c ++在创建新结构时如何为成员指针赋值NULL

Question

I'm writing a Run-length encoding project using linked list. Is it possible next to have value NULL every time i create a Node using new ?

I have

struct Node{
    int x;
    char c;
    Node* next;
}

and

class RLElist{
private:
    Node* startList;
public:
    /*some member functions here*/
}

I need it to be NULL so i can check if I've reached the last Node of the list.

Answer 1

Yes. And as usual with something that is possible, in this case too there is more than single way to do it.

---

If you call a new operator with value initilization semantics

Node* n = new Node();

than value initialization will be triggered and this will assign 0 value to each structure's data member if there is no user defined constructor in the structure.

---

You can also define a default constructor that will assign null to pointer ( and maybe do something else as well)

struct Node{
    int x;
    char c;
    Node* next;
    Node() : next( 0) {}
}

and use this as before

Node* n = new Node();  // your constructor will be called

---

And finally, you can initialize pointer in the place of it's declaration

struct Node{
    int x;
    char c;
    Node* next = nullptr;
};

Answer 2

There are different options:

Add a constructor that value-initializes the pointer (which leaves it zero-initialized):

struct Node{
    Node() : next() {}  // you can also value initialize x and c if required.
    int x;
    char c;
    Node* next;
};

Initialize at the point of declaration:

struct Node{
    int x;
    char c;
    Node* next = nullptr;
};

Value-initialize the new ed object:

node* Node = new Node(); // or new Node{};