Strange C++ thread function invocation
I have the following:
class DThread
{
virtual void run()=0;
_beginthreadex(NULL,0,tfunc,this,0,&m_UIThreadID); // class itself being passed as param to thread function...
static u开发者_运维百科nsigned int __stdcall tfunc(void* thisptr)
{
static_cast<DThread*>(thisptr)->run();
return 0;
}
//other stuff
}
The run function is implemented in a derived class.
Why is the function that's being called in the thread being called through a cast this
pointer? Is this good practise?
Can't it just be called directly?
The actual function needing to run is in the derived class.
My question is
_beginthreadex
expects a (stdcall) C style function, it cannot use a C++ member function as it has no knowledge of C++. The way to get a member function running is to pass a pointer to an object and call the member function inside that function. Such a function is often called a trampoline.
Thread functions are not 'class-aware'. Your implementation makes them class aware so that the derived 'run' function will have access to class members.
Most platform-level thread APIs are bare-bones C and take a plain pointer to function to run in new thread. This means in C++ that function has to be either a free function or a static member. Neither of these give access to any class instance. The workaround for building statefull thread classes is to exploit additional "pass-through" argument of the thread creation call (that's usually a pointer that is later passed to the function executed in the new thread) and give it a pointer to the class itself, i.e. this
. The static function could then call a [virtual] member, say run()
or something like that.
The _beginthreadex function is a C-function. It doesn't know anything about C++. To access a C++ member from within the thread function you need to cast it.
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