Re: What's the connection between objects and threads?

"Szabolcs Ferenczi" <szabolcs.ferenczi@gmail.com> wrote in message
news:f99c4d57-48cd-4623-84ee-9503b7a4544a@2g2000hsn.googlegroups.com...
On May 18, 3:23 pm, James Kanze <james.ka...@gmail.com> wrote:


The pseudo-code I post over on 'comp.programming.threads' is all about the
__experimental__ synchronization algorihtms that I am brainstorming. We have
been over this before Szabolcs. If I come up with an idea, I try to make
sure to post it on c.p.t. If there is a typo in the pseudo-code, I promptly
document them and show the corrections. Anyway...


Unfortunately, wrt this thread, your so-called help is only going to lead
the OP down the wrong path. You don't seem to get it. AFAICT, the OP wanted
to create a thread base class for users to derive from. Something like:
____________________________________________________________________
class thread_base {
  pthread_t m_tid;
  virtual void on_entry() = 0;

  thread_base() {
  }

  virtual ~thread_base() throw() = 0;
};

thread_base::~thread_base() throw() {}
____________________________________________________________________

The OP cannot start the thread from the constructor of the base class! This
is what I was referring to. When threads are involved there is a major
race-condition because the thread can be started and call the on_entry pure
virtual function which results in UB. Also, even in the absence of threads
there still can be a problem. Here is a VERY simple example:
____________________________________________________________________
#include <cstdio>
#include <cstdlib>

static void call_abstract_virtual_indirect(class base*);

class base {
  friend void call_abstract_virtual_indirect(class base*);
  virtual void on_action() = 0;

public:
  base() {
    call_abstract_virtual_indirect(this);
  }

  virtual ~base() throw() = 0;
};

base::~base() throw() {}

void call_abstract_virtual_indirect(base* _this) {
  _this->on_action();
}

class derived : public base {
  int const m_ctor;

public:
  derived() : m_ctor(123456789) {

  }

private:
  void on_action() {
    std::printf("(%p)::derived::on_action()\n", (void*)this);
    if (m_ctor != 123456789) {
      std::printf("(%p)::derived::on_action() - NOT CONSTRUCTED!\n",
(void*)this);
      std::getchar();
      std::abort();
    }
  }
};

int main() {
  {
    derived d;
  }

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
  std::puts("\n\n____________________________________________\n\
Press <ENTER> to exit...");
  std::getchar();
  return 0;
}

____________________________________________________________________

The above demonstrates one reason why its generally NOT a good idea to start
a thread in a constructor. Your failure to understand this is only going to
confuse the OP. Your advise is flawed in that respect. The problem produces
undefined-behavior. It can just seg-fault, or it might print "... NOT
CONSTRUCTED!", or whatever. Therefore, I guess the moral of the story is "Do
NOT call abstract virtual functions in constructors!"

:^o