Volatile

Definition

The volatile variable is one whose value may change due to an external event.

Usually, we can only change the value of a variable within our code. Let’s say there is an external I/O event that tries to change the value of the variable. This would not be allowed.

However, it would be possible if the variable was volatile. A volatile variable can be declared using the volatile keyword.

volatile int myInt{2011}

We can find the keyword in Java and C# as well.

volatile vs std::atomic

What do the volatile keywords in C# and Java have in common with the volatile keyword in C++? Nothing!

It’s so easy in C++.

1. volatile is for special objects, on which optimized read or write operations are not allowed.

2. std::atomic defines atomic variables, which are meant for thread-safe reading and writing. It’s so easy, but the confusion starts exactly here. The volatile keyword in Java and C# is equivalent to std::atomic in C++. In other words, volatile has no multithreading semantics in C++.

volatile is typically used in embedded programming to denote objects that can change independently of the regular program flow. These are, for example, objects that represent an external device (memory-mapped I/O). Because these objects can change independent of the regular program flow, their values will be written directly in the main memory. Hence, there is no optimized storing in caches.

Example