Makefiles for C++

1. Understanding the Project Structure

In this example, we have a C++ project with the following files:

main.cpp — Contains the main() function that drives the program.
Class.hpp — The header file declaring class members and methods.
Class.cpp — The implementation file defining methods declared in Class.hpp.

2. Making the Makefile

Steps to create a Makefile:

Create a new file named Makefile in the same folder as your .cpp and .hpp files.
Define the compiler and compilation flags:

CXX = g++
CXXFLAGS = -std=c++17 -g -Wall -O2

Important: The Makefile must be placed in the same directory as your source files (.cpp) and header files (.hpp) for the compilation process to work correctly.

Explanation:

-std=c++17 — Use the C++17 standard.
-g — Include debugging information.
-Wall — Enable all warnings.
-O2 — Optimize performance.

3. Defining the Target and Object Files

PROG ?= main
OBJS = Class.o main.o

Explanation: PROG is the name of the executable, and OBJS are the compiled object files.

Essentially, we want to create an executable that can later be run using ./main. To achieve this, we first compile individual source files into object files (.o), which are then linked together to produce the final executable.

4. Compiling Source Files

The rule for compiling source files:

.cpp.o:
$(CXX) $(CXXFLAGS) -c -o $@ $<

Explanation: This rule tells make how to compile a .cpp file into a corresponding .o (object) file.

$(CXX) — Represents the compiler (e.g., g++).
$(CXXFLAGS) — Compiler flags that control the compilation process, such as optimization and warnings.
-c — Indicates that the source file should be compiled into an object file without linking.
-o $@ — Specifies the output file, where $@ is the target (the object file).
$< — Represents the first prerequisite (the source file being compiled).

5. Linking Object Files

The rule for linking the object files into the final executable:

$(PROG): $(OBJS)
    $(CXX) $(CXXFLAGS) -o $@ $(OBJS)

6. Adding Clean and Rebuild Rules

To clean and rebuild the project:

clean:
    rm -rf *.o main

rebuild: clean all

7. Final Makefile

CXX = g++
CXXFLAGS = -std=c++17 -g -Wall -O2

PROG ?= main
OBJS = Class.o main.o

all: $(PROG)

.cpp.o:
    $(CXX) $(CXXFLAGS) -c -o $@ $<

$(PROG): $(OBJS)
    $(CXX) $(CXXFLAGS) -o $@ $(OBJS)

clean:
    rm -rf *.o main

rebuild: clean all

8. Using the Makefile

To build the project:
```
make
```
To clean the project:
```
make clean
```
To rebuild from scratch:
```
make rebuild
```

9. Extending the Makefile

Modifying the Makefile to include a subclass:

PROG ?= program
OBJS = Class.o Subclass.o main.o

clean:
    rm -rf *.o program

# The rest of the Makefile remains the same.

10. References

Author: Georgina Woo