Conditional Compilation In C: #ifdef #else #endif

In this video tutorial lets learn about preprocessor command or directives like #ifdef, #else and #endif. These directives are used for conditional compilation.

How Does #ifdef Work?

The block of code between #ifdef and #endif works only if the macro name is defined orelse the compiler will skip the entire block of code from compiling.

When To Use #ifdef

1. When we want the compiler to skip certain part of the source code: For this we could even use multi-line comment, but if we already had multi-line comments in the code, we can’t enclose the source code and multi-line comments with another multi-line comment. Nesting of multi-line comments are not allowed in C programming language. So instead of commenting the code we could make use of #ifdef directive.

2. To write portable code: For example, we could write code both for iOS and non-iOS devices. Using #ifdef we could check the device OS and based on that deliver specific set of codes. For this we can use #ifdef

Video Tutorial: Conditional Compilation In C: #ifdef #else #endif

Source Code: Conditional Compilation In C: #ifdef #else #endif

#include<stdio.h>

#define iOS

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #else
        printf("This is code for Android Devices\n");
    #endif // iOS

    return 0;
}

Output:
This is iOS Code

#include<stdio.h>

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #else
        printf("This is code for Android Devices\n");
    #endif // iOS

    return 0;
}

Output:
This is code for Android Devices

In above programs, if iOS macro name is defined, then the set of code present inside #ifdef block gets compiled and executed. If macro name iOS is not defined, then the code inside #else block gets compiled and executed.

#include<stdio.h>
#define iOS

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #endif // iOS

    printf("This is code for all non iOS Devices\n");

    return 0;
}

Output:
This is iOS Code
This is code for all non iOS Devices

#include<stdio.h>

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #endif // iOS

    printf("This is code for all non iOS Devices\n");

    return 0;
}

Output:
This is code for all non iOS Devices

Note: using #else is optional. You can just use #ifdef and #endif.

This is similar to if else condition, but here the block of code which do not match the criteria doesn’t even get compiled. It’s treated like regular comments.

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