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What Is Weak Compare and Exchange In Modern C++?

Since the C++11 standard, the Concurrency Support Library includes built-in support for threads (std::thread) with atomic operations (std::atomic). C++11 provides both weak and strong compare-and-exchange operations in multi-threading applications. Since C++11, weak compare and exchange are used in modern C++ standards such as C++14, C++17, C++20, and in other new standards. In this post, we explain weak compare and exchange with simple examples.

What is weak compare and exchange in C++?

The Weak Compare and Exchange template atomically compares the value pointed to by the atomic object with the value pointed to by expected. This feature comes with C++11 and is used in other modern C++ standards. and performs the following operations based on the comparison results:

  • If the comparison result is true (bitwise-equal), the function replaces the value pointed to by the atomic object with the value desired.
  • If the comparison result is false, the function updates the value in expected with the value pointed by the atomic object

There are two most common syntaxes for the compare_exchange_weak, first,

and in weak compare and exchange atomic operations, we can use std::compare_exchange_weak template with memory order types.

Here is the syntax for the compare_exchange_weak with memory orders,

here is how we can use it,

Note that, providing both compare_exchange_strong and compare_exchange_weak allow us to decide whether we want the library to handle spurious failures (using compare_exchange_strong) or if we want to handle it in our own code (using compare_exchange_weak). The compare_exchange_strong needs extra overhead to retry in the case of failure. For details please see Load-link/store-conditional in Wikipedia.

Is there a simple example of weak compare and exchange in C++?

Here is a simple example about weak compare and exchange in C++.

Let’s assume we have a thread function myf1() that ensures value is changed after a weak compare and exchange. This is how we can do this,

Is there a full example of weak compare and exchange in modern C++?

Let’s assume we have a thread function myf1() that ensures value is changed after a weak compare and exchange. Let’s change after 1 second with myf2(). Here is the full example about weak compare and exchange in C++.

The output will be as follows.

As you see weak compare-and-exchange operation is an atomic operation that can be used in multi-threaded applications to achieve synchronization. For more information about this strong and weak compare and exchange feature, see Strong Compare and Exchange Proposal document.

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About author

Dr. Yilmaz Yoru has 35+ years of coding with more than 30+ programming languages, mostly C++ on Windows, Android, Mac-OS, iOS, Linux, and some other operating systems. He graduated and received his MSc and PhD degrees from the Department of Mechanical Engineering of Eskisehir Osmangazi University. He is the founder and CEO of ESENJA LLC Company. His interests are Programming, Thermodynamics, Fluid Mechanics, Artificial Intelligence, 2D & 3D Designs, and high-end innovations.
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