#define vs const

 Earlier, #define is the only way to define constants.  Later const came in, which is also used to define constants.  Most of the programmers prefer to use const over #define for the following reasons

1. Syntax check will be done for const but not for #define. The #define directive is not checked for syntax errors until the error macro is used in code block.
2. #define can only define simple constant.  But, const can define complex constants as shown below.

struct student {

         char name[100];

         int age;

         float percentage;

};

const struct student obj = {"Victor", 20, 89};

Consider the following example,

  #include <stdio.h>
  #define SUM 10+
  int main() {
        int a;
        printf("Hello world\n");
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out
  Hello world

In the above C program, we have a syntax error in macro definition(#define SUM 10+). But, it wasn't reported to us since we haven't used it anywhere in our code.  Let us see what happen if we use the same macro inside the code block

  #include <stdio.h>
  #define SUM 10+
  int main() {
        int a = SUM;  // SUM is the macro with syntax error
        printf("Hello world\n");
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ gcc pgm.c 
  pgm.c: In function ‘main’:
  pgm.c:4: error: expected expression before ‘;’ token

In the above program, the definition for macro SUM has syntax error and the same is assigned to integer variable a.  During preprocessing 10+ would be assigned to variable a, compiler detects the same as error during compilation process.

Let us see how const behaves when there is some syntax error.

  #include <stdio.h>
  int main() {
        const int a = 10+; 
        printf("Hello world\n");
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ gcc pgm.c 
  pgm.c: In function ‘main’:
  pgm.c:4: error: expected expression before ‘;’ token

Syntax error with const is reported during compilation itself.