Unknown CMake Qt5 command

Question

I am currently trying to build a project using CMake. This is the output that I get

CMake Warning at ext/quazip/CMakeLists.txt:4 (FIND_PACKAGE):
  By not providing "FindQt5.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "Qt5", but
  CMake did not find one.

  Could not find a package configuration file provided by "Qt5" with any of
  the following names:

    Qt5Config.cmake
    qt5-config.cmake

  Add the installation prefix of "Qt5" to CMAKE_PREFIX_PATH or set "Qt5_DIR"
  to a directory containing one of the above files.  If "Qt5" provides a
  separate development package or SDK, be sure it has been installed.


CMake Error at ext/quazip/CMakeLists.txt:19 (qt5_wrap_cpp):
  Unknown CMake command "qt5_wrap_cpp".

I have Qt Creator installed and I am not sure why I am getting this error. I looked around on the web and they states that I should add

set(QT_USE_QTXML true)
FIND_PACKAGE(Qt5)

after the #project in ext/quazip/CMakeLists.txt I have done that however I am still getting this error any suggestions ?

Update: So I added the following to my top level CMakeLists.txt

find_package( Qt5Core REQUIRED )
find_package( Qt5Widgets REQUIRED )
find_package( Qt5Gui REQUIRED )
find_package( Qt5OpenGL REQUIRED )
find_package( Qt5Multimedia REQUIRED )
find_package( Qt5WebKitWidgets REQUIRED )
find_package( Qt5Xml REQUIRED )

I cleared the cache and then configured. At that point I get the following error.

CMake Error at CMakeLists.txt:3 (find_package):
  By not providing "FindQt5Core.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "Qt5Core", but
  CMake did not find one.

  Could not find a package configuration file provided by "Qt5Core" with any
  of the following names:

    Qt5CoreConfig.cmake
    qt5core-config.cmake

  Add the installation prefix of "Qt5Core" to CMAKE_PREFIX_PATH or set
  "Qt5Core_DIR" to a directory containing one of the above files.  If
  "Qt5Core" provides a separate development package or SDK, be sure it has
  been installed.


Configuring incomplete, errors occurred!

Answer 1

To use Qt5 in CMakeLists.txt you should use (a combination of) those functions:

In top level CMakeLists.txt

#### Qt5
find_package( Qt5Core REQUIRED )
find_package( Qt5Widgets REQUIRED )
find_package( Qt5Gui REQUIRED )
find_package( Qt5OpenGL REQUIRED )
find_package( Qt5Multimedia REQUIRED )
find_package( Qt5WebKitWidgets REQUIRED )
find_package( Qt5Xml REQUIRED )
set( CMAKE_AUTOMOC ON )  # to automoc remove the need to wrap by hand

EDIT: The Qt5 cmake modules are provided by Qt5 itself, so you need to tell cmake where qt can be found. See the Qt5 CMake doc about that.

Answer 2

So finally, with some searching, I was able to get CANDevStudio -- which was also having this Qt5 problem -- working on Ubuntu 20.04 by:

Step 1: Manually download and install Qt from their website (requires free account creation): https://download.qt.io/archive/qt/

According to: This Website, it may be necessary to remove any other versions of libqt5:

sudo apt purge libqt5svg5-def
sudo apt purge libqt5serialbus5-dev

Step 2: Build the Software with the DCMAKE_PREFIX_PATH flag set to the directory of the newly installed software (in my case, I put it in my home directory):

cmake .. -DCMAKE_PREFIX_PATH=/home/<username>/Qt5.12.0/5.12.0/gcc_64

Source: CanDevStudio's Help Page