build_systems.md

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Build systems

Today:

• Intro to build systems
• Build commands in a script
• Build commands in Makefile

📺 Watch the related YouTube video!

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Special symbols used in slides

• 🎨 - Style recommendation
• 🎓 - Software design recommendation
• 😱 - Not a good practice! Avoid in real life!
• ✅ - Good practice!
• ❌ - Whatever is marked with this is wrong
• 🚨 - Alert! Important information!
• 💡 - Hint or a useful exercise
• 🔼1️⃣7️⃣ - Holds for this version of C++(here, 17) and above
• 🔽1️⃣1️⃣ - Holds for versions until this one C++(here, 11)

Style (🎨) and software design (🎓) recommendations mostly come from Google Style Sheet and the CppCoreGuidelines

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Let's set up our C++ files (again)

blah.h

#pragma once
namespace tools {
void PrintHello();
} // namespace tools

blah.cpp

#include <blah.h>
#include <iostream>
namespace tools {
void PrintHello() { std::cout << "Hello!" << std::endl; }
} // namespace tools

main.cpp

#include <blah.h>
int main() {
  tools::PrintHello();
  return 0;
}

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It's annoying to compile libraries by hand, isn't it?

• There are many commands we have to type in the right order
• If we mess up their order --- the libraries don't compile!
• We can definitely do better!
• 🤔 Naive solution: we could write a bash script! build.sh
  

  # Building an executable from main.cpp using library blah.cpp
  c++ -std=c++17 -c blah.cpp -I . -o blah.o
  ar rcs libblah.a blah.o
  c++ -std=c++17 main.cpp -L . -I . -lblah -o main

• Now imagine building many libraries and executables or only rebuilding when the code changes
• We have to write lots of bash code to support this
• To avoid that, people have written multiple build systems!

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Make - a basic build system

• Stores the commands in a Makefile file
• Commands are executed as a chain of dependencies
• 🚨 Use tabs (↹) not spaces (⎵) to indent code in this file!
• Has block structure and simple syntax (for simple cases 😉)
  

  target: dependencies
    command
    command
    command

• Run make target from a folder with Makefile
• Can have many targets with many dependencies
• If target file exists and is not older than its dependencies --- nothing happens!
• 💡 This allows us to avoid doing the work twice!
• 💡 A very good make tutorial is here

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A Makefile to build C++ code

all: main

main: main.cpp libblah.a
	c++ -std=c++17 main.cpp -I . -L . -lblah -o main

libblah.a: blah.o
	ar rcs libblah.a blah.o

blah.o: blah.cpp
	c++ -std=c++17 -c blah.cpp -I . -o blah.o

clean:
	rm -f blah.o libblah.a main

• Run this with make from the same folder
• 💡 Follow the order of operations here! What happens when?
• 💡 Try to run make again! What happens now?
• 💡 Change a file and re-run make again! Did it re-build?
• 💡 Run make clean to remove the generated files

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Problems with Makefiles 🤔

• Makefiles are very powerful but can get pretty complex
• The syntax is finicky and a bit too detailed, not really abstract
• Writing a Makefile for a big project can be hard
• Supporting different configurations also gets complex

Alternatives?

• Use a different build system, e.g. ninja, bazel etc.
• And/or use a build generator, like CMake, Meson, etc.

Stay tuned!

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