Assign a pointer to a struct that is a member of a struct of the same type to another pointer to a struct of the same type

Question

This question sounds super confusing even for me, and it may seem obvious or already answered, but I have searched a lot and although I found interesting things, I didn\'t find

Answer 1

In c++, when you say struct Node, Node [immediately] becomes a type. So, you could say:

struct Node {
    char keyLine[100];
    int occurrences;
    Node *leftChild;
    Node *rightChild;
    Node *parent;
};

struct Tree {
    Node *root;
};

int
insertNode(Tree *myTree, Node *newNode)
{
    Node *currentNode = myTree->root;
    if (caseSenCmpString(newNode->keyLine, currentNode->keyLine) == -1) {
        currentNode = currentNode->leftChild;
    }
}

---

But, in c, it is merely in the "tag" namespace and does not define a type. Thus, you want:

typedef struct Node {
    char keyLine[100];
    int occurrences;
    struct Node *leftChild;
    struct Node *rightChild;
    struct Node *parent;
} Node;

typedef struct Tree_struct {
    Node *root;
} Tree;

int
insertNode(Tree *myTree, Node *newNode)
{
    Node *currentNode = myTree->root;
    if (caseSenCmpString(newNode->keyLine, currentNode->keyLine) == -1) {
        currentNode = currentNode->leftChild;
    }
}

---

As an alternative, you can use a forward declaration:

// forward declaration
struct Node;
typedef struct Node Node;

struct Node {
    char keyLine[100];
    int occurrences;
    Node *leftChild;
    Node *rightChild;
    Node *parent;
};

typedef struct Tree_struct {
    Node *root;
} Tree;

int
insertNode(Tree *myTree, Node *newNode)
{
    Node *currentNode = myTree->root;
    if (caseSenCmpString(newNode->keyLine, currentNode->keyLine) == -1) {
        currentNode = currentNode->leftChild;
    }
}

---

Note that the struct name does not have to match the type name:

// forward declaration
struct Node_struct;
typedef struct Node_struct Node;

struct Node_struct {
    char keyLine[100];
    int occurrences;
    Node *leftChild;
    Node *rightChild;
    Node *parent;
};

typedef struct Tree_struct {
    Node *root;
} Tree;

int
insertNode(Tree *myTree, Node *newNode)
{
    Node *currentNode = myTree->root;
    if (caseSenCmpString(newNode->keyLine, currentNode->keyLine) == -1) {
        currentNode = currentNode->leftChild;
    }
}

---

To allow cross linking of your two structs, we could do:

// forward declaration
struct Node_struct;
typedef struct Node_struct Node;

struct Tree_struct;
typedef struct Tree_struct Tree;

struct Node_struct {
    char keyLine[100];
    int occurrences;
    Node *leftChild;
    Node *rightChild;
    Node *parent;
    Tree *tree;
};

struct Tree_struct {
    Node *root;
};

int
insertNode(Tree *myTree, Node *newNode)
{
    Node *currentNode = myTree->root;
    if (caseSenCmpString(newNode->keyLine, currentNode->keyLine) == -1) {
        currentNode = currentNode->leftChild;
    }
}