In today’s video tutorial lets see how to use #undef preprocessor directive.
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Page Contents
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
[youtube https://www.youtube.com/watch?v=1FwlFR9PQDE]
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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