Compilation Process

When you click Run, watch the pipeline animate. Your .cpp file travels through 5 stages before becoming a program.

Preprocessor → Compiler → Assembler → Linker → Executable.

The preprocessor runs before the compiler. It handles all lines starting with #:

• #include — pastes the entire header file content inline
• #define — does text substitution (replaces every PI with 3.14159)
• #ifdef / #endif — conditionally includes blocks of code

The preprocessor never sees types or logic — it is pure text manipulation.

The compiler reads your C++ and checks it for correctness — types, syntax, undeclared variables — then translates it to assembly language.

• A missing semicolon → compiler error
• A wrong type (e.g. passing a string where int is expected) → compiler error
• An undeclared variable → compiler error

Try editing the code to create a mistake and see the error message.

Large projects have many .cpp files. Each is compiled to an object file (.o) independently. The Linker combines all object files into the final executable.

A linker error looks like: undefined reference to foo. This means you declared a function but never defined it. The compiler was happy, but the linker could not find the implementation.

Professional C++ projects split code into header files (.h) and source files (.cpp):

• .h — declarations (what exists)
• .cpp — definitions (what it does)

#include "math.h" pastes the declarations so the compiler knows about them, and the linker connects the implementations at link time.