Hello-world example

We start with a simple C++ program:

#include <iostream>

int main()
{
    std::cout << "Hello World!" << std::endl;
}

Create a fresh directory and save the C++ code to a file called hello.cpp.

We wish to compile this code to hello.x.

For this we create a file called CMakeLists.txt which contains:

# stop if cmake version is below 3.0
cmake_minimum_required(VERSION 3.0 FATAL_ERROR)

# project name and enable C++ support
project(hello LANGUAGES CXX)

# require C++11
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# we define the executable
add_executable(hello.x "")

# and its sources
target_sources(
  hello.x
  PRIVATE
    hello.cpp
  )

Your directory should look like this:

$ ls

CMakeLists.txt	hello.cpp

Now we create a build directory (out of source compilation), change to it, and configure the project:

$ mkdir build
$ cd build/
$ cmake ..

-- The CXX compiler identification is GNU 7.2.0
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/user/example/build

Now we are ready to compile the code:

$ make

Scanning dependencies of target hello.x
[ 50%] Building CXX object CMakeFiles/hello.x.dir/hello.cpp.o
[100%] Linking CXX executable hello.x
[100%] Built target hello.x

Done. In the following we will learn what happened here behind the scenes.

Have you used git init? Good! You probably want to ignore the build/ directory.

Name and location of the build directory

We have configured and compiled the code like this:

$ mkdir build
$ cd build/
$ cmake ..
$ make

But there is nothing special about build/ - we could have done this instead:

$ mkdir -p /tmp/debug
$ cd /tmp/debug
$ cmake /path/to/source
$ make

Benefits of out-of-source compilation

• CMake looks for CMakeLists.txt and processes this file
• CMake puts everything into ${PROJECT_BINARY_DIR} and does not pollute ${PROJECT_SOURCE_DIR}
• We can build different binaries with the same source:
  • Sequential and parallel builds
  • Debug build or optimized build
  • Production and debugging compilations