OutofSourceBuilds

Out-of-source builds, often written as out-of-tree builds, are a method of constructing software in a separate directory outside the source tree. The source tree remains pristine, while build outputs, intermediate files, and generated artifacts reside in the separate build directory. This approach is widely used in C and C++ projects but is supported by other languages and build systems as well.

To perform an out-of-source build, create a dedicated build directory outside the source tree, then invoke the build system from there, pointing to the source directory as needed. Examples: with CMake, create build/ and run cmake -S . -B build or cmake -S source -B build; then cmake --build build. Autotools typically uses a separate build directory by creating one, changing into it, and running ../configure from there, followed by make. Meson: meson setup builddir --buildtype=Release; ninja -C builddir. This approach ensures the source is not modified by the build process and artifacts are isolated from source files.

Benefits include keeping the source tree clean, enabling multiple independent builds (for example, Debug vs Release) without duplicating sources, simplifying version control operations, and facilitating clean packaging and patching since artifacts are isolated from sources. It also makes it easier to test changes without contaminating the main source layout and supports multiple configurations in parallel.

Challenges include occasional project-specific requirements for in-tree builds, potential complexities with path discovery and include or library search paths, and some tooling quirks on certain platforms. Some environments or IDEs may assume a fixed layout, and cross-compilation workflows may require careful configuration of the build or install prefixes.

Best practices involve adding the build directory to version control ignore lists, documenting the build steps for contributors, and using consistent directory naming. For reproducibility, record exact tool versions and configurations, and consider separate build directories per configuration to avoid cross-contamination. Overall, out-of-source builds are a standard technique in modern build workflows when supported by the build system.