const volatile C++

An object marked as const volatile will not be allowed to change by the code where it is declared. The error will be raised due to the const qualifier. The volatile part of the qualifier means that the compiler cannot optimize the code with respect to the object.

In an embedded system this is typically used to access hardware registers that can be read and are updated by the hardware, so it makes no sense to be able to write to the register via the code. An example might be the status register of a serial port.Various bits will indicate a status like if a character is waiting to be read. Each read to this status register could result in a different value depending on what else has occurred in the serial port hardware. It makes no sense to write to the status register but you need to make sure that each  read of the register results in an actual read of the hardware.

Below is an illustration :

//We assume that the below declared pointers point to the correct
//hardware addresses
unsigned int const volatile *status_reg;
unsigned char const volatile *recv_reg;

#define CHAR_READ 0x01

int get_next_char()
{
 while((*status_reg & CHAR_READ) == 0);
 return *recv_reg;
}