Volatile
In this lesson, we'll understand how volatile variables behave.
We'll cover the following
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++.
-
volatile
is for special objects, on which optimized read or write operations are not allowed. -
std::atomic
defines atomic variables, which are meant for thread-safe reading and writing. It’s so easy, but the confusion starts exactly here. Thevolatile
keyword in Java and C# is equivalent tostd::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 #
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