#undef Directive: C Program

In today’s video tutorial lets see how to use #undef preprocessor directive.

Related Read:
C Program To Find Area of Circle Using Macro Expansion
Conditional Compilation In C: #if #elif #else #endif

What Does #undef Do?

#undef removes or undefines a macro name which is previously created or defined using #define directive.

Syntax

#undef MACRO_NAME

Note: If a Macro template is associated with a Macro expansion, you need not mention the macro expansion to undefine the macro. You simple undefine using this syntax: #undef MACRO_NAME

Video Tutorial: #undef Preprocessor Directive: C Program

Source Code: #undef Directive: C Program

#include<stdio.h>

#define iOS
#undef iOS

int main()
{
    #if defined(iOS)
        printf("I Love Apple Devices\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
Macro iOS has been undefined

In above source code we’re undefining the Macro iOS immediately after defining it. So defined(iOS) returns false or 0. So code inside #else block gets compiled and executed.

#include<stdio.h>

#define iOS

int main()
{
    #if defined(iOS)
        printf("I Love Apple Devices\n");
        printf("Gift me a latest iPhone!\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
I Love Apple Devices
Gift me a latest iPhone!

Here Macro iOS is defined, so defined(iOS) returns true. That’s why block of code inside #if directive gets compiled and executed.

#include<stdio.h>

#define iOS

int main()
{
    #undef iOS
    #if defined(iOS)
        printf("I Love Apple Devices\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
Macro iOS has been undefined

Even though #define iOS is defined at global level you can undefine it inside main() method.

Note: defined() is a preprocessor operator which returns true if the passed macro name is defined orelse returns false or zero if its not defined.

Instead of defined() preprocessor operator you could even use #ifdef or #ifndef to check if the Macro template or Macro name is defined or not defined.

#include<stdio.h>

#define iOS

int main()
{
    int flag = 1;

    do
    {
        #if defined(iOS)
            printf("I Love Apple Devices\n");
            printf("This displays all your iPhone Device Details\n");
        #else
            printf("I'm inside do while block.\n");
        #endif // defined

        printf("Enter your choice? (0/1)\n");
        scanf("%d", &flag);

    }while(flag);


    #undef iOS

    #if defined(iOS)
        printf("I Love Apple Devices\n");
        printf("This displays all your iPhone Device Details\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
I Love Apple Devices
This displays all your iPhone Device Details
Enter your choice? (0/1)
1
I Love Apple Devices
This displays all your iPhone Device Details
Enter your choice? (0/1)
1
I Love Apple Devices
This displays all your iPhone Device Details
Enter your choice? (0/1)
0
Macro iOS has been undefined

In above program we are defining iOS macro globally. Inside main method we use do while loop and keep executing #if #else #endif directives. Since iOS is defined, the block of code inside #if gets executed each time. Once user enters 0 as input, the control exits do while loop execution. Next it encounters #undef iOS, so defined(iOS) now returns false or 0, hence code inside #else directive gets executed.

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