Typedef/struct declarations

Question

What is the difference between these two declarations, if someone could explain in detail:

typedef struct atom {
  int element;
  struct atom *next;
};

and

typedef struct {
  int element;
  struct atom *next;
} atom;

Answer 1

This is Normal structure declaration

  struct atom {
      int element;
      struct atom *next;
    };    //just declaration

creation of object

 struct atom object; 

---

  struct atom {
      int element;
      struct atom *next;
    }object;    //creation of object along with structure declaration

---

And

This is Type definition of struct atom type

typedef  struct atom {
  int element;
  struct atom *next;
}atom_t;  //creating new type

Here atom_t is alias for struct atom

creation of object

atom_t object;      
struct atom object; //both the ways are allowed and same

Answer 2

The purpose of typedef is to give a name to a type specification. The syntax is:

typedef <specification> <name>;

After you've done that, you can use <name> much like any of the built-in types of the language to declare variables.

In your first example, you the <specification> is everything starting with struct atom, but there's no <name> after it. So you haven't given a new name to the type specification.

Using a name in a struct declaration is not the same as defining a new type. If you want to use that name, you always have to precede it with the struct keyword. So if you declare:

struct atom {
    ...
};

You can declare new variables with:

struct atom my_atom;

but you can't declare simply

atom my_atom;

For the latter, you have to use typedef.

Note that this is one of the notable differences between C and C++. In C++, declaring a struct or class type does allow you to use it in variable declarations, you don't need a typedef. typedef is still useful in C++ for other complex type constructs, such as function pointers.

You should probably look over some of the questions in the Related sidebar, they explain some other nuances of this subject.