Thread Creation

28 April 2016

Contents[Show]

Thread creation is easy. Call std::thread, and a new thread will be created. The thread gets a work package and starts it immediately. The creator of the thread (the Parent) has to take care of the created thread (the child). The parent should wait until their child completes their task or has to detach himself from the child. The child thread can get its payload task arguments by copy or by reference.

That was too fast. So the details will follow.

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Creation and execution of a thread

Now, a more formal approach: a thread gets a Callable and starts it immediately.

This sentence needs a few notes.

A Callable is an entity that behaves like a function. It can be a function, a function object, or a lambda function.
A function object is an instance of a class for which the call operator is overloaded. The key difference between functions and function objects is that a function object can have a state.
A lambda (anonymous function) is a pure function body without a name. It can be invoked just in place. A lambda function can capture its calling context. That's why they are often called closures.

After the theory, a small example.

// createThread.cpp

#include <iostream>
#include <thread>

void helloFunction(){
  std::cout << "Hello C++11 from function." << std::endl;
}

class HelloFunctionObject  {
  public:
    void operator()() const {
      std::cout << "Hello C++11 from a function object." << std::endl;
    }
};


int main(){

  std::cout << std::endl;

  // thread executing helloFunction
  std::thread t1(helloFunction);

  // thread executing helloFunctionObject
  HelloFunctionObject helloFunctionObject;
  std::thread t2(helloFunctionObject);

  // thread executing lambda function
  std::thread t3([]{std::cout << "Hello C++11 from lambda function." << std::endl;});

  // ensure that t1, t2 and t3 have finished before main terminates
  t1.join();
  t2.join();
  t3.join();

  std::cout << std::endl;

};

All threads - t1, t2, and t3 - write their messages to the console. The work package of thread t2 is a function object (lines 10 - 15), and the work package of thread t3 is a lambda function (line 29). In lines 32 - 34, the Main thread or Parent waits until his children are done.

Let's have a look at the output. This is more interesting.

createThread

The two programs' execution results differ in two aspects. First, child threads will be executed in a different order. Second, the output is a little bit of a mess. So, in the second run, the line break of the function helloFunction happens after the lambda function call.

What's next?

The next article will be about the lifetime of a thread. (Proofreader Alexey Elymanov)

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