Conditional compilation

Learn conditional compilation with #ifdef, #ifndef and #endif.

Conditional compilation: #ifdef, #ifndef, #endif

#ifdef compiles code if a macro is defined; #ifndef compiles if it is not. #endif closes the block.

#define DEBUG

#ifdef DEBUG
    printf("debug");
#endif

Here if DEBUG is defined, the printf is compiled; otherwise it is skipped. This is useful to remove debug code from production builds. With #ifndef you can run custom code when a macro is missing. This allows producing different builds from a single source.

Conditional definition with #ifndef

#ifndef compiles its content if a macro is not defined. This is useful to give a constant a default value.

#ifndef VERSION
    #define VERSION 1
#endif

Here if VERSION was not previously defined, it becomes 1. The compiler can give a different value from the command line with -DVERSION=2. This is also used as include guard in header files; we will see it later. It is an important tool for writing flexible and portable code.

Exercise Conditional print with the DEBUG macro

Define a macro with #define DEBUG. In main use #ifdef DEBUG to print printf("debug\n"); close the block with #endif.

Show example solution

#define DEBUG
#ifdef DEBUG
printf("debug\n");
#endif

Test yourself

Fill in the blank: #ifdef block

#ifdef DEBUG
    printf("debug");
____

Show answer

#ifdef DEBUG
    printf("debug");
#endif

Match directive to meaning

#ifdef X

If X is defined
#ifndef X

If X is not defined
#endif

Ends the block
#define X

Defines X
What does #endif do?
- Starts an #ifdef block
- Terminates the program
- Declares a variable
- Ends an #ifdef block
Show answer

Ends an #ifdef block — #endif closes an #ifdef or #ifndef block.

Next lesson: Macro functions

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