Typedef

Introduction

The typedef mechanism allows the creation of aliases for other types. It does not create new types. People often use typedef to improve the portability of code, to give aliases to structure or union types, or to create aliases for function (or function pointer) types.

In the C standard, typedef is classified as a ‘storage class’ for convenience; it occurs syntactically where storage classes such as static or extern could appear.

Syntax

• typedef existing_name alias_name;

Remarks

Disadvantages of Typedef

typedef could lead to the pollution of namespace in large C programs.

Disadvantages of Typedef Structs

Also, typedef’d structs without a tag name are a major cause of needless imposition of ordering relationships among header files.

Consider:

#ifndef FOO_H
#define FOO_H 1

#define FOO_DEF (0xDEADBABE)

struct bar; /* forward declaration, defined in bar.h*/

struct foo {
    struct bar *bar;
};

#endif

With such a definition, not using typedefs, it is possible for a compilation unit to include foo.h to get at the FOO_DEF definition. If it doesn’t attempt to dereference the bar member of the foo struct then there will be no need to include the bar.h file.

Typedef vs #define

#define is a C pre-processor directive which is also used to define the aliases for various data types similar to typedef but with the following differences:

• typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well.
• typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.
• Note that #define cptr char * followed by cptr a, b; does not do the same as typedef char *cptr; followed by cptr a, b;. With the #define, b is a plain char variable, but it is also a pointer with the typedef.