Conditional compilation - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is conditional compilation in C?

Conditional compilation allows the compiler to include or exclude parts of the code based on certain conditions, usually using preprocessor directives like #if, #ifdef, and #ifndef.

beginner

What does the #ifdef directive do?

#ifdef checks if a macro is defined. If it is, the code following it up to #endif is included in compilation.

beginner

How do you exclude code when a macro is defined?

Use #ifndef MACRO_NAME to include code only if the macro is NOT defined.

beginner

What is the purpose of #else in conditional compilation?

#else provides an alternative block of code to compile if the condition in #if or #ifdef is false.

intermediate

Give an example of conditional compilation to compile code only on Windows.

Example:

#ifdef _WIN32
// Windows-specific code
#endif

This compiles the code inside only if _WIN32 is defined, which usually means the program is being compiled on Windows.

Which directive checks if a macro is NOT defined?

A #else

B #ifndef

C #if

D #ifdef

#ifndef means "if not defined" and includes code only if the macro is not defined.

What happens to code inside #ifdef MACRO if MACRO is not defined?

A It is excluded from compilation

B It is always compiled

C It causes a compilation error

D It is compiled twice

If the macro is not defined, the code inside #ifdef is skipped.

Which directive can be used to provide an alternative code block if a condition is false?

A #include

B #endif

C #define

D #else

#else is used to specify code that compiles when the #if or #ifdef condition is false.

What is the role of #endif in conditional compilation?

A It includes a file

B It defines a macro

C It ends a conditional directive block

D It comments out code

#endif marks the end of a conditional compilation block started by #if, #ifdef, or #ifndef.

Which directive would you use to check a condition with a value, not just if a macro is defined?

A #if

B #ifdef

C #ifndef

D #else

#if allows checking expressions or values, unlike #ifdef which only checks if a macro is defined.

Explain how conditional compilation helps in writing cross-platform C programs.

Think about how you can write code that works on Windows and Linux by including only the right parts.

Describe the difference between #ifdef and #if directives.

One checks presence, the other checks a condition.

Practice

1. What does the #ifdef directive do in C conditional compilation?

easy

A. Includes the code block only if the specified macro is defined.

B. Includes the code block only if the specified macro is not defined.

C. Always includes the code block regardless of any macro.

D. Excludes the code block if the specified macro is defined.

Solution

1. Step 1: Understand #ifdef usage

   The #ifdef directive checks if a macro is defined.

2. Step 2: Effect on code inclusion

   If the macro is defined, the code inside the #ifdef block is included during compilation.

3. Final Answer:

   Includes the code block only if the specified macro is defined. -> Option A

4. Quick Check:

   #ifdef means "if defined" [OK]

Hint: Remember: #ifdef means "if defined" [OK]

Common Mistakes:

• Confusing #ifdef with #ifndef
• Thinking it always includes code
• Assuming it excludes code if macro is defined

2. Which of the following is the correct syntax to check if a macro DEBUG is NOT defined?

easy

A. #ifdef DEBUG

B. #ifndef DEBUG

C. #if DEBUG

D. #if !DEBUG

Solution

1. Step 1: Identify the directive for 'not defined'

   The directive #ifndef means "if not defined".

2. Step 2: Match with macro DEBUG

   Using #ifndef DEBUG checks if DEBUG is not defined.

3. Final Answer:

   #ifndef DEBUG -> Option B

4. Quick Check:

   #ifndef means "if not defined" [OK]

Hint: Use #ifndef to check if macro is NOT defined [OK]

Common Mistakes:

• Using #ifdef instead of #ifndef
• Using #if !DEBUG which is invalid for macro existence
• Confusing #if with #ifndef

3. What will be the output of this code if FEATURE is defined?

#include <stdio.h>
#define FEATURE

int main() {
#ifdef FEATURE
    printf("Feature is enabled\n");
#else
    printf("Feature is disabled\n");
#endif
    return 0;
}

medium

A. Feature is enabled

B. Feature is disabled

C. Compilation error

D. No output

Solution

1. Step 1: Check if FEATURE is defined

   The code has #define FEATURE, so FEATURE is defined.

2. Step 2: Evaluate #ifdef FEATURE block

   Since FEATURE is defined, the code inside #ifdef FEATURE runs, printing "Feature is enabled".

3. Final Answer:

   Feature is enabled -> Option A

4. Quick Check:

   Defined macro triggers #ifdef block [OK]

Hint: If macro defined, #ifdef block runs [OK]

Common Mistakes:

• Ignoring the #define line
• Thinking #else block runs
• Assuming compilation error due to macro

4. Identify the error in this code snippet:

#include <stdio.h>

int main() {
#ifdef DEBUG
    printf("Debug mode\n");
#else
    printf("Release mode\n");
#endif
    return 0;
}

medium

A. printf statements are invalid

B. Incorrect use of #ifdef directive

C. Missing #define DEBUG before #ifdef

D. Missing closing brace for main function

Solution

1. Step 1: Check code structure

   The code opens int main() { but does not have a closing } brace.

2. Step 2: Verify directives and syntax

   The #ifdef and #else directives are correctly used; missing #define DEBUG is not an error, just changes output.

3. Final Answer:

   Missing closing brace for main function -> Option D

4. Quick Check:

   Every function needs matching braces [OK]

Hint: Check all braces match in code blocks [OK]

Common Mistakes:

• Assuming missing #define is an error
• Thinking #ifdef usage is wrong
• Ignoring missing closing brace

5. You want to compile a program that prints "Debug mode" only if the macro DEBUG is defined, otherwise it prints "Normal mode". Which code snippet correctly achieves this using conditional compilation?

hard

A. #if !DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

B. #ifndef DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

C. #ifdef DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

D. #if DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

Solution

1. Step 1: Understand the requirement

   Print "Debug mode" only if DEBUG is defined, else print "Normal mode".

2. Step 2: Choose correct directive

   #ifdef DEBUG checks if DEBUG is defined and includes the first print statement; #else handles the other case.

3. Final Answer:

   #ifdef DEBUG printf("Debug mode\n"); #else printf("Normal mode\n"); #endif -> Option C

4. Quick Check:

   Use #ifdef for defined macro check [OK]

Hint: Use #ifdef for defined, #else for else [OK]

Common Mistakes:

• Using #ifndef which reverses logic
• Using #if DEBUG without macro value
• Using #if !DEBUG which is invalid