typedef and #define

While browsing I found someone asking “difference between typedef and #define”. I thought a moment, why the person is asking difference between two different entities altogether.  Typedef is used to give different names to existing data types [int, char, float etc]. Whereas #define is a preprocessor directive which replaces identifier with token.

Look at below c programs.

Program 1

#include <stdio.h>

 

#define type int

 

type main()

{

        type count=10;

        printf("count = %d",count);

        return 0;

}

 

Program 2

#include <stdio.h>

 

typedef int type;

 

type main()

{

        type count=10;

        printf("count = %d",count);

        return 0;

}

 

In the above two programs does typedef and #define behave differently?

 

Both are replacing word "type" with “int”.Finally the end result is same. But the replacement process happens in different phases. #define replaces word "type" with ” int” in preprocessing stage, typedef replaces word "type" with “int” in compilation stage.

 

I was trying below programs while thinking of difference between typedef and #define.

 

Program 1

 

#include <stdio.h>

typedef int int

 

int main()

{

        int count=10;

        printf("count = %d",count);

        return 0;

}

 

If you compile this program it ends up in error.

error: two or more data types in declaration specifiers

error: two or more data types in declaration specifiers

error: function definition declared typedef

 

Program 2

 

#include <stdio.h>

 

#define int int

 

int main()

{

        int count=10;

        printf("count = %d",count);

        return 0;

}

 

If you compile this program, it compiles successfully and displays correct output. So we can add this difference too.