Preprocessor, Debug Macros & Compilation

The preprocessor runs before compilation, transforming your source code. Understanding it helps you write shorter contest code and debug faster.

The C++ Preprocessor & Macros

What is a Macro?

A macro is a text substitution that happens before compilation:

#define ll long long
#define vi vector<int>
#define pii pair<int,int>

ll n;       // becomes: long long n;
vi a;       // becomes: vector<int> a;
pii p;      // becomes: pair<int,int> p;

Macros with Parameters

#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()

sort(all(a));       // sort(a.begin(), a.end())
int n = sz(a);      // int n = (int)(a).size();

Macros vs Inline Functions

For competitive programming, macros are fine for type aliases and shorthand. For anything with logic, prefer inline functions.

Preprocessor Directives

#include <bits/stdc++.h>  // include entire standard library
#define MOD 1000000007     // text replacement
#undef MOD                 // undo a #define
#ifdef DEBUG               // conditional compilation
    cerr << "debug: " << x << "\n";
#endif
#ifndef ONLINE_JUDGE       // if not on a judge
    freopen("input.txt", "r", stdin);
#endif

Include Guards

Header files use include guards (or #pragma once) to prevent double inclusion:

#pragma once
// header content here

Compilation Pipeline & G++ Compiler Flags

Compilation Steps

1. Preprocessing: #include, #define, #ifdef are resolved
2. Compilation: C++ code → assembly
3. Assembly: assembly → object file (.o)
4. Linking: object files → executable

Essential Flags

g++ -O2 -std=c++17 -Wall -Wextra -o solution solution.cpp

Debug Build

# Debug: no optimization, debug symbols, sanitizers
g++ -O0 -std=c++17 -g -fsanitize=address,undefined -DLOCAL -o debug solution.cpp

# Contest: optimized
g++ -O2 -std=c++17 -o solution solution.cpp

Fast I/O and O2

Always compile with -O2 in contests. Judges use it, so your solution's time complexity assumes optimized code. Without -O2, even correct solutions may TLE.

Summary

• Macros are text substitution — use for type aliases (#define ll long long) and shorthand
• #define has no type safety; prefer using for type aliases when possible
• Always compile with -O2 in contests
• Use -fsanitize=address,undefined during development to catch bugs
• #ifndef ONLINE_JUDGE lets you use freopen for local testing without affecting judge submissions