Typedef

The typedef keyword (short for type definition) in C does not create a new data type. Instead, it creates an alias (a new name) for an existing data type. This is primarily used to enhance code readability, making complex declarations shorter and more intuitive.

1. Creating Aliases for Basic Types

The simplest use of typedef is to provide a more meaningful or shorter name for a standard data type.

Syntax: typedef existing_type new_alias_name;

// Creates 'UINT' as an alias for 'unsigned int'
typedef unsigned int UINT; 

// Now you can declare variables using the alias:
UINT data_count = 50; 

2. Simplifying Structures and Unions

The most common use of typedef is to simplify the declaration of structures and unions. Without typedef, every time you declare a variable, you must use the full struct or union keyword along with the name.

A. Without typedef

struct Student {
    int id;
    float gpa;
};

// Must use 'struct Student' every time:
struct Student freshman;

B. With typedef (Preferred Method)

By using typedef on the structure definition, you can use the new alias directly, making structure usage look like a built-in type.

// Define the struct and immediately give it an alias 'Student'
typedef struct {
    int id;
    float gpa;
} Student; // <-- 'Student' is the alias name

// Now you can declare variables simply:
Student sophomore;

3. Simplifying Complex Pointers

Typedef is invaluable for simplifying the syntax of complex pointer declarations, especially pointers to functions (a concept covered later).

A. Pointer to Function without typedef

// This is a function pointer named 'op_ptr' that points to a function
// that takes two integers and returns an integer.
int (*op_ptr)(int, int); 

B. Pointer to Function with typedef

You can use typedef to create an alias for the entire pointer type, drastically improving readability.

// Defines 'IntOperation' as the TYPE of a pointer to a function 
// that returns int and takes (int, int).
typedef int (*IntOperation)(int, int); 

// Now you can declare the pointer variable cleanly:
IntOperation op_ptr; 

4. typedef vs. #define

While both can create aliases, they operate at different stages and are fundamentally different:

Example of #define pitfall:

// This creates 'PTR' as the text 'int *'
#define PTR int *

PTR a, b; // This is expanded to: int *a, b;
// 'a' is a pointer, but 'b' is a regular integer!

This error does not occur with typedef:

typedef int *IntPtr; // Creates a type alias for 'pointer to int'

IntPtr a, b; // Both 'a' and 'b' are correctly declared as pointers to int