Re: [Boost-users] initialization of boost::thread::id object

Subject: Re: [Boost-users] initialization of boost::thread::id object
From: Boost lzw (boostlzw_at_[hidden])
Date: 2010-06-08 19:57:33

Thanks a lot for the answer. In fact, I am trying to associate data
(std::vector<int> and Widget objects) with a boost::mutex by wrapping its
interface [ lock(), try_lock() and unlock() ] as follows:

class Widget { ... }; // data to be associated with

class MyData {
public:
   MyData(std::vector<int>& v) : v_(v), w_(new Widget) { }

   ~MyData() {
        m_.~mutex();
      delete w_;
 }

 bool is_held() { // test if current thread has locked the thread
      return id_ == boost::this_thread::get_id();
 }

 std::vector<int>& v() {
     assert( is_held() );  // lock before accessing data
     return v_;
 }

 Widget* w() {
     assert( is_held() );  // lock before accessing data
     return w_;
 }

 void lock() {
     id_ = boost::this_thread::get_id();
     m_.lock();
 }

 bool trylock() {
     bool b = m_.try_lock();
     if (b)
       id_ = boost::this_thread::get_id();
     return b;
 }

 void unlock() {
       id_ = 0;  // reset it, does not compile
       m_.unlock();
  }

private:
     boost::mutex m_;
     std::vector<int>& v_;
     Widget* w_;
     boost::thread::id id_;
};
In the MyData::unlock(), I want to reset id_ to some initial value, but it
does not compile. What should I do with it?

Thanks in advance,
Robert

On Tue, Jun 8, 2010 at 12:55 PM, Anthony Williams <anthony.ajw_at_[hidden]>wrote:

> Boost lzw <boostlzw_at_[hidden]> writes:
>
> > class X {
> > public:
> >       X() : id_(0) // does not compile
> >      { }
> >
> >      void f() {
> >            if (operation_failed)
> >                id_ = 0;   // does not compile
> >           else
> >                id_ = boost::this_thread::get_id();   // get which thread
> is
> > sucessfully executing f()
> >       }
> >       bool compare() { // compare current thread with the one that has
> > sucessfully executed f()
> >           return id_ == boost::this_thread::get_id();
> >       }
> > private:
> >    boost::thread::id id_;
> > };
> >
> > My questions are:
> >
> > How can I set the id_ to an initial value (e.g., 0 here) for compare()
> function
> > to get correct result
>
> The default constructor for thread::id yields a "not any thread" value.
>
> > , assuming multiple threads might be executing f() and
> > compare() at the same time? Is there a way to do this:
> >         atomic<boost::thread::id> id_;
> > to make them thread-safe?
>
> If you want to protect a value for safe access from multiple threads,
> use a mutex.
>
> Anthony
> --
> Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
> just::thread C++0x thread library             http://www.stdthread.co.uk
> Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
> 15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976
>
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