How do I insert a element into a std::unordered_map<int, vector<Object*>>

Question

I'm trying to create a hash of arrays of pointers to my object.

The hash key is an int for the type of the object, and the array is a list of the objects to render.

What I'm trying to do is:

unordered_map<int, vector<Object*> > drawQueue;
drawQueue.clear(); // new empty draw queue

for ( ... ) {
   drawQueue.at(type).push_back(my_obj);
}
 

So I'm not familiar enough with the nuances of the STL stuff, since I get an exception saying out_of_bounds, which is what happens when the key doesn't exist.

So I figured I need to create the key first, and then add to the vector:

if (drawQueue.count(type)) {
    // key already exists
    drawQueue.at(type).push_back(my_obj);
} else {
    //key doesn't exist
    drawQueue.insert(type, vector<Object*>); // problem here
    drawQueue.at(type).push_back(my_obj);
}

But now I'm really lost, as I don't know how to create/initialise/whatever an empty vector to the insert of the unordered_map ...

Or am I doing this the entirely wrong way?

Answer 1

You are not using insert in the proper way. This should work:

drawQueue.insert(std::make_pair(type, std::vector<Object*>()));

If using C++11, the previous statement can be simplified to:

drawQueue.emplace(type, std::vector<Object*>());

By using this approach the element is constructed in-place (ie, no copy or move operations are performed).

I also include links to the documentation forinsert and emplace .

Answer 2

I think this is an easy approach. My example will create an unordered_map string as key and integer vector as values.

unordered_map<string,vector<int>> keys;
keys["a"] = vector<int>(); // Initialize key with null vector
keys["a"].push_back(1); // push values into vector.
keys["a"].push_back(5);    
for(int i : keys["a"] ){
    cout << i << "\t";
}

Answer 3

I think you could simplify it by

drawQueue[type].push_back(my_obj);

The operator [] would do the insert for you if the key is not found.