Question

I am a new programmer in C/C++ having programmed in Java for quite a while. I am currently understanding some C code. Here I am seeing some macro definitions like:

/* Flags for ds_flags */ 
#define DSF_OVER        (1<<0)  
#define DSF_DISPLAY     (1<<1)    
#define DSF_CALLFLOW    (1<<2) 

I am not able to understand why do we have to define these macros in such a manner. What is the advantage gained in these rather than in defining like:

#define DSF_OVER        0
#define DSF_DISPLAY     1    
#define DSF_CALLFLOW    2

Answer 1

Some times the position of the bits represent some bit operations like in your case:

#define DSF_OVER        (1<<0)  is 1
#define DSF_DISPLAY     (1<<1)  is 2  
#define DSF_CALLFLOW    (1<<2)  is 4 (This is not 3)

If you were to add a new item later, you will do it as

#define DSF_OVER       1
#define DSF_DISPLAY    2  
#define DSF_CALLFLOW   4
#define DSF_CALLNEW    5.

You cant have a value 5 here since it is two bits enabled (101 in binary). To avoid such potential errors it is always safe to use macros using >>. It can’t produce a value of 5 (101) by error in when the next bit should be 1000 in binary.

It is all about programming convenience to produce error free code.