Why should we typedef a struct so often in C?

Question

I have seen many programs consisting of structures like the one below ``` typedef struct { int i; char k; } elem; elem user; ``` Why is it needed so often? Any specific reason or applicable area?

Accepted Answer

As Greg Hewgill said, the typedef means you no longer have to write `struct` all over the place. That not only saves keystrokes, it also can make the code cleaner since it provides a smidgen more abstraction.

Stuff like

```
typedef struct {
  int x, y;
} Point;

Point point_new(int x, int y)
{
  Point a;
  a.x = x;
  a.y = y;
  return a;
}
```

becomes cleaner when you don't need to see the "struct" keyword all over the place, it looks more as if there really is a type called "Point" in your language. Which, after the `typedef`, is the case I guess.

Also note that while your example (and mine) omitted naming the `struct` itself, actually naming it is also useful for when you want to provide an opaque type. Then you'd have code like this in the header, for instance:

```
typedef struct Point Point;

Point * point_new(int x, int y);
```

and then provide the `struct` definition in the implementation file:

```
struct Point
{
  int x, y;
};

Point * point_new(int x, int y)
{
  Point *p;
  if((p = malloc(sizeof *p)) != NULL)
  {
    p->x = x;
    p->y = y;
  }
  return p;
}
```

In this latter case, you cannot return the Point by value, since its definition is hidden from users of the header file. This is a technique used widely in GTK+, for instance.

UPDATE Note that there are also highly-regarded C projects where this use of `typedef` to hide `struct` is considered a bad idea, the Linux kernel is probably the most well-known such project. See Chapter 5 of The Linux Kernel CodingStyle document for Linus' angry words. :) My point is that the "should" in the question is perhaps not set in stone, after all.

Why should we typedef a struct so often in C?

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