Question

I am going to store some objects keyed by various numbers. Most numbers will have no objects, some will have 1, and some will have multiple.

std::map<int, std::vector<MyObject>> myObjects;
// or...
std::vector<std::vector<MyObject>> myObjects;

std::vector<MyObject> GetObjectsForNumber( int number )
{
    // how best to do this?

    if ( -check if there is a vector for the number- )
    {
        return myObjects[number];
        // or...
        return myObjects.at(number);
    }
    else
    {
        // return empty vector?
    }
}

Should I use a map or vector, and how should I implement the function?

Answer 1

What you are looking for is probably the multimap, see http://www.cplusplus.com/reference/stl/multimap/.

But you should point out what exactly your goals are – memory efficiency, performance? Also, how is the “distribution” of the values over the keys? If it’s an important decision, you should prototype.

P.S.: Don’t write

std::vector<std::vector<MyObject>> myObjects;

but rather

std::vector<std::vector<MyObject> > myObjects;  //note the space between the > >

GCC will interpret >> as operator>> otherwise.