Multithreading in C++
Multithreading is a feature that allows concurrent execution of two or more parts of a program for maximum utilization of the CPU. Each part of such a program is called a thread. So, threads are lightweight processes within a process.
Multithreading support was introduced in C++11. Prior to C++11, we had to use POSIX threads or <pthreads> library. 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.
Syntax:
std::thread thread_object (callable);
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 any of the five:
- A Function Pointer
- A Lambda Expression
- A Function Object
- Non-Static Member Function
- Static Member Function
After defining the callable, we pass it to the constructor.
Launching Thread Using Function Pointer
A function pointer can be a callable object to pass to the std::thread constructor for initializing a thread. The following code snippet demonstrates how it is done.
Example:
C++
void foo(param) { Statements; } // The parameters to the function are put after the comma std:: thread thread_obj(foo, params); |
Launching Thread Using Lambda Expression
std::thread object can also be launched using a lambda expression as a callable. The following code snippet demonstrates how this is done:
Example:
C++
// Define a lambda expression auto f = [](params) { Statements; }; // Pass f and its parameters to thread // object constructor as std:: thread thread_object(f, params); |
Launching Thread Using Function Objects
Function Objects or Functors can also be used for launching a thread in C++. The following code snippet demonstrates how it is done:
Example:
C++
// Define the class of function object class fn_object_class { // Overload () operator void operator()(params) { Statements; } } // Create thread object std:: thread thread_object(fn_object_class(), params) |
Note: We always pass parameters of the callable separately as arguments to the thread constructor.
Launching Thread Using Non-Static Member Function
We can also launch the thread using the non-static member function of a class. The following snippet demonstrates how to do it.
C++
// defining clasc class Base { public : // non-static member function void foo(param) { Statements; } } // object of Base Class Base b; // first parameter is the reference to the functionn // and second paramter is reference of the object // at last we have arguments std:: thread thread_obj(&Base::foo, &b, params); |
Launching Thread Using Static Member Function
We can also launch the threads using static member functions.
C++
// defining class class Base { public : // static member function static void foo(param) { Statements; } } // object of Base Class Base b; // first parameter is the reference to the function // and rest are arguments std:: thread thread_obj(&Base::foo, 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:
C++
int main() { // Start thread t1 std:: thread t1(callable); // Wait for t1 to finish t1.join(); // t1 has finished do other stuff Statements; } |
A Complete C++ Program For Multithreading
A C++ program is given below. It launches three threads from the main function. Each thread is called using one of the callable objects specified above.
C++
// C++ 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" ; } }; // class definition class Base { public : // non-static member function void foo() { cout << "Thread using non-static member function " "as callable" << endl; } // static member function static void foo1() { cout << "Thread using static member function as " "callable" << endl; } }; // Driver code 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 // lambda expression as callable thread th3(f, 3); // object of Base Class Base b; thread th4(&Base::foo, &b); thread th5(&Base::foo1); // 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(); // Wait for thread t4 to finish th4.join(); // Wait for thread t5 to finish th5.join(); return 0; } |
Output (Machine Dependent)
Threads 1 and 2 and 3 operating independently
Thread using function pointer as callable
Thread using function pointer as callable
Thread using function pointer as callable
Thread using non-static member function as callable
Thread using function object as callable
Thread using function object as callable
Thread using function object as callable
Thread using lambda expression as callable
Thread using lambda expression as callable
Thread using lambda expression as callable
Thread using static member function as callable
Note: To compile programs with std::thread support use g++ -std=c++11 -pthread.
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