Difference between typedef and #define

Difference between #define and typedef

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

• The typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, like you can define 1 as ONE etc.

 

• The typedef interpretation is performed by the compiler where as #define statements are processed by the pre-processor.

 

• #define should not be terminated with semicolon, but typedef should be terminated with semicolon.

 

typedef char* PTR_t;
#define PTR_d char*

 

 

• #define will just copy-paste the definition values at the point of use, while typedef is actual definition of a new type
  

 

typedef char* PTR_t;
#define PTR_d char*

PTR_t p1,p2;		// In typedef usage, Both p1 and p2 are become char*
PTR_d ptr1,ptr2;	// In macro usage, only ptr1 becomes of type char* while ptr2 is char only

 

 

• typedef follows the scope rule which mean if a new type is defined in a scope(inside a function), then the new type name will only be visible till the scope is there.
                                But in preprocessor case, when preprocessor encounters a #define, then it will replace all the occurrences, after that (No scope rule is followed). See below example:
  

 

#include <stdio.h>

void print()
{
	#define ONE 1
	
	typedef int integer;
	
	printf("%d",ONE);
}
 
int main()
{
     printf("%d",ONE);      // Print 1 successfully
     
     integer a; 	// Gives compiler error    
  
     return 0;
}

 

 

• There are certain types of definitions which you can only define using typedef only and not #define like below
  

typedef unsigned int U_INT32;      // We cant define this with #define, because it contain more that one word in definition( unsigned and int here) .

 

 

• There is some benefit of typedef when we define structure with typedef like this
  

 

typedef struct
{
      int x;
      char y;
      float z;
}MyStruct;

//Now define new objects

MyStruct A; // You dont need to use struct repeatedly when you define structure again.

 

 

Problems on typedef and #define

This problems will clear some doubts and give you practical idea

1. What is the output of this C code?

#include <stdio.h>

typedef struct student
{
    char *a;
}stu;

void main()
{
    struct student s;
    s.a = "hey";
    printf("%s", s.a);
}

 

a) Compile time error
b) Varies
c) he
d) hey

 

2. What is the output of this C code?

#include <stdio.h>

typedef struct p
{
    int x, y;
};

int main()
{
    p k1 = {1, 2};
    printf("%d\n", k1.x);

	return 0;
}

 

a) Compile time error
b) 1
c) 0
d) Depends on the standard

 

3. The correct syntax to use typedef for struct is.

a) typedef struct temp
    {
        int a;
    }TEMP;

b) typedef struct
    {
        int a;
     }TEMP;

c) struct temp
    {
        int a;
    };
    typedef struct temp TEMP;

d) All of the mentioned

 

4. typedef int (*PFI)(char *, char *)creates
a) type PFI, for pointer to function (of two char * arguments) returning int
b) Error
c) type PFI, function (of two char * arguments) returning int
d) type PFI, for pointer

 

5. For the following expression to work, which option should be selected.
    string p = “HELLO”;
a) typedef char [] string;
b) typedef char *string;
c) Both (a) and (b)
d) Such expression cannot be generated in C 

 

6. What is the output of this C code?

#include<stdio.h>

typedef char* charPtr_t;
#define charPtr_d char* 

int main()
{
	charPtr_t a,b;
	charPtr_d x,y;
	
	printf("%d,%d,%d,%d",sizeof(a),sizeof(b),sizeof(x),sizeof(y));
	
	return 0;
}

 

a) Compiler error
b) 8,8,8,8
c) 1,1,1,1
d) 8,8,8,1

 Answers: (1: d) , (2: a) , (3: d) ,(4: a ) , (5: b), (6: d)