how to use c++ with cubeIDE?

You will have to wait until someone does this in ST first, then write a PDF flow with SCREEN captures showing the exact flow on how this is down. Unfortunately the STM32CubeIDE is very new (early 2019 release) and very little documentation is released other than one video (5:37 minutes) and it does not talk about C++.

My input is to start your design using SW4STM32 where there is lots of documentation including the UM1766 manual showing the many available IDE tools. There is documentation for using these other IDEs with STM32CubeMX.

I suggest SW4STM32 as it is the closest in view as the STM32CubeIDE. Once ST gets more PDFs on the website you will be able to move your code to the STMCubeIDE.

please provide a detailed description what you have done and what the error is. C++ works for me.