c++ threads - parallel processing

I was wondering how to execute two processes in a dual-core processor in c++.

I know threads (or multi-threading) is not a built-in feature of c++.

There is threading support in Qt, but I did not understand anything from their reference. :(

So, does anyone know a simple way for a beginner to do it. Cross-platform support (like Qt) would be very he开发者_如何学Golpful since I am on Linux.

Try the Multithreading in C++0x part 1: Starting Threads as a 101. If you compiler does not have C++0x support, then stay with Boost.Thread

Take a look at Boost.Thread. This is cross-platform and a very good library to use in your C++ applications.

What specifically would you like to know?

The POSIX thread (pthreads) library is probably your best bet if you just need a simple threading library, it has implementations both on Windows and Linux.

A guide can be found e.g. here. A Win32 implementation of pthreads can be downloaded here.

Edit: Didn't see you were on Linux. In that case I'm not 100% sure but I think the libraries are probably already bundled in with your GCC installation.

I'd recommend using the Boost libraries Boost.Thread instead. This will wrap platform specifics of Win32 and Posix, and give you a solid set of threading and synchronization objects. It's also in very heavy use, so finding help on any issues you encounter on SO and other sites is easy.

You can search for a free PDF book "C++-GUI-Programming-with-Qt-4-1st-ed.zip" and read Chapter 18 about Multi-threading in Qt.

Concurrent programming features supported by Qt includes (not limited to) the following:

• Mutex
• Read Write Lock
• Semaphore
• Wait Condition
• Thread Specific Storage

However, be aware of the following trade-offs with Qt:

• Performance penalties vs native threading libraries. POSIX thread (pthreads) has been native to Linux since kernel 2.4 and may not substitute for < process.h > in W32API in all situations.
• Inter-thread communication in Qt is implemented with SIGNAL and SLOT constructs. These are NOT part of the C++ language and are implemented as macros which requires proprietary code generators provided by Qt to be fully compiled.

If you can live with the above limitations, just follow these recipes for using QThread:

1. #include < QtCore >
2. Derive your own class from QThread. You must implement a public function run() that returns void to contain instructions to be executed.
3. Instantiate your own class and call start() to kick off a new thread.

Sameple Code:

#include <QtCore>

class MyThread : public QThread {
public:
    void run() {
        // do something
    }
};

int main(int argc, char** argv) {
    MyThread t1, t2;
    t1.start(); // default implementation from QThread::start() is fine
    t2.start(); // another thread
    t1.wait();  // wait for thread to finish
    t2.wait();
    return 0;
}

As an important note in c++14, the use of concurrent threading is available:

#include<thread>

class Example
{
  auto DoStuff() -> std::string
  { 
    return "Doing Stuff";
  }

  auto DoStuff2() -> std::string
  {
    return "Doing Stuff 2";
  }
};

int main() 
{
  Example EO;
  std::string(Example::*func_pointer)();
  func_pointer = &Example::DoStuff;
  std::future<string> thread_one = std::async(std::launch::async, func_pointer, &EO); //Launching upon declaring

  std::string(Example::*func_pointer_2)();
  func_pointer_2 = &Example::DoStuff2;
  std::future<string> thread_two = std::async(std::launch::deferred, func_pointer_2, &EO);
  thread_two.get(); //Launching upon calling
}

Both std::async (std::launch::async, std::launch::deferred) and std::thread are fully compatible with Qt, and in some cases may be better at working in different OS environments.

For parallel processing, see this.