Forward Declaration

Sometimes you have a struct that may need to reference itself, or be used recursively.

For example, consider a Node struct that can contain other Nodes. This might be useful for building a linked list or a tree:

typedef struct Node {
  int value;
  node_t *next;
} node_t;

The problem here is that node_t is not defined yet, so the compiler will complain. To fix this, we can add a forward declaration. A forward declaration lets the compiler know about the existence of a struct type before it's fully defined:

typedef struct Node node_t;

typedef struct Node {
  int value;
  node_t *next;
} node_t;

Note that the forward declaration must match the eventual definition, so you can't do something like this:

typedef struct Node node_t;

typedef struct BadName {
  int value;
  node_t *next;
} node_t;