Chapter 130: Build Systems

C++, like C, has a long and varied history regarding compilation workflows and build processes. Today, C++ has various popular build systems that are used to compile programs, sometimes for multiple platforms within one build system. Here, a few build systems will be reviewed and analyzed.

Section 130.1: Generating Build Environment with CMake

CMake generates build environments for nearly any compiler or IDE from a single project definition. The following examples will demonstrate how to add a CMake file to the cross-platform "Hello World" C++ code.

CMake files are always named "CMakeLists.txt" and should already exist in every project's root directory (and possibly in sub-directories too.) A basic CMakeLists.txt file looks like:

cmake_minimum_required(VERSION 2.4) project(HelloWorld) add_executable(HelloWorld main.cpp)

See it live on Coliru.

This file tells CMake the project name, what file version to expect, and instructions to generate an executable called "HelloWorld" that requires main.cpp.

Generate a build environment for your installed compiler/IDE from the command line:

> cmake .

Build the application with:

> cmake --build .

This generates the default build environment for the system, depending on the OS and installed tools. Keep source code clean from any build artifacts with use of "out-of-source" builds:

>mkdir build

>cd build

>cmake ..

>cmake --build .

CMake can also abstract the platform shell's basic commands from the previous example:

>cmake -E make_directory build

>cmake -E chdir build cmake ..

>cmake --build build

CMake includes generators for a number of common build tools and IDEs. To generate makefiles for Visual Studio's nmake:

>cmake -G "NMake Makefiles" ..

>nmake

Section 130.2: Compiling with GNU make

Introduction

The GNU Make (styled make) is a program dedicated to the automation of executing shell commands. GNU Make is one specific program that falls under the Make family. Make remains popular among Unix-like and POSIX-like operating systems, including those derived from the Linux kernel, Mac OS X, and BSD.

GNU Make is especially notable for being attached to the GNU Project, which is attached to the popular GNU/Linux operating system. GNU Make also has compatible versions running on various flavors of Windows and Mac OS X. It is also a very stable version with historical significance that remains popular. It is for these reasons that GNU Make is often taught alongside C and C++.

Basic rules

To compile with make, create a Makefile in your project directory. Your Makefile could be as simple as:

Makefile

#Set some variables to use in our command

#First, we set the compiler to be g++ CXX=g++

#Then, we say that we want to compile with g++'s recommended warnings and some extra ones. CXXFLAGS=-Wall -Wextra -pedantic

#This will be the output file

EXE=app

SRCS=main.cpp

#When you call `make` at the command line, this "target" is called.

#The $(EXE) at the right says that the `all` target depends on the `$(EXE)` target.

#$(EXE) expands to be the content of the EXE variable

#Note: Because this is the first target, it becomes the default target if `make` is called without target

all: $(EXE)

#This is equivalent to saying

#app: $(SRCS)

#$(SRCS) can be separated, which means that this target would depend on each file.

#Note that this target has a "method body": the part indented by a tab (not four spaces).

#When we build this target, make will execute the command, which is:

#g++ -Wall -Wextra -pedantic -o app main.cpp

#I.E. Compile main.cpp with warnings, and output to the file ./app

$(EXE): $(SRCS)

@$(CXX) $(CXXFLAGS) -o $@ $(SRCS)

#This target should reverse the `all` target. If you call

#make with an argument, like `make clean`, the corresponding target

#gets called.

clean:

@rm -f $(EXE)

NOTE: Make absolutely sure that the indentations are with a tab, not with four spaces. Otherwise, you'll get an error of Makefile:10: *** missing separator. Stop.

To run this from the command-line, do the following:

$ cd ~/Path/to/project $ make

$ ls

app main.cpp Makefile

$ ./app Hello World!

$ make clean $ ls

main.cpp Makefile

Incremental builds

When you start having more files, make becomes more useful. What if you edited a.cpp but not b.cpp? Recompiling b.cpp would take more time.

With the following directory structure:

.

+-- src

|+-- a.cpp

|+-- a.hpp

+-- Makefile

This would be a good Makefile:

Makefile

CXX=g++

CXXFLAGS=-Wall -Wextra -pedantic

EXE=app

SRCS_GLOB=src/*.cpp

SRCS=$(wildcard $(SRCS_GLOB))

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

all: $(EXE)

$(EXE): $(OBJS)

@$(CXX) -o $@ $(OBJS)

depend: .depend

.depend: $(SRCS)

@-rm -f ./.depend

@$(CXX) $(CXXFLAGS) -MM $^>>./.depend

clean:

-rm -f $(EXE) -rm $(OBJS) -rm *~

-rm .depend

include .depend