[0] != nullptr)
{
    nodes[quadrant]->retrieveObjects(p, relevantObjects);
}

relevantObjects.insert(relevantObjects.end(), storedObjects.begin(), storedObjects.end());


return relevantObjects; }

You discard any nodes from the recursive calls (by not using the function's return value), also consider passing the vector by reference all the way down and then only returning it at the top level (splitting the internal recursive function from the public one that returns the vector) — Borgleader, Jan 12 '17 at 14:28

score 0 · Accepted Answer · answered Jan 12 '17 at 14:49

In your recursive function std::vector<PTR> Tree::retrieveObjects(PTR p, std::vector<PTR> relevantObjects), you pass parameter relevantObjects by value; with each call of nodes[quadrant]->retrieveObjects(p, relevantObjects), a copy of vector relevantObjects is made, an the function then operates on the copy.

Within you function, you do not make use of the result of [quadrant]->retrieveObjects(p, relevantObjects), such that the operations on the respective copy of relevantObjects get lost; Your function will return a copy of the very first input to relevantObjects after having executed relevantObjects.insert(relevantObjects.end(), storedObjects.begin(), storedObjects.end());.

To solve the problem, simply make parameter relevantObjects a "call by reference", i.e. change your signature to std::vector<PTR> Tree::retrieveObjects(PTR p, std::vector<PTR> &relevantObjects), and it should work.

BTW: if you pass the vector by reference, it is unnecessary to return the result; a signature like void Tree::retrieveObjects(PTR p, std::vector<PTR> &relevantObjects) is sufficient.

Thanks! That was so obvious and yet I missed to see it. – marete Jan 12 '17 at 15:14 — marete, Jan 12 '17 at 15:14

Quad tree recursive retrieve

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