How to get current disk number/name which contains running Windows/Linux?

Let's assume that a PC contains several storage devices and we have C++/Qt utility which contains 2 blackbox functions (we don't consider their implementation here):

QString get_disk_serial(int drive_number); //for Windows 

QString get_disk_serial(const QString& device); //for Linux. For example, "/dev/sda"

I need to know (using C++) which disk is used in the current session (what disk number is active/what device name is active). In my context active disk is a disk which contains the partition, which in turn contains running operating system.