05_pattern_rules

Lesson 05 — Pattern Rules

What you'll learn

How to write one rule that compiles any .cpp file into a .o file, eliminating per-file rules entirely.

The problem

In lesson 04, we still wrote a separate rule for each .o file. With 50 source files, that's 50 nearly-identical rules. Pattern rules collapse them into one.

The Makefile

SRCS = main.cpp vec.cpp
OBJS = $(SRCS:.cpp=.o)        # text substitution: main.o vec.o

$(TARGET): $(OBJS)
	$(CXX) -o $@ $^

%.o: %.cpp                     # pattern rule
	$(CXX) $(CXXFLAGS) -c -o $@ $<

How pattern rules work

The % character is a wildcard called the stem. When Make needs to build vec.o:

1. It matches vec against the % in %.o
2. It looks for vec.cpp (substituting the stem into %.cpp)
3. It runs the recipe with $@ = vec.o and $< = vec.cpp

This works for any .cpp → .o pair. Adding a new source file only requires adding it to SRCS.

Substitution references

OBJS = $(SRCS:.cpp=.o)

This replaces .cpp with .o in each word of SRCS. So main.cpp vec.cpp becomes main.o vec.o. No manual synchronization needed.

Try it

cd 05_pattern_rules
make            # one pattern rule compiles both files
./main          # a + b = (4, 6), a . b = 11
make clean

Files

File	Purpose
vec.hpp	Declares a Vec2 struct with operator+ and dot()
vec.cpp	Implements them
main.cpp	Creates two vectors and operates on them
Makefile	Pattern rule %.o: %.cpp handles all compilations