C++ Preprocessor Directives

C++ preprocessor visualized: macros, header guards, conditional compilation, and #include directives explained interactively.

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

Inspect macro expansion, header inclusion and directives before the compiler parses the translation unit.

Preprocessing Pipeline

??=Trigraph ReplacementLegacy character replacement

\ Line SplicingJoin continued lines

[ ]TokenizationBreak into tokens

#Macro ExpansionReplace macros

<>Include ProcessingInsert header files

#ifConditional CompilationProcess conditionals

Inspecting the original source for simple macro.

Original Code · inspecting

#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.i    # Preprocess only
g++ -dM -E main.cpp          # Show all defined macros
g++ -H main.cpp              # Show included headers

Authored preprocessing sketches omit header contents and compiler line markers. Trigraph replacement is a legacy phase removed from standard C++ in C++17.

Key Directives

Macros (#define)

C++

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

Include Guards

C++

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

// Or modern:
#pragma once

Conditional Compilation

C++

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

Common Pitfalls

Macro Expansion Can Repeat Side Effects

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

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