Typedef Declarations

A typedef declaration is a declaration with typedef as the storage class. The declarator becomes a new type. You can use typedef declarations to construct shorter or more meaningful names for types already defined by C or for types that you have declared. Typedef names allow you to encapsulate implementation details that may change.

A typedef declaration is interpreted in the same way as a variable or function declaration, but the identifier, instead of assuming the type specified by the declaration, becomes a synonym for the type.

Syntax

• declaration :
  declaration-specifiers init-declarator-list _opt ;
• declaration-specifiers :
  storage-class-specifier declaration-specifiers _opt
  type-specifier declaration-specifiers _opt
  type-qualifier declaration-specifiers _opt
• storage-class-specifier :
  typedef
• type-specifier :
  void
  char
  short
  int
  long
  float
  double
  signed
  unsigned
  struct-or-union-specifier
  enum-specifier
  typedef-name
• typedef-name :
  identifier

Note that a typedef declaration does not create types. It creates synonyms for existing types, or names for types that could be specified in other ways. When a typedef name is used as a type specifier, it can be combined with certain type specifiers, but not others. Acceptable modifiers include const and volatile.

Typedef names share the name space with ordinary identifiers (see Name Spaces for more information). Therefore, a program can have a typedef name and a local-scope identifier by the same name. For example:

typedef char FlagType;

int main()
{
}

int myproc( int )
{
    int FlagType;
}

When declaring a local-scope identifier by the same name as a typedef, or when declaring a member of a structure or union in the same scope or in an inner scope, the type specifier must be specified. This example illustrates this constraint:

typedef char FlagType;
const FlagType x;

To reuse the FlagType name for an identifier, a structure member, or a union member, the type must be provided:

const int FlagType;  /* Type specifier required */

It is not sufficient to say

const FlagType;      /* Incomplete specification */

because the FlagType is taken to be part of the type, not an identifier that is being redeclared. This declaration is taken to be an illegal declaration like

int;  /* Illegal declaration */

You can declare any type with typedef, including pointer, function, and array types. You can declare a typedef name for a pointer to a structure or union type before you define the structure or union type, as long as the definition has the same visibility as the declaration.

Typedef names can be used to improve code readability. All three of the following declarations of signal specify exactly the same type, the first without making use of any typedef names.

typedef void fv( int ), (*pfv)( int );  /* typedef declarations */

void ( *signal( int, void (*) (int)) ) ( int );
fv *signal( int, fv * );   /* Uses typedef type */
pfv signal( int, pfv );    /* Uses typedef type */

Examples

The following examples illustrate typedef declarations:

typedef int WHOLE; /* Declares WHOLE to be a synonym for int */

Note that WHOLE could now be used in a variable declaration such as WHOLE i; or const WHOLE i;. However, the declaration long WHOLE i; would be illegal.

typedef struct club
{
    char name[30];
    int size, year;
} GROUP;

This statement declares GROUP as a structure type with three members. Since a structure tag, club, is also specified, either the typedef name (GROUP) or the structure tag can be used in declarations. You must use the struct keyword with the tag, and you cannot use the struct keyword with the typedef name.

typedef GROUP *PG; /* Uses the previous typedef name
                      to declare a pointer            */

The type PG is declared as a pointer to the GROUP type, which in turn is defined as a structure type.

typedef void DRAWF( int, int );

This example provides the type DRAWF for a function returning no value and taking two int arguments. This means, for example, that the declaration

DRAWF box;

is equivalent to the declaration

void box( int, int );