README.md

Lesson 03 — Variables

What you'll learn

How to use Make variables to eliminate repetition and make your Makefile maintainable.

The problem

In lesson 02, g++, -std=c++20, and -Wall -Wextra appeared in every rule. If you want to switch to clang++ or add -O2, you'd have to change multiple lines. Variables fix this.

The Makefile

CXX      = g++
CXXFLAGS = -std=c++20 -Wall -Wextra
LDFLAGS  =
TARGET   = main
SRCS     = main.cpp greet.cpp
OBJS     = main.o greet.o

$(TARGET): $(OBJS)
	$(CXX) -o $(TARGET) $(OBJS) $(LDFLAGS)

main.o: main.cpp greet.hpp
	$(CXX) $(CXXFLAGS) -c main.cpp

greet.o: greet.cpp greet.hpp
	$(CXX) $(CXXFLAGS) -c greet.cpp

Standard variable names

These are conventions used by virtually all Makefiles:

Variable	Purpose	Typical value
CXX	C++ compiler	g++, clang++
CXXFLAGS	Compile flags	-std=c++20 -Wall -Wextra
LDFLAGS	Linker flags	-L/usr/local/lib
LDLIBS	Libraries to link	-lm -lpthread
TARGET	Output name	main, app
SRCS	Source files	main.cpp greet.cpp
OBJS	Object files	main.o greet.o

Variables are referenced with $(VAR) or ${VAR}. They're expanded as plain text substitution — no types, no scoping.

Overriding from the command line

You can override any variable when invoking Make:

make CXX=clang++                    # use clang instead of g++
make CXXFLAGS="-std=c++20 -O2"     # different flags

Command-line values override what's in the Makefile.

Try it

cd 03_variables
make            # builds with g++
./main          # Hello, Make variables!
make clean      # removes main and .o files

Files

File	Purpose
greet.hpp	Declares greet()
greet.cpp	Implements it
main.cpp	Calls greet()
Makefile	Uses variables for compiler, flags, and file lists