C++ Preprocessor Directives

The C++ Preprocessor

The preprocessor is your first line of code transformation, running before compilation begins. It handles macros, includes, and conditional compilation through simple text substitution.

Key Directives

Macros (#define)

#define PI 3.14159
#define SQUARE(x) ((x) * (x))
#define MAX(a,b) ((a) > (b) ? (a) : (b))

Include Guards

#ifndef HEADER_H
#define HEADER_H
// Header content
#endif

// Or modern:
#pragma once

Conditional Compilation

#ifdef DEBUG
  #define LOG(x) std::cout << x
#else
  #define LOG(x)
#endif

Common Pitfalls

• Missing parentheses: #define DOUBLE(x) x * 2 → Use ((x) * 2)
• Side effects: SQUARE(i++) expands to ((i++) * (i++))
• Name collisions: Macros are global, use unique names

Best Practices

1. Prefer const and constexpr over macros
2. Use ALL_CAPS for macro names
3. Always parenthesize macro parameters
4. Document macro behavior
5. Consider inline functions instead

Next Steps

• Explore AST & Parsing
• Learn about Compilation Pipeline
• Master Templates for type-safe alternatives