Chapter 1. Hello, world of concurrency in C++!

This chapter covers

What is meant by concurrency and multithreading

Why you might want to use concurrency and multithreading in your applications

Some of the history of the support for concurrency in C++

What a simple multithreaded C++ program looks like

These are exciting times for C++ users. Thirteen years after the original C++ Standard was published in 1998, the C++ Standards Committee gave the language and its supporting library a major overhaul. The new C++ Standard (referred to as C++11 or C++0x) was published in 2011 and brought with it a swath of changes that made working with C++ easier and more productive. The Committee also committed to a new "train model" of releases, with a new C++ Standard to be published every three years. So far, we've had two of these publications: the C++14 Standard in 2014, and the C++17 Standard in 2017, as well as several Technical Specifications describing extensions to the C++ Standard.

One of the most significant new features in the C++11 Standard was the support of multithreaded programs. For the first time, the C++ Standard acknowledged the existence of multithreaded applications in the language and provided components in the library for writing multithreaded applications. This made it possible to write multithreaded C++ programs without relying on platform-specific extensions and enabled you to write portable multithreaded code with guaranteed behavior. It also came at a time when programmers were increasingly looking to concurrency in general, and multithreaded programming in particular, to improve application performance. The C++14 and C++17 Standards have built upon this baseline to provide further support for writing multithreaded programs in C++, as have the Technical Specifications. There's a Technical Specification for concurrency extensions, and another for parallelism, though the latter has been incorporated into C++17.

This book is about writing programs in C++ using multiple threads for concurrency and the C++ language features and library facilities that make it possible. I'll start by explaining what I mean by concurrency and multithreading and why you would want to use concurrency in your applications. After a quick detour into why you might not want to use it in your applications, we'll go through an overview of the concurrency support in C++, and we'll round off this chapter with a simple example of C++ concurrency in action. Readers experienced with developing multithreaded applications may wish to skip the early sections. In subsequent chapters, we'll cover more extensive examples and look at the library facilities in more depth. The book will finish with an in-depth reference to all the C++ Standard Library facilities for multithreading and concurrency.

So, what do I mean by concurrency and multithreading?

1.1. What is concurrency?

At the simplest and most basic level, concurrency is about two or more separate activities happening at the same time. We encounter concurrency as a natural part of life; we can walk and talk at the same time or perform different actions with each hand, and we each go about our lives independently of each other-you can watch football while I go swimming, and so on.