volatile(讀書筆記)

The grammar of volatile is the same with that of const, but volatile means "this data may
be changed besides the compiler's eyeshot". For some reason, may be multitask, multithread,
interrupt, this data are changed by the enviroment, so we declare it volatile to tell
compiler not to do any optimization on it.
    If the complier says, "I have read the data into a register, and haven't attained to it
since that", in common, it doesn't need to read it any more. But if it was specified by vo-
latile, the compiler must read it again.

//volatile.cpp
//The volatile keyword

class Comm
{
    const volatile unsigned char byte;
    volatile unsigned char flag;
    enum {bufsize = 100};
    unsigned char buf[bufsize];
    int index;
public:
    Comm();
    void isr() volatile;
    char read(int index) const;   
};

Comm::Comm() : index(0), byte(0), flag(0) {}

//Only a demo: won't actually work
//as an interrupt service routine:
void Comm::isr() volatile
{
    flag = 0;
    buf[index++] = byte;
    //Wrap to beginning of buffer:
    if(index >= buffsize)    index = 0;
}

char Comm::read(int index) const
{
    if(index < 0 || index >= bufsize)
        return 0;
    return buf[index];
}

int main()
{
    volatile Comm port;
    port.isr(); //OK
    port.read(0); //Error, read() not volatile
}