C++ Preprocessor

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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.

C++ Preprocessor Visualizer

Preprocessing Pipeline

??=

Trigraph Replacement

Legacy character replacement

\

Line Splicing

Join continued lines

[ ]

Tokenization

Break into tokens

#

Macro Expansion

Replace macros

<>

Include Processing

Insert header files

#if

Conditional Compilation

Process conditionals

Original Code

#define PI 3.14159
#define SQUARE(x) ((x) * (x))

float area = PI * SQUARE(radius);

After Preprocessing

float area = 3.14159 * ((radius) * (radius));

💡 Explanation: Macros are replaced with their definitions. Note the parentheses in SQUARE to ensure correct precedence.

Preprocessor Commands

g++ -E main.cpp -o main.iPreprocess only

g++ -dM -E main.cppShow all defined macros

g++ -H main.cppShow included headers

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

Prefer const and constexpr over macros
Use ALL_CAPS for macro names
Always parenthesize macro parameters
Document macro behavior
Consider inline functions instead

Next Steps

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

Table of Contents

C++ Preprocessor Visualizer

Preprocessing Pipeline

Preprocessor Commands