Multithreading support was introduced in C+11. Prior to C++11, we had to use POSIX threads or p threads library in C. While this library did the job the lack of any standard language provided feature-set caused serious portability issues. C++ 11 did away with all that and gave us std::thread. The thread classes and related functions are defined in the thread header file.

std::thread is the thread class that represents a single thread in C++. To start a thread we simply need to create a new thread object and pass the executing code to be called (i.e, a callable object) into the constructor of the object. Once the object is created a new thread is launched which will execute the code specified in callable.

A callable can be either of the three

• A function pointer
• A function object
• A lambda expression

After defining callable, pass it to the constructor.

#include<thread>

std::thread thread_object(callable)

Launching thread using function pointer

The following code snippet demonstrates how this is done

void foo(param)
{
    // Do something
}
  
// The parameters to the function are put after the comma
std::thread thread_obj(foo, params);

Launching thread using lambda expression

The following code snippet demonstrates how this is done

// Define a lamda expression
auto f = [](params) {
    // Do Something
};
  
// Pass f and its parameters to thread 
// object constructor as
std::thread thread_object(f, params);

We can also pass lambda functions directly to the constructor.

std::thread thread_object([](params) {
    // Do Something
};, params);

Launching threads using function objects

The following code snippet demonstrates how this is done

// Define the class of function object
class fn_object_class {
    // Overload () operator
    void operator()(params)
    {
        // Do Something
    }
}
  
// Create thread object
std::thread thread_object(fn_class_object(), params)

Waiting for threads to finish

Once a thread has started we may need to wait for the thread to finish before we can take some action. For instance, if we allocate the task of initializing the GUI of an application to a thread, we need to wait for the thread to finish to ensure that the GUI has loaded properly.

To wait for a thread use the std::thread::join() function. This function makes the current thread wait until the thread identified by *this has finished executing.

For instance, to block the main thread until thread t1 has finished we would do

int main()
{
    // Start thread t1
    std::thread t1(callable);
  
    // Wait for t1 to finish
    t1.join();
  
    // t1 has finished do other stuff
  
    ...
}

A Complete C++ Program

A C++ program is given below. It launches three thread from the main function. Each thread is called using one of the callable objects specified above.

// CPP program to demonstrate multithreading
// using three different callables.
#include <iostream>
#include <thread>
using namespace std;
  
// A dummy function
void foo(int Z)
{
    for (int i = 0; i < Z; i++) {
        cout << "Thread using function"
               " pointer as callable\n";
    }
}
  
// A callable object
class thread_obj {
public:
    void operator()(int x)
    {
        for (int i = 0; i < x; i++)
            cout << "Thread using function"
                  " object as  callable\n";
    }
};
  
int main()
{
    cout << "Threads 1 and 2 and 3 "
         "operating independently" << endl;
  
    // This thread is launched by using 
    // function pointer as callable
    thread th1(foo, 3);
  
    // This thread is launched by using
    // function object as callable
    thread th2(thread_obj(), 3);
  
    // Define a Lambda Expression
    auto f = [](int x) {
        for (int i = 0; i < x; i++)
            cout << "Thread using lambda"
             " expression as callable\n";
    };
  
    // This thread is launched by using 
    // lamda expression as callable
    thread th3(f, 3);
  
    // Wait for the threads to finish
    // Wait for thread t1 to finish
    th1.join();
  
    // Wait for thread t2 to finish
    th2.join();
  
    // Wait for thread t3 to finish
    th3.join();
  
    return 0;
}

• Output (Machine Dependent)

Threads 1 and 2 and 3 operating independently                                                       
Thread using function pointer as callable                                                           
Thread using lambda expression as callable                                                          
Thread using function pointer as callable                                                           
Thread using lambda expression as callable                                                          
Thread using function object as  callable                                                          
Thread using lambda expression as callable                                                          
Thread using function pointer as callable                                                          
Thread using function object as  callable                                                           
Thread using function object as  callable

Note:

To compile programs with std::thread support use

g++ -std=c++11 -pthread

Source: https://www.geeksforgeeks.org/multithreading-in-cpp/