Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

volatile

The syntax of volatile is identical to that for const, but volatile means “This data may change outside the knowledge of the compiler.” Somehow, the environment is changing the data (possibly through multitasking, multithreading or interrupts), and volatile tells the compiler not to make any assumptions about that data, especially during optimization.

If the compiler says, “I read this data into a register earlier, and I haven’t touched that register,” normally it wouldn’t need to read the data again. But if the data is volatile, the compiler cannot make such an assumption because the data may have been changed by another process, and it must reread that data rather than optimizing the code to remove what would normally be a redundant read.

You create volatile objects using the same syntax that you use to create const objects. You can also create const volatile objects, which can’t be changed by the client programmer but instead change through some outside agency. Here is an example that might represent a class associated with some piece of communication hardware: 

//: C08:Volatile.cpp
// The volatile keyword

class Comm {
  const volatile unsigned char byte;
  volatile unsigned char flag;
  static const int 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 {
  if(flag) flag = 0;
  buf[index++] = byte;
  // Wrap to beginning of buffer:
  if(index >= bufsize) 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
} ///:~

As with const, you can use volatile for data members, member functions, and objects themselves. You can only call volatile member functions for volatile objects.

The reason that isr( ) can’t actually be used as an interrupt service routine is that in a member function, the address of the current object ( this) must be secretly passed, and an ISR generally wants no arguments at all. To solve this problem, you can make isr( ) a static member function, a subject covered in a future chapter.

The syntax of volatile is identical to const, so discussions of the two are often treated together. The two are referred to in combination as the c-v qualifier.

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