Std::atomic_ref in C++

The C++ function std::atomic_ref is an effective tool for safe, lock-free concurrent programming. It was included in C++20's introduction of the C++ Standard Library. Because this class offers a reference-like interface to atomic objects, it eliminates the need for explicit locking techniques like mutexes and permits multiple threads to safely access and modify shared data. Although std::atomic manages atomic objects directly, std::atomic_ref acts upon pre-existing non-atomic variables, giving atomic access to them. For this reason, it's especially helpful when working with legacy code or in scenarios where changing the variable type directly to make it atomic isn't possible.

Std::atomic_ref has the important benefit of enforcing atomicity guarantees without needlessly sacrificing performance. In order to ensure effective concurrent access to shared data, it makes use of hardware support for atomic operations whenever feasible. With std::atomic_ref, programmers can create safe and effective concurrent code that reduces the possibility of data races and guarantees consistent behavior in multi-threaded environments.

On the object it references, the std::atomic_ref class template performs atomic operations. The object referenced by std::atomic_ref is regarded as an atomic object for the duration of the object's lifetime. For more information on data races, refer to the memory model. If one thread writes to an atomic object while another reads from it, the behavior is well-defined. Furthermore, atomic object accesses have the ability to create inter-thread synchronization and arrange non-atomic memory accesses according to the specifications provided by std::memory_order.

All std::atomic_refs that reference an object must have lifetimes greater than the object itself. An object can be accessed only through these std::atomic_ref instances, even though any std::atomic_ref instance referencing the object exists. None of the std::atomic_ref objects' subobjects may be referenced by any other std::atomic_ref object at the same time.

Defined in header <atomic>

Declaring std::atomic_ref:

If we want to use the std::atomic_ref function, include the <atomic> header file.

Syntax:

It has the following syntax:

Parameters:

• Type: It is the type of the shared variable.
• shared_var: It is the shared variable to be accessed atomically.

Example 1:

Let us take an example to illustrate the std::atomic_ref() in C++.

Output:

 
Final coordinates: (30000, 30000)   

Explanation:

• In this example, we define a basic Point structure with two integer members, x and y.
• For atomic access, we create a std::atomic_ref named atomicPoint and an instance of Point named point.
• Within the lambda function modifyPoint, every thread iteratively increases the atomicPoint's x and y coordinates.
• Multiple threads are created to change the Point structure simultaneously.
• Each thread increases x and y by 10,000 times, so once all threads have completed running, we print the point structure's final coordinates, which should be (30000, 30000).

Example 2:

Let us take another example to illustrate the std::atomic_ref() in C++.

Output:

 
Final counter value: 40000   

Explanation:

• In this example, our counter variable's initial value is 0.
• We construct an atomicCounter std::atomic_ref to enable atomic access to the counter.
• Every thread repeatedly increases the atomicCounter by 1 inside the lambda function incrementCounter.
• Concurrently incrementing the counter requires the creation of multiple threads.
• Once every thread has completed its execution, we use the atomicCounter object to output the counter's final value.

Conclusion:

In C++, std::atomic_ref provides an effective mechanism to allow safe, lock-free concurrent programming. It performs this by giving atomic objects a reference-like interface, which eliminates the need for explicit locking techniques like mutexes and permits multiple threads to safely access and modify shared data. When working with legacy code or in scenarios where directly changing the variable type to make it atomic is impractical, this feature is especially helpful. The std::atomic_ref uses hardware support for atomic operations to guarantee effective concurrent access to shared data. By utilizing it, programmers can create safe and effective concurrent code that reduces the possibility of data races and guarantees consistent behavior in multi-threaded settings.