To create a new C++ Project, Go to File > New > Project... > C/C++ > C++ Project. Enter the project name, then select as project type : Executable > Ac6 STM32 MCU Project and as Toolchains : Ac6 STM32 MCU GCC.
Your source files extension have to be .cpp to be built with the G++ Compiler.
For more details, follow this documentation page : http://www.openstm32.org/Creating+a+new+project
(Instead of selecting “C Project”, choose “C++ Project”).
If your have created and imported your project from CubeMX, to convert your C project into C++ project: Right-click on your project, then select “Convert to C++” (ensure you have the latest version of System Workbench, this feature has been fixed recently (Help > Check for Updates...)).
Rename the source file extension into .cpp for the C++ code.
Hope it answer your problem,