What C++ is and how compiling works

Why learn C++?

C++ was created by Bjarne Stroustrup in the 1980s as "C with classes". Today it runs inside things you use every day: the Chrome and Firefox browsers, game engines like Unreal Engine, databases like MySQL, stock-exchange trading systems, car software and the firmware in many devices. People choose C++ when a program must be very fast or must control memory and hardware closely.

C++ is also the most popular language in competitive programming (Codeforces, CodeChef, ICPC) because it is fast and its standard library has ready-made data structures. Many Indian college placement tests accept C++, so it is a strong first or second language for students.

Compiled, not interpreted

A computer's processor only understands machine code: numbers that mean "add these", "jump there". Python reads your code line by line while it runs, using a program called an interpreter. C++ works differently. A compiler reads your whole source file first and translates it into machine code, saved as an executable file (a program you can run, like hello.exe on Windows or ./hello on Linux and macOS).

The steps inside "compiling"

1. Preprocessing: lines starting with #, like #include <iostream>, are handled first. #include pastes the contents of another file (a header) into yours.
2. Compiling: each .cpp file is turned into an object file (.o or .obj) of machine code.
3. Linking: the linker joins your object files with the libraries you used (like the code behind std::cout) into one executable.

Usually one command does all three steps. You only notice them when something goes wrong: a compile error means your code broke a language rule; a linker error (for example "undefined reference to ...") means you declared a function but the linker could not find its body.

Getting a compiler

g++ --version
g++ -std=c++20 -Wall -Wextra -O2 hello.cpp -o hello
./hello

What the flags mean

• -std=c++20 chooses the version of the C++ language. This course uses C++17 and C++20, which every modern compiler supports.
• -Wall -Wextra turn on warnings: hints about code that is legal but probably wrong. Always use them while learning.
• -O2 asks the compiler to optimise (make the program faster). Leave it out while debugging.
• -o hello names the output file. Without it, g++ makes a.out (Linux/macOS) or a.exe (Windows).
• -g adds debug information so a debugger like gdb can show your source lines.

Common mistakes

• "g++ is not recognized": the compiler is installed but its folder is not in PATH. Reopen the terminal after installing, or add the bin folder to PATH.
• Editing the .cpp file and running the old program: you must compile again after every change.
• Ignoring warnings: a warning like "unused variable" or "comparison of integers of different signs" often points to a real bug.