无法取消引用插入向量中的类对象指针

Question

struct Tree_Node
{
    typedef int node_value_type;

    Tree_Node(node_value_type key) : value(key) {}

public:
    node_value_type value = 0;
    Tree_Node* right = nullptr;
    Tree_Node* left = nullptr;
};

After inserting pointers to the above class into a vector, trying to pop an element and dereference it gives me an ``Access Violation Reading Location'' exception in Visual C++.

vector<Tree_Node*> node_list(2);

Tree_Node* node1 = new Tree_Node(5);

node_list.push_back(node1);

cout << node_list.front()->value;

However, the following works fine:

Tree_Node* node2 = node1;

cout << node2->value << endl;

What am I missing out on here?

Please note that I am adding and accessing the vector elements inside the same scope. Additionally, I am also aware of possible memory leaks and cleaning up. I just wish to know why the above code isn't working

Answer 1

vector<Tree_Node*> node_list(2);

it creates a vector with two null pointers.

node_list.push_back(node1);

it inserts a third element

cout << node_list.front()->value;

it tries to access the first, which is a null pointer

---

To reserve space without inserting elements use:

vector<Tree_Node*> node_list;
node_list.reserve(2);

But I'm almost sure that the default ctor reserves place for more than 2 elements.

You can also update the two allocated elements:

vector<Tree_Node*> node_list(2);
node_list[0] = node1;
node_list[1] = node2;

And if you want to make sure that your container contains only two elements use an std::pair or (if you can use C++11) an std::array .