Why use Macros in C? [duplicate]

Answer 1

We used this type of macro in our code:

// convenience macros for implementing field setter/getter functions
#ifndef CREATE_GET_SET
#define CREATE_GET_SET(PREFIX, FIELD_NAME,FIELD_DATATYPE) \
  protected:\
    FIELD_DATATYPE PREFIX ## FIELD_NAME;\
  public:\
  inline FIELD_DATATYPE get ## _ ## FIELD_NAME(void) const\
  { \
    return(PREFIX ## FIELD_NAME); \
  } \
  inline void set ## _ ## FIELD_NAME(FIELD_DATATYPE p) \
  { \
    PREFIX ## FIELD_NAME = p; \
  }
#endif

within a class/structure you would define a variable:

CREATE_GET_SET(_, id, unsigned int);

This would define your variable and create the generic getter/setter for the code. It just makes for cleaner, consistent code generation for get/set. Sure, you can write it all out, but that's a lot of boilerplate type code NOTE: this is just one of a couple macros. I didn't post them all. You wouldn't handle say "char *" this way (where you want the set to strncpy or strcpy the data). This was just a simple demo of what you could do with a macro and some simple types.

Answer 2

The compiler often knows details about the target machine, so you can use conditional compilation to have one piece of code for big endian processors and another for little endian processors. That keeps the code from being bloated with code that's designed for a different processor.

A similar case is when you have assembly code for one particular system but C code for other systems.

Answer 3

Macros can have many different uses other than function like things.

It is very useful to use macros for anything magic number or string related.

#define MY_MAGIC_NUM 57

/*MY_MAGIC_NUM used all through out the code*/

Answer 4

Sometimes you want to do logging, but only in debug mode. So you write something like:

#ifdef DEBUG
#define LOG_MSG(x) printf(x);
#else
#define LOG_MSG(X)
#endif

Sometimes you just want to turn something off or on based on a compile-time switch. E.g., my company does some co-branding of our product and partners ask to turn things off. So we'll do something like:

#ifndef SPECIAL_PARTNER_BUILD
    DoSomethingReallyAwesome();
#endif

Then build with -DSPECIAL_PARTNER_BUILD when giving the partner drops.

Among many other possible reasons.

Sometimes you just want to save typing for things you do over and over again. Some people take this to far (cough MFC cough) and write what amounts to their own language in Macros to abstract away a difficult API. This makes debugging a frikkin' nightmare.

Answer 5

In performance intense scenarios, you can use macros to create "automatic" loop unrolling. Modern compilers probably do a better job with this though, but that used to be a useful application for it.