CODEWITHSUNDEEP
cstructtypedef
wandergeek
wandergeek

Reputation: 21

typedef confusion in C

I'm having a hard time understanding the typedefs in this C structure. 

typedef struct node {
   int   value;
   list  rest;
} node;

typedef struct node *list;

What's the difference between the "node" typedef declaration and the "list" declaration? Why is list prefaced as a pointer? Isn't "node" a pointer as well? Why can't I simply say "typedef struct node list" and omit the asterisk? I've been searching everywhere and I can't really find a satisfactory answer.

Upvotes: 2

Views: 456

Answers (5)

Matteo Italia
Matteo Italia

Reputation: 126957

The first typedef defines node as an alias for struct node, to allow you to refer to it simply as node without writing struct node every time (in C "regular" type names and struct names live in two different namespaces). It's equivalent to:

struct node
{
    int value;
    struct node* rest;
};

typedef struct node     node;

The second typedef, instead, defines list as an alias for node *, i.e. defines the type list as a pointer to a node structure.

(by the way, personally I find that this is very bad style: hiding pointers inside typedefs is almost always a bad idea; one can argue that a pointer to the first element in a list could be identified as the list, but the usage of list even for the rest pointer is IMHO not very nice)

Upvotes: 5

Dan Fego
Dan Fego

Reputation: 14034

node is a struct, which is (confusingly) named the same thing as struct node. list is a pointer to struct node. So the following are equivalent:

struct node *a;
node *a;
list a;

Upvotes: 1

Paul R
Paul R

Reputation: 213200

node is just a synonym for struct node

list is a pointer to a struct node (or node)

Each node instance contains a list pointer which enables you to build data structures such as (singly) linked lists, etc.

Upvotes: 0

wallyk
wallyk

Reputation: 57804

The first declaration says that a node is the structure. The second says that a list is a pointer to a node.

So, this would be proper code using those declarations:

list  x;
node  n;

x = &n;

Declaring an item using typedef struct node list is not correct. The typedef statement declares a new type. The keyword typedef is not part of the type's name.

Upvotes: 1

Ed Swangren
Ed Swangren

Reputation: 124790

node is not a pointer; it is a struct. list is a typedef for a pointer to node.

You typedef a struct in C to avoid typing struct node ... everywhere.

Whether or not this is good practice is questionable (most will agree that hiding a pointer type behind a typedef, unless it is truly opaque, is a bad idea), but that's the gist of it.

Upvotes: 1

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