列表向量

Question

So I am trying to write a function that takes two arguments, list<item> myList , and int num_items and creates a vector of new lists. Each item has data member ID, which is the index in the vector of which list the item should be added to. The ID of any item is guaranteed to be no greater than (num_items - 1). Here is my code:

vector<list<item>> createVector(list<item> myList, int num_items) {
   vector<list<item>> myVector;
   myVector.reserve(num_items);

   for(item& i : myList)
      myVector[i.ID].push_back(i);

   return myVector;
}

This is causing a segmentation fault whenever I try to call push_back() to add an item to a list in the vector. I know how to implement the function using pointers, however the type returned by the function must be an actual vector of lists not pointers to them, so I can't use dynamic memory allocation. Any help would be greatly appreciated. Thanks.

Answer 1

reserve() merely allocates a block of memory. It does not create the list objects that you are calling push_back() on. You need to use resize() instead of reserve() to create them:

myVector.resize(num_items);

Or better, pass the count to the constructor directly instead:

vector<list<item>> myVector(num_items);

Answer 2

reserve only reserves space for that many vector elements; it doesn't create any elements, so you can't access them. You want to use resize (or just pass the size as a constructor argument), to populate the vector with valid, empty lists.