struct and typedef

Question

Are the following equivalent in C?

// #1
struct myStruct {
    int id;
    char value;
};

typedef struct myStruct Foo;

// #2
typedef struct {
    int id;
    char value;
} Foo;

If not, which one should I use and when?

(Yes, I have seen this and this .)

Answer 1

The second option cannot reference itself. For example:

// Works:
struct LinkedListNode_ {
    void *value;
    struct LinkedListNode_ *next;
};

// Does not work:
typedef struct {
    void *value;
    LinkedListNode *next;
} LinkedListNode;

// Also Works:
typedef struct LinkedListNode_ {
    void *value;
    struct LinkedListNode_ *next;
} LinkedListNode;

Answer 2

No, they're not exactly equivalent.

In the first version Foo is a typedef for the named struct myStruct .

In the second version, Foo is a typedef for an unnamed struct .

Although both Foo can be used in the same way in many instances there are important differences. In particular, the second version doesn't allow the use of a forward declaration to declare Foo and the struct it is a typedef for whereas the first would.

Answer 3

The first form allows you to refer to the struct before the type definition is complete, so you can refer to the struct within itself or have mutually dependent types:

struct node {
  int value;  
  struct node *left;
  struct node *right;
};

typedef struct node Tree;

or

struct A;
struct B;

struct A {
  struct B *b;
};

struct B {
  struct A *a;
};

typedef struct A AType;
typedef struct B Btype;

You can combine the two like so:

typedef struct node {
  int value;
  struct node *left;
  struct node *right;
} Tree;

typedef struct A AType;  // You can create a typedef 
typedef struct B BType;  // for an incomplete type

struct A {
  BType *b;
};

struct B {
  AType *a;
};