C #undef

How does #undef work?

Preprocessors aim to refer to the programs processed in our source code before the code even enters the compiler. The user writes the source code, and this source code is then sent for preprocessing to the preprocessors that generate an expanded source file, having the name same as that of the program. The expanded file is forwarded for compilation to the compiler to create an object code of the library functions.

Now when the #undef macro_name command is encountered by the preprocessor, it removes the macro from memory such that it can be used again. We may also use the #ifdef and #endIf directive, which helps us to check if any macro name exists or not.

Example 1

Code

#include <stdio.h>
#define num 10
int s1=num*num;
#undef num
#define num 11
int s2=num*num;
int main() 
{
	printf("Value of square with first value of num variable is = %d",s1);
	printf("\n");
	printf("Value of square with second value of num variable is = %d",s2);
	return 0;
}

Explanation

In the above code, we declared a num variable with a value of 10 and then undefine it by using the undef directive. We then again define the variable with value 11 and then see how the value of variables s1 and s2 change.

Output

Value of square with first value of num variable is = 100
Value of square with second value of num variable is = 121

Example 2

Code

#include <stdio.h>
#define num 7
int square1=num1*num1;
#undef num1
int main() 
{
	printf("Value of constant num that has been removed using #undef directive  = %d",num1);
	return 0;
}

Explanation

In the above code, we see what happens when we try to access a constant or a macro that is defined using #define but later has been removed using #undef directive.

Output

Main.c.3:13: error: ‘num1’ undeclared here (not in a function)
Int square1=num1*num1;

You can also read about dynamic array in c, Short int in C Programming and Tribonacci Series

Frequently Asked Questions

What are the different types of preprocessors?

There are 4 categories of preprocessors which includes macros, file inclusion, conditional compilation, and other directives.

Why are preprocessor directives?

All the commands used for the preprocessor are known as preprocessor directives and all preprocessor directives are defined using #.

Recommended Topic -  singleton design pattern in c#

Conclusion

This article extensively discussed C #undef. We hope that this blog has helped you enhance your knowledge regarding C #undef and if you would like to learn more, check out our articles here.

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