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Macros in C

Macros in C

A macro is a piece of code with a given name. When the name is used, it is replaced by the content of the macro. The #define keyword is used to define new macros. It's followed by a name and a content. By convention, macro names are written in uppercase. There are two type of macros: Object-like macros and Function-like macros.

Object-like Macros

#define PI 3.14

If you use the macro this way:

printf("Value of PI: %d", PI);

Is the same as write this:

printf("Value of PI: %d", 3.14);

Undefining Macros

After defining macros you can also undefine them at any point. just Type

#undefine PI

This is used to use macros only for specific lines of code and again undefine it.

Function-like Macros

Function-like uses the same #define keyword. The difference is that you use a pair of parentheses after the function name.

#define hello_world() printf("Hello World!")

So calling:

hello_world();

You get:

printf("Hello World!");

You can set parameters too:

#define hello(X) printf("Hello " X "!")

Now calling:

hello("World");

You get the equivalent of:

printf("Hello World!");

Special Operators in Macros

One can use the special operators # (stringize) and ## (concatenate) in macros for achieving unique functionality.

Stringizing Operator (#)

A macro's parameter preceded by a # is converted and treated as a string token.
For example, we can define ERROR and WARN macros that print a LOG message.
While the LOG message gets prefixed with either an ERR or a WARN, respectively.

#define LOG(level, message) printf(#level ": " #message "\n")
#define ERROR(msg) LOG(FAIL, msg)
#define WARN(msg) LOG(WARN, msg)

Now, one can use it as

ERROR(Invalid settings);  // Output-> FAIL: Invalid settings
WARN(Upper threshold);    // Output-> WARN: Upper threshold

Concatenation (or token-pasting) Operator (##)

Using concatenation the parameters can be joined together to form one single token. Token-pasting is much more powerful in the sense that the resulting token could be an object defined in the C program.

#define NUM(x) number_##x

void foo() {
  int number_one = 10;
  int number_two = 15;

  printf("%d + %d = %d\n", NUM(one), NUM(two), NUM(one) + NUM(two));
  // Output-> 10 + 15 = 25
}

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