C typedef

typedef gives a type a new, shorter or clearer name, like a nickname for a long title. It makes code easier to read.

In this page:

What is typedef?
Aliasing Standard Types
Aliasing Structures
Aliasing Pointers
Best Practices for typedef

Syntax

typedef existing_type new_name;

new_name variable_name = value;

What is typedef?

typedef unsigned long ulong; lets you write ulong instead of the more verbose unsigned long everywhere in your code, which becomes especially valuable for long, unwieldy type names involving pointers or function signatures.

Example: What is typedef?

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Create a type alias with typedef
typedef unsigned long ulong;
// Program execution starts in main()
int main() {
	ulong x = 100;
	// Print formatted text to the screen
	printf("%lu", x);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Aliasing Standard Types

For example, typedef int Age; doesn't create a genuinely distinct type the way some languages' type systems would -- Age is fully interchangeable with int, but naming it this way documents the variable's intended meaning at a glance.

Example: Aliasing Standard Types

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Create a type alias with typedef
typedef int Age;
// Program execution starts in main()
int main() {
	Age myAge = 25;
	// Print formatted text to the screen
	printf("%d", myAge);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Aliasing Structures

typedef struct {int x; int y;} Point; lets you later declare variables with just Point p; instead of the more verbose struct Point p; -- this shorthand is extremely common in real-world C codebases and libraries.

Example: Aliasing Structures

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Create a type alias with typedef
typedef struct {int x; int y;} Point;
// Program execution starts in main()
int main() {
	Point p = {3, 4};
	// Print formatted text to the screen
	printf("%d %d", p.x, p.y);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Aliasing Pointers

typedef int* IntPtr; lets you write IntPtr values[10]; instead of int* values[10];, though this particular use is somewhat controversial since it can obscure that a variable is actually a pointer, making code harder to read for some.

Example: Aliasing Pointers

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Create a type alias with typedef
typedef int* IntPtr;
// Program execution starts in main()
int main() {
	// Declare x and set it to 5
	int x = 5;
	IntPtr ptr = &x;
	// Print formatted text to the screen
	printf("%d", *ptr);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Best Practices for typedef

Following this convention (like typedef struct {...} Point;) makes it immediately visually clear in code that Point is a typedef'd type rather than a built-in one, helping readers distinguish your custom types from standard ones like int or char at a glance.

Example: Best Practices for typedef

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Create a type alias with typedef
typedef struct {int x; int y;} Point;
// Program execution starts in main()
int main() {
	Point p = {1, 2};
	// Print formatted text to the screen
	printf("%d %d", p.x, p.y);
	// Return 0 to signal that the program finished successfully
	return 0;
}

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