How do I use Qt headers in a CMake sub-project?

[tuzepoito]

I'm using Windows. I have a CMake project that uses Qt and I wish to divide it into subprojects for clarity.

However, if I compile a class outside of the root folder, I get an error that CMake cannot find the Qt headers.

For example, let's say I have the following project tree :

root/
-->gui/
   -->CMakeLists.txt
   -->mainwindow.cpp
   -->mainwindow.h
-->CMakeLists.txt
-->main.cpp

CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)

project(testcmake VERSION 0.1 LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

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

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(QT NAMES Qt6 Qt5 COMPONENTS Widgets REQUIRED)
find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)

add_subdirectory(gui)

set(PROJECT_SOURCES main.cpp)

add_executable(testcmake ${PROJECT_SOURCES})

target_link_libraries(testcmake PRIVATE gui Qt${QT_VERSION_MAJOR}::Widgets)

main.cpp:

#include "gui/mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  MainWindow w;
  w.show();
  return a.exec();
}

gui/CMakeLists.txt:

set(GUI_SRC mainwindow.cpp mainwindow.h)
add_library(gui ${GUI_SRC})

gui/mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

class QPushButton;

class MainWindow : public QMainWindow
{
  Q_OBJECT

public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();

private:
  QPushButton *m_button;
};
#endif // MAINWINDOW_H

gui/mainwindow.cpp:

#include "mainwindow.h"

#include <QPushButton>

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
{
  m_button = new QPushButton("Hello CMake!", this);
  m_button->resize(150, 50);
}

MainWindow::~MainWindow()
{
}

In this example, if I put class MainWindow in the root folder, the project compiles. But MainWindow class is in a subfolder, I get this error:

[build] ...\build\gui\gui_autogen\include_debug\ewiega46ww\../../../../../gui/mainwindow.h(4): fatal error C1083: Impossible d'ouvrir le fichier include : 'QMainWindow' : No such file or directory [...\build\gui\gui.vcxproj]
[build]   mainwindow.cpp
[build] ...\gui\mainwindow.h(4): fatal error C1083: Impossible d'ouvrir le fichier include : 'QMainWindow' : No such file or directory [...\build\gui\gui.vcxproj]

Which means, at the preprocessing step, the compiler cannot find any Qt header file, because the source file is in a sub-folder.
How do I let CMake use the Qt headers, which are retrieved from the find_package command in the root folder?

[mchinand]

I've been bitten by this before; forgetting to add a library to a target_link_libraries() statement and getting 'include' errors during compilation. Then, not checking the target_link_libraries() statements; wrongly think they would only come into play during linking.

[Christian Ehrlicher]

If you want to use a library you have to link against this library with target_link_libraries()

[mchinand]

I've been bitten by this before; forgetting to add a library to a target_link_libraries() statement and getting 'include' errors during compilation. Then, not checking the target_link_libraries() statements; wrongly think they would only come into play during linking.

[tuzepoito]

OK, I think I have a solution, or maybe a workaround. I added linking to the Qt libraries at the end of gui/CMakeLists.txt:

set(GUI_SRC mainwindow.cpp mainwindow.h)
add_library(gui ${GUI_SRC})
target_link_libraries(gui PUBLIC Qt${QT_VERSION_MAJOR}::Widgets)

and then it compiles.
Even though it's just a library, I guess I still need to link the Qt libraries so my code can use the header files.

[mchinand]

Your gui sub-project still needs to link to the libraries, it's not just so it can find the header files. In the compile/build output you should see a link step.

[Christian Ehrlicher]

@tuzepoito said in How do I use Qt headers in a CMake sub-project?:

I think I have a solution, or maybe a workaround.

Why workaround? If you want to use a library, you have to link against it what you're doing now - how should this be a workaround?