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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.

Why Macros?

The C compiler will go through your code and replace every occurrence of a macro with it's value. This begs the question, what is the point of using macros? The answer: macros are a tool for the programmer, not for the program.

If you have the number 365 hard-coded into your program, it becomes difficult for you and other people who may look at your code to understand what it means. If you #define DAYSINYEAR 365 and use that instead of the number, the code makes a lot more sense.

It's also beneficial if you have many instances of the same thing. If you'd used 2018 as the year in multiple places in your program, and then the year changed to 2019, you would have to go and find every single line containing 2018 and change it, hoping you didn't miss any. With the macro #define YEAR 2018, you can simply change the value of the macro and be confident all of the values have been updated accordingly.

Defining macros

The #define keyword is used to define new macros. It's followed by a name and a content, but no equals sign. By convention, macro names are written in uppercase.

#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!");

An example to explain how code is replaced by macros

#define add(a,b) a+b

Now calling :

int x = 2*add(3,5)*4;

Here, the value of x will be 26 because the expression will become 23+54 which evaluates to give 26. Sometimes we think that the macro will give us 3+5 i.e. 8 and the expression will become 284 but this is not the case. This shows how the code is replaced exactly.

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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