如何删除节点指针

Question

This is schoolwork. I haven't seen anything that really answers this directly, so I'm having a hard time fixing it. I have to create a linked node implementation of a max heap and I'm having difficulty with the deletion of a node after removing a value.

My Code:

template<class ItemType>
BinaryHeapNode<ItemType>* LinkedMaxHeap<ItemType>::getLastNode()
{
    BinaryHeapNode<ItemType>* lastNode = rootPtr->getRightSiblingPtr();
    BinaryHeapNode<ItemType>* prevLastNode = rootPtr;
    while(lastNode != nullptr)
{
    prevLastNode = lastNode;
    lastNode = lastNode->getRightSiblingPtr();      
}
return prevLastNode;
}

template<class ItemType>
bool LinkedMaxHeap<ItemType>::removeValue(ItemType value)
{
    BinaryHeapNode<ItemType>* tempNode = rootPtr;
    for (int i = 0; i < itemCount; i++)
    {
        if(tempNode->getItem() == value)
        {
            tempNode->setItem(getLastNode()->getItem());//set item
            delete getLastNode();                       //delete last node
            getLastNode() = nullptr;                    //set last node null
            getLastNode()->setRightSiblingPtr(nullptr); //last node should be different
            itemCount--;                                //set it's sibling to null
            heapRebuild(tempNode);
        }

        tempNode = tempNode->getRightSiblingPtr();
    }

    return true;
}

My issue is with getLastNode() = nullptr. VS is telling me that getLastNode() isn't an lvalue. That doesn't make sense to me because getLastNode is returning a pointer to a BinaryHeapNode, but it can't set that pointer to nullptr?

I thought this might be a problem with my logic of pointers (which is shaky at best) so I thought changing getLastNode() to return just a node would help. That did not. So I tried messing with the & operator and returning an address of the last node. Needless to say I haven't found the solution yet. If anyone can provide some sort of direction it would be appreciated. I'm just not entirely sure why it doesn't work.

EDIT:

Edited the code based on what arynaq mentioned. The errors went away, but now I have a bunch of linker errors I have to fix before I can test it. Will this code do what I want? I feel like it is just going to delete nodeToDelete and not get rid of the node in the heap.

template<class ItemType>
bool LinkedMaxHeap<ItemType>::removeValue(ItemType value)
{
    BinaryHeapNode<ItemType>* tempNode = rootPtr;
    BinaryHeapNode<ItemType>* nodeToDelete = getLastNode();
    for (int i = 0; i < itemCount; i++)
    {
        if(tempNode->getItem() == value)
        {
            tempNode->setItem(nodeToDelete->getItem());
            delete &nodeToDelete;
            nodeToDelete = nullptr;
            getLastNode()->setRightSiblingPtr(nullptr);
            itemCount--;
            heapRebuild(tempNode);
        }

        tempNode = tempNode->getRightSiblingPtr();
    }

    return true;
}

Answer 1

Ok, I'll try to help by explaining some things about pointers. Hopefully this will clarify some misconceptions and help you with your assignment.

When you get a copy of the pointer like so: mypointer* p = get_pointer(); and then you delete that, you are deleting the memory. But when you assign nullptr to this local variable, it wont affect the "source" of your pointer.

Here is a detailed example, showing where things can go wrong. If you never set v[0] to nullptr .

#include <iostream>
#include <vector>

struct Object {
    ~Object() {
        std::cout << "Object destructor." << std::endl;
    }
    int val = 42;
};

struct OtherObj {
    int val = 322;
};

void print_vec(const std::vector<Object*>& v) {
    for (const auto& x : v) {
        std::cout << x << std::endl;
    }
}

int main(int, char**) {
    // Init vector and print addresses.
    std::vector<Object*> v(2);
    print_vec(v);

    // Init objects in vector and printit.
    for (auto& x : v) {
        x = new Object();
    }
    print_vec(v);

    // Get a copy of a pointer and delete that. All good so far.
    Object* pointer_to_delete = v[0];
    delete pointer_to_delete;

    // Assign nullptr to the temporary local pointer.
    // Does nothing to the pointer in the vector.
    pointer_to_delete = nullptr;

    // Print the vector to prove it.
    print_vec(v);

    // On a non debug build, the memory will still have the last value.
    // Careful! Cause of headaches here. This should be set to nullptr.
    std::cout << v[0]->val << std::endl; // "No problem", certainly not nullptr.

    // Now that we allocate a new object, v[0] will be overwritten.
    OtherObj* bad_bad_boy = new OtherObj();
    // Print the address of the new object, to show it was created at
    // the old v[0] address.
    std::cout << bad_bad_boy << std::endl;

    // Bad things ensue...
    std::cout << v[0]->val << std::endl;

    return 0;
}

The output on clang is :

0x0
0x0
0x7ffa21c026c0
0x7ffa21c026d0
Object destructor.
0x7ffa21c026c0
0x7ffa21c026d0
42
0x7ffa21c026c0
322

As you can see, setting the local pointer to nullptr is not enough! I hope this clears up some things for you :)

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