# Compiling, Linking and Warnings — C++

Source: https://www.skillbyai.com/en/cpp/f-toolchain

> Compile and link programs with g++ or clang++ and use warnings and standards flags.

## Translation units and the build pipeline

Building a C++ program has several stages. The **preprocessor** handles `#include` (pasting header contents in) and macros. The **compiler** translates each **source file** (`.cpp`), together with the headers it includes, called a **translation unit**, into an **object file** (`.o` or `.obj`) of machine code. The **linker** combines object files and libraries into an **executable**, resolving references to functions defined in other files. Common compilers are **GCC** (`g++`), **Clang** (`clang++`) and **MSVC** (`cl` on Windows). Always choose a language standard (`-std=c++20` or `-std=c++23`) and enable **warnings**: `-Wall -Wextra -Wpedantic` catch many bugs, and `-Werror` makes warnings fail the build. Use `-O2` for optimised builds and `-g` for debug information. For anything beyond a few files, a build system such as **CMake** (covered later) generates the compiler commands for you.

## Compiling one and several files

Separate compilation means only changed files are recompiled.

```bash
# one file, C++20, warnings on, debug info
g++ -std=c++20 -Wall -Wextra -Wpedantic -g hello.cpp -o hello
./hello

# several files: compile each to an object file, then link
g++ -std=c++20 -Wall -Wextra -c geometry.cpp -o geometry.o
g++ -std=c++20 -Wall -Wextra -c main.cpp     -o main.o
g++ geometry.o main.o -o app

# optimised release build
clang++ -std=c++23 -O2 -DNDEBUG main.cpp geometry.cpp -o app
```

## Chapters, printing and binding

Each source file is a chapter typeset separately (compiling). Binding the chapters into one book, with page references between chapters resolved, is linking. A broken cross-reference shows up only at binding time, just like an "undefined reference" linker error.

**Quiz:** What does the linker do?

- [ ] Expands #include directives
- [ ] Checks syntax of each source file
- [ ] Runs the program
- [x] Combines object files and libraries into an executable, resolving references between them

*Answer:* Combines object files and libraries into an executable, resolving references between them. The linker resolves symbols across object files and libraries to produce the final binary.