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From: Ioannis Vranos <ivr@remove.this.grad.com>
Newsgroups: comp.lang.ada
Subject: Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Fri, 25 Mar 2005 05:27:09 +0200
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--->> At first, in this and other messages you had set the follow-ups to
--->> comp.lang.ada which means we can not follow the discussion
--->> anymore!]


Martin Krischik wrote:

 > Yes, and process/system boundaries when Annex E is implemented.
 >
 > Martin



ISO C++ defines no threading facilities so far (but *care has been 
taken* to be easy implementing the algorithms and container operations 
as thread-safe).

The only multithreading I know is the .NET lock-based multithreading. I 
can't understand how in such an environment you can expect to catch 
exceptions reliably. I can only assume that Ada's built in 
multithreading mechanism is not lock-based.



Here is a .NET example:


#using <mscorlib.dll>

using namespace System;
using namespace System::Threading;


class SomeException
{};


__gc class SomeClass
{
  int index;

  //...

  public:

  // ...


  void DoSomething()
  {
     Monitor::Enter(this);

     throw SomeException();

    // Modify index

     Monitor::Exit(this);
  }

  void DoSomethingElse()
  {
     Monitor::Enter(this);

     // Modify index

     Monitor::Exit(this);
   }

// ...
};


int main() try
{
     SomeClass *ps= __gc new SomeClass;


     Thread *pthread1= __gc new Thread ( __gc new ThreadStart(ps, 
&SomeClass::DoSomething) );

     Thread *pthread2= __gc new Thread ( __gc new ThreadStart(ps, 
&SomeClass::DoSomethingElse) );


     //Start execution of ps->DoSomething()
     pthread1->Start();

     //Start execution of ps->DoSomethingElse()
     pthread2->Start();
}

catch(SomeException)
{
     Console::WriteLine("SomeException caught in main thread!\n");
}


How can you expect to catch the exception in the main thread for 
example, since it terminates immediately after the two calls in the try 
block?



The proper way in .NET (this isn't production-level code BTW):



#using <mscorlib.dll>

using namespace System;
using namespace System::Threading;

class SomeException
{};

__gc class SomeClass
{
  int index;

  //...

  public:

  // ...


  void DoSomething() try
  {
     Monitor::Enter(this);

     throw SomeException();

    // Modify index

     Monitor::Exit(this);
  }

  ==> Handle inside the specific thread locally
  catch(SomeException)
  {
         // ...
  }

  void DoSomethingElse()
  {
     Monitor::Enter(this);

     // Modify index

     Monitor::Exit(this);
   }

// ...
};


int main() try
{
     SomeClass *ps= __gc new SomeClass;


     Thread *pthread1= __gc new Thread ( __gc new ThreadStart(ps, 
&SomeClass::DoSomething) );

     Thread *pthread2= __gc new Thread ( __gc new ThreadStart(ps, 
&SomeClass::DoSomethingElse) );


     //Start execution of ps->DoSomething()
     pthread1->Start();

     //Start execution of ps->DoSomethingElse()
     pthread2->Start();
}

catch(Exception *pe)
{
     Console::WriteLine("{0}", pe->Message);
}




-- 
Ioannis Vranos

http://www23.brinkster.com/noicys