Re: Virtual function call

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 22 Jan 2008 03:42:39 -0800 (PST)

Message-ID:

<14a47440-38d6-4fe5-aecb-a95cb65c24bd@c23g2000hsa.googlegroups.com>

On Jan 21, 10:43 pm, "Erazem Polutnik" <erazem_polut...@hotmail.com>
wrote:

i have a strange problem, when calling virtual function, using
SDL Thread library. Here is situation:

CThread::CThread
{
SDL_CreateThread(RunProc,this);
}

virtual bool CThread::IsRunning()
{
return false;
}

There's a fundamental conceptual problem here already. At least
I think there's one---I'm not sure what SDL_CreateThread
actually does. But if it starts the thread (i.e. if it has
semantics something like pthread_create), then invoking it in
the constructor of a base class is a serious error---which may
or may not be immediately apparent in tests, depending on any
number of random variables in the surrounding system.

int SDLCALL CThread::RunProc(void *pParam)
{
    /*delay 100ms*/
   CThread *pt=(CThread *)pParam;
   while (pt->IsRunning()) {
      pt->DoRun();
   }
}

then I make:

class CMyThread : public CThread
{
    virtual bool IsRunning()
    {
        return true;
    }
};

After new CMyThread() I expect to function IsRunning to return
true, but it returns false as in CThread class If I add 100ms
delay everthing is working ok.

Those are the aleas of the scheduler. In general, thread
classes should NOT be polymorphic, unless they have a separate
function for starting the thread. Otherwise, you risk starting
the thread in a class before its constructor has finished. For
customization of a thread class, use the strategy pattern, not
the template method pattern.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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