Macros

• macros are very important tools in C/C++
• Consider them to be simple copy paste utility.
• Well, macros are a text processing feature. What happens once you build your program is that all occurrences of macros are “expanded” and replaced by the macro definitions.
• we define macro using #define <identifier> <value or function definition>
• So, what happens is, say we have #define maxvalue 3, so if in my code, I write maxvalue anywhere, it will be simply replaced by the value, so in this case 3

#include <stdio.h>

#define AGE 23

int main(int argc, char *argv[])
{
	printf("%d\n", AGE);
}

So, it is as good as writing:

#include <stdio.h>

int main(int argc, char *argv[])
{
	printf("%d\n", 23);
}

23 got copied in place of AGE. below is a example of a function

#include <stdio.h>

#define SUM(x, y) (x + y)

int main(int argc, char *argv[])
{
	int a = 5;
	int b = 10;
	int sum = SUM(a, b);
	printf("%d\n", sum);
}

So, it is as good as writing:

#include <stdio.h>

int main(int argc, char *argv[])
{
	int a = 5;
	int b = 10;
	int sum = (a + b);
	printf("%d\n", sum);
}

• to see this in action we can pass -E option to the gcc compiler, it will output the processed file.
• Can someone predict the output of main.c.
• Now run the command gcc -E main.c -o main_processed.c check the newly created file, ignore large list of extra lines add, go to the bottom. We can see SUM and AGE have been replaced by their values.
• Using macros can be extremely unsafe and they hide a lot of pitfalls which are very hard to find. However, as a C or C++ programmer, inevitably, you will encounter macros in your coding life. Even if you don’t use them in your own project, there is a high chance you will encounter them somewhere else, such as a library.