Question

For certain objects (elements) in the vector aArray, I want to create a mininum sorted heap.

1. I want to modify the members of aArray in other functions and call Heap member functions to update the min heap. This means that I should pass in a constant vector to the Heap structure, but I am having a lot of trouble with this am being swamped with errors. What is the proper way to pass constant objects to Heap constructor?

2. Also, when I call the function popHeap, I want to delete only the pointer to the root in the heap, but it deletes the object in vector aArray also. How can I fix this?

   #include <vector>
   #include <iostream>
   
   struct A
   {
       A(int av, int bv):a(av),b(bv){}
       int a, b;
   };
   
   struct Heap
   {
      Heap() : ptr(new std::vector<A>()) {}
   
      Heap(std::vector<A> *p) : ptr(p) {}
   
      void makeHeap()
      { // some code here }
   
      void popHeap()
      {
          ptr->erase(ptr.begin()+heapLoc[0]);  //DELETES aArray member! I only want to delete the pointer, not the object. 
          //some code here   
      }
   
      std::vector<A> *ptr;
      std::vector<int> heapLoc;
   };
   
   
   int main()
   {
        A a0(2,5), a1(4,2), a2(8,4), a3(0,3);  
        std::vector<A> aArray;  
        aArray.push_back(a0);  aArray.push_back(a1);  aArray.push_back(a2);   aArray.push_back(a3);
   
        for(int i=0; i<aArray.size(); ++i)
        {
           std::cout << "aArray[i].a = " << aArray[i].a << " "
                     << "aArray[i].b = " << aArray[i].b <<  "\n";
        } //All 4 objects outputted
   
        Heap h(&aArray);
        h.makeHeap();
   
        h.popHeap();
   
        for(int i=0; i<aArray.size(); ++i)
        {
           std::cout << "aArray[i].a = " << aArray[i].a << " "
                     << "aArray[i].b = " << aArray[i].b <<  "\n";
        } //Only 3 objects outputted. One gets deletated. 
   }
   

Answer 1

Copy the entire passed vector in the constructor (don’t just have a pointer to it). Allowing anything to modify the internal structure of something that needs to have a specific structure (as in sorted elements) to work is a bad idea. You can do this simply as follows:

class Heap
{
   ...
   Heap(std::vector<A> &input): vect(input) { };
   std::vector<A> vect;
}

You will need to either have a copy constructor or override the = operator for A for the above to work, but doing both doesn’t hurt. As in:

class A
{
   ...
   A(const A &o): a(o.a), b(o.b) {}; // Copy constructor
   void operator=(const A &o): a(o.a), b(o.b) {}; // = operator
}

Adding both pieces of code will copy all the elements in the vector in the heap’s constructor. Pointers won’t work for this since are always be shallow-copied, unless you explicitly deep-copy them, which is a bit more effort.

An alternative to this is always starting with an empty heap and having an add method (still not using pointers).

If you don’t want to copy elements, you can use the const keyword. As in use const A everywhere in your code where you used A.

Note 1 – I’d suggest getting rid of the makeHeap method and just doing that work in the constructor.

Note 2 – I used class above, and not struct. Don’t use struct in C++, use class instead. struct is more C.