You seem to be looking for the kernel driver for the GPIO pins.
That file may or may not be it but:
when does pi use that file?
A Linux device driver can be:
a) Compiled and linked into the kernel binary when you build the kernel. A disk driver for example needs to be built into the kernel so that it is available at start up. You can't read a disk with out a driver and you need to read the disk to continue boot up of the OS.
b) Compiled into a separate "kernel module" that can be loaded and unloaded from the kernel at run time. Such a driver might be for USB devices. The driver being loaded when a particular USB device is plugged in at run time.
What actually are you trying to do?
Memory in C++ is a leaky abstraction .