Every modern programming language supports concurrency, offering high-level APIs that allow the execution of many tasks simultaneously. C++ supports concurrency starting from C++11 and more sophisticated APIs got added further in C++14 and C++17. Though the C++ thread support library allows multithreading, it requires writing lengthy code using complex synchronizations; however, concurrency lets us execute independent tasks--even loop iterations can run concurrently without writing complex code. The bottom line is parallelization is made more easy with concurrency.  

The concurrency support library complements the C++ thread support library. The combined use of these two powerful libraries makes concurrent programming more easy in C++.

Let's write a simple Hello World program using C++ concurrency in the following file named main.cpp:

#include <iostream>
#include <future>
using namespace std;

void sayHello( ) {
  cout << endl << "Hello Concurrency support library!" << endl;
}

int main ( ) {
  future<void> futureObj = async ( launch::async, sayHello );
  futureObj.wait( );

  return 0;
}

Let's try to understand the main() function. Future is an object of the concurrency module that helps the caller function retrieve the message passed by the thread in an asynchronous fashion. The void in future<void> represents the sayHello() thread function that is not expected to pass any message to the caller, that is, the main thread function. The async class lets us execute a function in two modes, namely launch::async or launch::deferred mode.

The launch::async mode lets the async object launch the sayHello() method in a separate thread, whereas the launch::deferred mode lets the async object invoke the sayHello() function without creating a separate thread. In launch::deferred mode, the sayHello() method invocation will be different until the caller thread invokes the future::get() method.

The futureObj.wait() voice is used to block the main thread to let the sayHello() function complete its task. The future::wait() function is similar to thread::join() in the thread support library.