Re: Virtual function call

James Kanze <>
Tue, 22 Jan 2008 03:42:39 -0800 (PST)
On Jan 21, 10:43 pm, "Erazem Polutnik" <>

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


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()) {

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)
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