I built it all from source repositories
I tried qalculate-0.4.0 that time
Compiling any of both 2.0.0 or 2.4.0 libqalculate run smoothly with

./autogen.sh (if needed)
./configure
make
make install (as root)

cmake::

~/setups/pkg/qalculate-0.4.0/build$ cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release -DKDE_INSTALL_LIBDIR=lib -DKDE_INSTALL_USE_QT_SYS_PATHS=ON
-- The C compiler identification is GNU 6.3.0
-- The CXX compiler identification is GNU 6.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- 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
-- Looking for __GLIBC__
-- Looking for __GLIBC__ - found
-- Performing Test _OFFT_IS_64BIT
-- Performing Test _OFFT_IS_64BIT - Success
-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29")
-- Found Readline: /usr/include
-- Checking for module 'libqalculate>=0.9.8'
-- Found libqalculate, version 2.0.0 <============ or 2.4.0 instead
-- Configuring done
-- Generating done
-- Build files have been written to: /_path_/setups/pkg/qalculate-0.4.0/build
continu

then make::

~/setups/pkg/qalculate-0.4.0/build$ make
Scanning dependencies of target org.kde.plasma.qalculate-metadata-json
[ 16%] Generating org.kde.plasma.qalculate-metadata.json
Unknown property type for key "X-KDE-ParentApp" -> falling back to string
Unknown property type for key "X-Plasma-API" -> falling back to string
Unknown property type for key "X-Plasma-MainScript" -> falling back to string
Unknown property type for key "X-Plasma-RemoteLocation" -> falling back to string
Generated "/_path_/setups/pkg/qalculate-0.4.0/build/org.kde.plasma.qalculate-metadata.json"

[ 16%] Built target org.kde.plasma.qalculate-metadata-json
Scanning dependencies of target plasmoidplugin_automoc
[ 33%] Automatic moc for target plasmoidplugin
Generating moc source plasmoidplugin_automoc.dir/moc_plasmoidplugin_DZ66HZV5Z3E2K4.cpp
Generating moc source plasmoidplugin_automoc.dir/moc_qwrapper_S2JPYLPKINLUVR.cpp
Generating moc compilation plasmoidplugin_automoc.cpp
[ 33%] Built target plasmoidplugin_automoc
Scanning dependencies of target plasmoidplugin
[ 50%] Building CXX object plugin/CMakeFiles/plasmoidplugin.dir/plasmoidplugin.cpp.o
[ 66%] Building CXX object plugin/CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o
[ 83%] Building CXX object plugin/CMakeFiles/plasmoidplugin.dir/plasmoidplugin_automoc.cpp.o
[100%] Linking CXX shared library libplasmoidplugin.so
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::getExchangeRatesUpdateTime()':
qwrapper.cpp:(.text+0x60e): undefined reference to `Calculator::getExchangeRatesTime()'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::checkReturnState()':
qwrapper.cpp:(.text+0x80c): undefined reference to `Calculator::aborted()'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::printResultInBase(int, MathStructure&, QString&)':
qwrapper.cpp:(.text+0xc3a): undefined reference to `Calculator::print[abi:cxx11](MathStructure const&, int, PrintOptions const&)'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::runCalculation(std::__cxx11::basic_string, std::allocator > const&)':
qwrapper.cpp:(.text+0x141c): undefined reference to `Calculator::print[abi:cxx11](MathStructure const&, int, PrintOptions const&)'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::worker()':
qwrapper.cpp:(.text+0x2047): undefined reference to `Calculator::startControl(int)'
qwrapper.cpp:(.text+0x205b): undefined reference to `Calculator::stopControl()'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::QWrapper(QObject*)':
qwrapper.cpp:(.text+0x2557): undefined reference to `Calculator::useDecimalPoint(bool)'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::fileDownloaded(QNetworkReply*)':
qwrapper.cpp:(.text+0x2c6a): undefined reference to `Calculator::getExchangeRatesTime()'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `QWrapper::setDecimalSeparator(QString const&)':
qwrapper.cpp:(.text+0x261): undefined reference to `Calculator::useDecimalPoint(bool)'
CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o: In function `_GLOBAL__sub_I_qwrapper.cpp':
qwrapper.cpp:(.text.startup+0x33f): undefined reference to `Number::Number(long, long, long)'
collect2: error: ld returned 1 exit status
plugin/CMakeFiles/plasmoidplugin.dir/build.make:147: recipe for target 'plugin/libplasmoidplugin.so' failed
make[2]: *** [plugin/libplasmoidplugin.so] Error 1
CMakeFiles/Makefile2:155: recipe for target 'plugin/CMakeFiles/plasmoidplugin.dir/all' failed
make[1]: *** [plugin/CMakeFiles/plasmoidplugin.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2

123

Usually whenever libqalculate was upgraded, all that I had to do is to recompile the plasmoid. However, after I upgraded libqalculate to 2.0 and recompiling the plasmoid, I was unable to start plasmashell; there was a "malloc(): memory corruption" message even. I was able to get plasmashell starting again after removing the plasmoid.

The moral of the story is, that plasmoid 0.3x and libqalculate 2.0 do not seem to like each other.

I uploaded a new experimental version to github which should support libqalculate v2.0.0 much better.

https://github.com/dschopf/qalculate/archive/libqalculate-2.0-fix.zip

Thank you! I just used qalculate 0.4 and it works like a charm!

Are you sure this issue is related to the plasmoid? I have upgraded two different machines to libqalculate 2.0 and both are working fine.

You can use "plasmoidviewer --applet org.kde.plasma.qalculate" to run it separately from KDE and see some debug messages in the konsole.

No problems compiling and installing 0.3. Great update. Thanks!!

Thanks!! however, I cannot build 0.2

This is the relevant compiler output:

[ 66%] Building CXX object plugin/CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o
/home/eduardo/Downloads/build/qalculate-0.2/plugin/qwrapper.cpp: In constructor ‘QWrapper::QWrapper(QObject*)’:
/home/eduardo/Downloads/build/qalculate-0.2/plugin/qwrapper.cpp:46:31: error: ‘bind’ is not a member of ‘std’
m_thread = std::thread(std::bind(&QWrapper::worker, this));
^~~~
/home/eduardo/Downloads/build/qalculate-0.2/plugin/qwrapper.cpp:46:31: note: suggested alternative: ‘find’
m_thread = std::thread(std::bind(&QWrapper::worker, this));
^~~~
find
make[2]: *** [plugin/CMakeFiles/plasmoidplugin.dir/build.make:87: plugin/CMakeFiles/plasmoidplugin.dir/qwrapper.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:156: plugin/CMakeFiles/plasmoidplugin.dir/all] Error 2
make: *** [Makefile:139: all] Error 2

Just opened an isse on the github project page about this.

This issue has been fixed in v0.2.1.

Indeed! THanks!!

Can you please add a pre-compiled version, so it can be installed via GHNS?
I really missed this applet from KDE4

When I try using "cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release -DKDE_INSTALL_LIBDIR=lib -DKDE_INSTALL_USE_QT_SYS_PATHS=ON"
I got an error

"CMake Error at CMakeLists.txt:10 (find_package):
Could not find a package configuration file provided by "ECM" with any of
the following names:

ECMConfig.cmake
ecm-config.cmake

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

You probably are missing the Extra CMake Modules (ECM) package.
Just search your package manager for "ecm" or "extra cmake modules" and the
installation should be pretty easy and straightforward.

I have libqalculate installed but I cannot install this plasmoid. How can I do this?

I just added install instuctions to the description. If that doesn't work for you please let me know.

Thank you! Worked very well. I use it on the panel so I would suggest the following: a) change the icon (it's a sore bleed among the Plasma/Breeze minimalism) or either replace the panel version of the widget with a small input box to directly input expressions. Thanks!!

Using an input box as a widget seems to be rather difficult and not
properly supported by KDE. I will look into making the icon
configurable.

I missed this widget so much... Thanks for the good work!