README.md

Lesson 13 — Out-of-Source Builds

What you'll learn

How to keep all build artifacts (.o, .d, executables) in a separate build/ directory, keeping your source tree clean.

The problem

In lessons 01–12, object files and executables were created alongside source files. This makes it hard to tell what's source and what's generated. git status shows clutter. Cleaning requires knowing every artifact type.

The technique: BUILDDIR prefix

All output paths include a build/ prefix:

BUILDDIR = build
OBJS     = $(SRCS:%.cpp=$(BUILDDIR)/%.o)
TARGET   = $(BUILDDIR)/app

The source file src/main.cpp compiles to build/src/main.o — mirroring the directory structure inside build/.

Creating directories on the fly

The build/src/ directory doesn't exist initially. Each recipe creates it:

$(BUILDDIR)/%.o: %.cpp
	@mkdir -p $(@D)
	$(CXX) $(CXXFLAGS) -c -o $@ $<

Part	Meaning
@mkdir -p $(@D)	Create the directory part of the target path. @ suppresses echoing the command. -p creates parents and doesn't error if it exists
$(@D)	The directory part of $@. For build/src/main.o, this is build/src

Clean is trivial

clean:
	rm -rf $(BUILDDIR)

One command removes everything. No need to list individual file types.

This is what CMake does

CMake always uses out-of-source builds — cmake -B build creates the build directory and puts everything there. Now you understand why.

Try it

cd 13_out_of_source
make                    # all artifacts go into build/
ls build/src/           # main.o main.d point.o point.d
./build/app             # a = (0, 0), b = (3, 4), distance = 5
make clean              # rm -rf build/ — done
ls build/ 2>/dev/null   # directory is gone

Files

File	Purpose
include/point.hpp	Declares a Point struct with distance_to() and to_string()
src/point.cpp	Implements them
src/main.cpp	Computes distance between two points
Makefile	Out-of-source builds with BUILDDIR, $(@D), mkdir -p