Build systems with CMake | C++

Once a C++ project has multiple files, typing compiler commands by hand becomes fragile. A build system records how the project is built.

CMake is a common modern choice for C++ projects.

Translation units

Each .cpp file is compiled as a translation unit. Headers included by that file become part of the translation unit during preprocessing.

main.cpp + included headers -> main.o
parser.cpp + included headers -> parser.o

The linker combines object files into an executable or library.

Compiler errors happen while compiling a translation unit. Linker errors happen while connecting compiled pieces.

Small project layout

notes/
  CMakeLists.txt
  src/
    main.cpp
    note.cpp
    note.h

note.h declares the interface. note.cpp defines implementation. main.cpp uses it.

Minimal CMakeLists.txt

cmake_minimum_required(VERSION 3.20)
project(notes LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_executable(notes
    src/main.cpp
    src/note.cpp
)

target_compile_options(notes PRIVATE
    -Wall
    -Wextra
    -pedantic
)

This creates an executable target named notes.

Configure and build

From the project root:

cmake -S . -B build
cmake --build build

-S . points at the source directory. -B build chooses a separate build directory. Keeping generated files out of the source tree makes cleanup easier.

Common linker error

If a function is declared in a header but its .cpp file is not included in the target, the compiler may succeed but linking fails:

undefined reference to `make_note(...)'

That often means the definition is missing from the build target or the signature does not match.

What to carry forward

C++ compiles translation units separately
the linker combines compiled pieces
build systems record source files, options, and targets
CMake is a practical default for many C++ projects
out-of-source builds keep generated files separate

Next, you will learn how to debug compiler errors, crashes, and logic bugs.