There is nothing really changed when you switch a project to C++ (except that the menu entry “Convert to C++” now reads “Convert to C” ); the only change you will see is that the linker will now use arm-none-eabi-g++ instead of -gcc (and that in project properties build tools settings you will see the G++ compiler).
However there may be a problem at link time; it seems to be that, when converting your project from C to C++, you lose the link to the HAL library... That seems to be a small bug in the tool that we will investigate, although there is a quite simple workaround in th emean-time.
To correct it you should recreate this link by:
- project >> Properties >> C/C++ General >> Paths and Symbols >> References
- Here the HAL library shoudl be selected then you need first to deselect it the Apply your change
- Now reselect it and click OK
Now your project should compile and link correctly in C++ and you could add C++ code in it.