Getting started with CMake

CMake is a group of tools that allow to build, test, and package applications. Just like Qt, it is available on all major development platforms. It is also supported by various IDE's, including Qt Creator.

In this section we will show the most basic way to use Qt in a CMake project. We will walk through an example project that is utilizing Qt Widgets.

Build a C++ GUI executable

A CMake project is defined by files written in the CMake language. The main file is called CMakeLists.txt, and is usually placed in the same directory as the actual program sources.

Here is a typical CMakeLists.txt file for an application written in C++ and using Qt and Qt Widgets:

cmake_minimum_required(VERSION 3.16)

project(helloworld VERSION 1.0.0 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
set(CMAKE_AUTOUIC ON)

find_package(Qt6 COMPONENTS Widgets REQUIRED)

add_executable(helloworld
    mainwindow.ui
    mainwindow.cpp
    main.cpp
    resources.qrc
)

target_link_libraries(helloworld PRIVATE Qt6::Widgets)

Let's go through the content.

cmake_minimum_required(VERSION 3.16)

cmake_minimum_required() specifies the minimum CMake version that the application requires. Qt itself requires at least CMake version 3.16. If you use a Qt that was built statically - the default in Qt for iOS and Qt for WebAssembly - you need CMake 3.21.1 or newer.

project(helloworld VERSION 1.0.0 LANGUAGES CXX)

project() sets a project name and the default project version. The LANGUAGES argument tells CMake that the program is written in C++.

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

Qt 6 requires a compiler supporting C++ version 17 or newer. Enforcing this by setting the CMAKE_CXX_STANDARD, CMAKE_CXX_STANDARD_REQUIRED variables will let CMake print an error if the compiler is too old.

set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
set(CMAKE_AUTOUIC ON)

Qt applications typically make use of the Meta-Object Compiler (moc), Resource Compiler (rcc), and User Interface Compiler (uic) that come with Qt. Setting the CMAKE_AUTOMOC, CMAKE_AUTORCC, and CMAKE_AUTOUIC variables to ON will let CMake automatically set up rules so that the respective compilers are called transparently, when required.

find_package(Qt6 COMPONENTS Widgets REQUIRED)

This tells CMake to look up Qt 6, and import the Widgets module. There is no point in continuing if CMake cannot locate the module, so we do set the REQUIRED flag to let CMake abort in this case.

If successful, the module will set some CMake variables documented in Module variables. It furthermore imports the Qt6::Widgets target that we use below.

For find_package to be successful, CMake must find the Qt installation. There are different ways you can tell CMake about Qt, but the most common and recommended approach is to set the CMake cache variable CMAKE_PREFIX_PATH to include the Qt 6 installation prefix. Note that Qt Creator will handle this transparently for you.

add_executable(helloworld
    mainwindow.ui
    mainwindow.cpp
    main.cpp
    resources.qrc
)

add_executable() tells CMake that we want to build an executable (so not a library) called helloworld as a target. The target should be built from C++ code (mainwindow.cpp, main.cpp), a Qt Designer file (mainwindow.ui), and a Qt Resource System file (resources.qrc).

Note that you typically do not list header files here. This is different from qmake, where header files need to be explicitly listed so that they are processed by the Meta-Object Compiler (moc).

For less trivial projects, you may want to call qt_add_executable() instead. It is a wrapper around the built-in add_executable() command, providing additional logic to automatically handle things like linking of Qt plugins in static Qt builds, platform-specific customization of library names and so on.

target_link_libraries(helloworld PRIVATE Qt6::Widgets)

Finally, target_link_libraries tells CMake that the helloworld executable makes use of Qt Widgets by referencing the Qt6::Widgets target imported by the find_package() call above. This will not only add the right arguments to the linker, but also makes sure that the right include directories, compiler definitions are passed to the C++ compiler. The PRIVATE keyword is not strictly necessary for an executable target, but it is good practice to specify it. If helloworld was a library rather than an executable, then either PRIVATE or PUBLIC should be specified (PUBLIC if the library mentions anything from Qt6::Widgets in its headers, PRIVATE otherwise).

Further reading

The official CMake Documentation is an invaluable source for working with CMake.

The book Professional CMake: A Practical Guide provides a great introduction to the most relevant CMake features.

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