18 Defining Typedef Names

You can define a data type keyword as an alias for any type, and then use the alias syntactically like a built-in type keyword such as int. You do this using typedef, so these aliases are also called typedef names.

typedef is followed by text that looks just like a variable declaration, but instead of declaring variables it defines data type keywords.

Here’s how to define fooptr as a typedef alias for the type struct foo *, then declare x and y as variables with that type:

typedef struct foo *fooptr;

fooptr x, y;

That declaration is equivalent to the following one:

struct foo *x, *y;

You can define a typedef alias for any type. For instance, this makes frobcount an alias for type int:

typedef int frobcount;

This doesn’t define a new type distinct from int. Rather, frobcount is another name for the type int. Once the variable is declared, it makes no difference which name the declaration used.

There is a syntactic difference, however, between frobcount and int: A typedef name cannot be used with signed, unsigned, long or short. It has to specify the type all by itself. So you can’t write this:

unsigned frobcount f1;  /* Error! */

But you can write this:

typedef unsigned int unsigned_frobcount;

unsigned_frobcount f1;

In other words, a typedef name is not an alias for a keyword such as int. It stands for a type, and that could be the type int.

Typedef names are in the same namespace as functions and variables, so you can’t use the same name for a typedef and a function, or a typedef and a variable. When a typedef is declared inside a code block, it is in scope only in that block.

Warning: Avoid defining typedef names that end in ‘_t’, because many of these have standard meanings.

You can redefine a typedef name to the exact same type as its first definition, but you cannot redefine a typedef name to a different type, even if the two types are compatible. For example, this is valid:

typedef int frobcount;
typedef int frotzcount;
typedef frotzcount frobcount;
typedef frobcount frotzcount;

because each typedef name is always defined with the same type (int), but this is not valid:

enum foo {f1, f2, f3};
typedef enum foo frobcount;
typedef int frobcount;

Even though the type enum foo is compatible with int, they are not the same type.